Record the game view from a behavior into a video asset (#405)

A game can now film itself. A behavior starts and stops a take, and
the editor turns it into a video asset with the run's picture and the
sound the player hears. The exported game hands the file over as a
download, since there is no project to keep it in.

- New action nodes `Start recording` (Name, Resolution, FPS) and
  `Stop recording`. An empty name becomes `Recording <timestamp>`.
- Resolution is `Game view`, `720p` or `1080p`. For the fixed sizes
  the height is set and the width follows the camera.
- The picture is the camera only, without hints, overlays or the
  letterbox. It is drawn into a render texture of the target size,
  so a smaller window still gives full 1080p.
- `PictureReader` reads the texture back through pixel buffers with
  a fence. The editor never waits on the GPU, and a frame with all
  three buffers busy is dropped and counted.
- Nothing of the take reaches the visible canvas. A first version
  drew tiles into it and copied them out, and with the low-latency
  canvas that midway state flickered on screen.
- The sound is tapped behind the master limiter in the FX worklet
  and encoded as Opus.
- H.264 is chosen when the browser has it, VP9 otherwise. A small
  own Matroska writer packs both tracks, no new dependency. It names
  the frame rate and BT.709 so ffmpeg keeps both.
- A second `Start recording` is ignored with a warning in the log.
  A take goes on through `Load scene` and a restart, pauses with the
  editor and is saved when the run ends. It stops at 2 GB.
- The editor uploads the take on the existing video route and makes
  the asset once the run is over. The video tab gets a Download
  button for the converted MP4.
- `downloadBlob` in `src/ui` replaces the song editor's own copy.

Findings:
- `On start` runs before the game screen knows the world. A take
  therefore fixes its size on the first frame that has a camera.
- Without a frame rate in the file, ffmpeg turned 30 fps into 59.94.
- The worker's poster JPEG is off in colour for every upload (red
  255 comes out as 229, BT.601). Not touched here.

Tests:
- Unit recording + matroska: 21 green, plus the node catalog cases.
  The Matroska test reads the file back with its own EBML reader.
- e2e 67 (new) on melektaus-atx: ALL PASSED. 1080p30 from a
  1280x800 window gives a 1920x1080 MP4 with sound and no dropped
  frame. Every 4th pixel is checked, a green quarter catches an
  upside-down picture. A take ended by Esc is saved with the
  timestamp name.
- e2e 60j: the exported game downloads an `.mkv` with EBML magic.
- The flicker is gone on the user's machine without `?vsync`.
- Gate on melektaus-atx: CHECK EXIT 0, 299 files, 4586 tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -47,7 +47,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2010 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2016 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs

src/core/graph/NodeContext.ts

@@ -2,6 +2,7 @@ import type { AnimationPlayer, EaseShare } from '../model/animation';
 import type { Mat } from '../model/matrix';
 import type { Position, PropertyBag } from '../model/Properties';
 import type { FxParams } from '../soundfx/nodes';
+import type { RecordingSettings } from '../recording/recording';
 import { type ChainTransform, composeAlongChain } from '../model/walk';
 import { enabledAlongChain } from '../properties/enabled';
 import { localMatrixOf, positionOf, transformOf } from '../properties/transform';
@@ -391,4 +392,8 @@ export interface NodeContext {
   isPaused(): boolean;
   /** From the next frame on, only behaviors that run while paused are ticked. */
   setPaused(paused: boolean): void;
+  /** Starts recording the game view (#405); where it cannot, the log says why. */
+  startRecording(settings: RecordingSettings): void;
+  /** Ends the recording and saves it; without one, only the log hears of it. */
+  stopRecording(): void;
 }

src/core/graph/nodes/action/recordingNodes.ts

@@ -0,0 +1,66 @@
+import { RECORDING_FPS, RECORDING_RESOLUTIONS, recordingSettingsOf } from '../../../recording/recording';
+import type { NodeDef, PortDef } from '../../NodeRegistry';
+import { asString } from '../../values';
+import { exec } from '../shared';
+
+const choicePort = (
+  key: string,
+  label: string,
+  options: readonly { value: string; label: string }[],
+): PortDef => ({
+  key,
+  label: key,
+  type: 'string',
+  default: options[0]!.value,
+  field: { key, label, kind: 'enum', default: options[0]!.value, options: [...options] },
+});
+
+export const StartRecording: NodeDef = {
+  type: 'StartRecording',
+  label: 'Start recording',
+  category: 'action',
+  description:
+    'Records the game view as a video, with what the master plays: only the picture of the camera, ' +
+    'none of what lies over it. "name" is what the recording is saved as; left empty, it is named ' +
+    'after the moment it starts. "resolution" Game view records at the size the picture has on ' +
+    'screen when it starts, 720p and 1080p at that height with the width of the camera. The recording ' +
+    'goes on through Load scene and Restart game, holds while the editor pauses the run, and ends with ' +
+    'Stop recording or with the run. In the editor it becomes a video asset of the project, once the ' +
+    'server has converted it; a released game downloads it as an MKV file. A second start while one ' +
+    'records is ignored and says so in the log.',
+  inputs: [
+    exec(),
+    {
+      key: 'name',
+      label: 'name',
+      type: 'string',
+      field: { key: 'name', label: 'Name', kind: 'text', default: '' },
+    },
+    choicePort('resolution', 'Resolution', RECORDING_RESOLUTIONS),
+    choicePort('fps', 'FPS', RECORDING_FPS),
+  ],
+  outputs: [exec()],
+  params: [],
+  run: (ctx, inputs) => {
+    ctx.startRecording(
+      recordingSettingsOf(asString(inputs.name), asString(inputs.resolution), asString(inputs.fps)),
+    );
+    return { next: 'exec' };
+  },
+};
+
+export const StopRecording: NodeDef = {
+  type: 'StopRecording',
+  label: 'Stop recording',
+  category: 'action',
+  description:
+    'Ends the recording Start recording began and saves it. Without one running it does nothing but ' +
+    'say so in the log.',
+  inputs: [exec()],
+  outputs: [exec()],
+  params: [],
+  run: (ctx) => {
+    ctx.stopRecording();
+    return { next: 'exec' };
+  },
+};

src/core/graph/nodes/index.ts

@@ -33,6 +33,7 @@ import { StopSong } from './action/StopSong';
 import { SetZ } from './action/SetZ';
 import { Spawn } from './action/Spawn';
 import { StopGame } from './action/StopGame';
+import { StartRecording, StopRecording } from './action/recordingNodes';
 import { StopSound } from './action/StopSound';
 import { CellToWorld, SetTile, TileAt, TileLiteral, WorldToCell } from './action/tileNodes';
 import { Abs } from './data/Abs';
@@ -323,6 +324,8 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   Log,
   StopGame,
   RestartGame,
+  StartRecording,
+  StopRecording,
   PauseGame,
   ResumeGame,
   IsGamePaused,

src/core/recording/matroska.ts

@@ -0,0 +1,374 @@
+/**
+ * Matroska (`.mkv`), the file a recording of the game view is made into (#405): the encoded frames
+ * of a `VideoEncoder` and the Opus packets of an `AudioEncoder`, in clusters as they come. Only the
+ * head waits for the end – it holds what the encoders say about themselves once they ran, and the
+ * length – so the clusters can leave the writer long before.
+ */
+import { OPUS_RATE } from '../music/ogg';
+
+export const AVC_CODEC = 'V_MPEG4/ISO/AVC';
+export const VP9_CODEC = 'V_VP9';
+export const OPUS_CODEC = 'A_OPUS';
+
+/** Milliseconds per tick of a block's time: the usual 1 ms, so a cluster spans 32 s at most. */
+const TIMESTAMP_SCALE = 1_000_000;
+/** What a cluster may span before the next one starts, inside the ±32767 ticks of a block. */
+const CLUSTER_SPAN_MS = 30_000;
+const CLUSTER_MOST_BYTES = 8 * 1024 * 1024;
+/** How far one track may run ahead of a silent other before its blocks are written anyway. */
+const INTERLEAVE_WAIT_MS = 1000;
+/** What Opus asks a player to decode before a point it seeks to (RFC 7845: 80 ms). */
+const OPUS_PRE_ROLL_NS = 80_000_000;
+
+const ID = {
+  ebml: 0x1a45dfa3,
+  ebmlVersion: 0x4286,
+  ebmlReadVersion: 0x42f7,
+  ebmlMaxIdLength: 0x42f2,
+  ebmlMaxSizeLength: 0x42f3,
+  docType: 0x4282,
+  docTypeVersion: 0x4287,
+  docTypeReadVersion: 0x4285,
+  segment: 0x18538067,
+  info: 0x1549a966,
+  timestampScale: 0x2ad7b1,
+  duration: 0x4489,
+  muxingApp: 0x4d80,
+  writingApp: 0x5741,
+  tracks: 0x1654ae6b,
+  trackEntry: 0xae,
+  trackNumber: 0xd7,
+  trackUid: 0x73c5,
+  trackType: 0x83,
+  flagLacing: 0x9c,
+  codecId: 0x86,
+  codecPrivate: 0x63a2,
+  defaultDuration: 0x23e383,
+  codecDelay: 0x56aa,
+  seekPreRoll: 0x56bb,
+  video: 0xe0,
+  pixelWidth: 0xb0,
+  pixelHeight: 0xba,
+  colour: 0x55b0,
+  matrixCoefficients: 0x55b1,
+  range: 0x55b9,
+  transferCharacteristics: 0x55ba,
+  primaries: 0x55bb,
+  audio: 0xe1,
+  samplingFrequency: 0xb5,
+  channels: 0x9f,
+  cluster: 0x1f43b675,
+  timestamp: 0xe7,
+  simpleBlock: 0xa3,
+} as const;
+
+export interface MatroskaVideo {
+  codec: typeof AVC_CODEC | typeof VP9_CODEC;
+  width: number;
+  height: number;
+  /** H.264 needs its `avcC`, the `description` the encoder hands out with its first frame. */
+  private: Uint8Array | null;
+  /** The frame rate asked for: without it a player guesses one from the times of the frames. */
+  fps: number;
+  colour: MatroskaColour;
+}
+
+/** How the pixels are to be read, by the numbers of ITU-T H.273 that Matroska takes. */
+export interface MatroskaColour {
+  matrix: number;
+  transfer: number;
+  primaries: number;
+  fullRange: boolean;
+}
+
+/** What Chrome's encoders make of a canvas when they do not say: BT.709 in the broadcast range. */
+const BT709: MatroskaColour = { matrix: 1, transfer: 1, primaries: 1, fullRange: false };
+
+const MATRICES: Record<string, number> = { rgb: 0, bt709: 1, bt470bg: 5, smpte170m: 6, 'bt2020-ncl': 9 };
+const TRANSFERS: Record<string, number> = {
+  bt709: 1,
+  smpte170m: 6,
+  linear: 8,
+  'iec61966-2-1': 13,
+  pq: 16,
+  hlg: 18,
+};
+const PRIMARIES: Record<string, number> = { bt709: 1, bt470bg: 5, smpte170m: 6, bt2020: 9, smpte432: 12 };
+
+/**
+ * The colour a `VideoColorSpaceInit` names, as a WebCodecs encoder reports it; what it leaves out is
+ * taken from BT.709. Without it a player falls back on its own guess, and ffmpeg's is BT.601: a red
+ * of 255 came back as 233.
+ */
+export function colourOf(
+  space: {
+    matrix?: string | null;
+    transfer?: string | null;
+    primaries?: string | null;
+    fullRange?: boolean | null;
+  } | null,
+): MatroskaColour {
+  return {
+    matrix: MATRICES[space?.matrix ?? ''] ?? BT709.matrix,
+    transfer: TRANSFERS[space?.transfer ?? ''] ?? BT709.transfer,
+    primaries: PRIMARIES[space?.primaries ?? ''] ?? BT709.primaries,
+    fullRange: space?.fullRange ?? BT709.fullRange,
+  };
+}
+
+export interface MatroskaAudio {
+  channels: number;
+  /** The OpusHead, the encoder's `description`. */
+  private: Uint8Array | null;
+  /** Samples at 48 kHz the decoder throws away at the start, as the OpusHead says. */
+  preSkip: number;
+}
+
+export interface MatroskaHead {
+  video: MatroskaVideo;
+  audio: MatroskaAudio | null;
+  durationMs: number;
+  app: string;
+}
+
+/** The track a block belongs to: the video is always the first, the sound the second. */
+export type MatroskaTrack = 1 | 2;
+
+interface Block {
+  track: MatroskaTrack;
+  ms: number;
+  key: boolean;
+  data: Uint8Array;
+}
+
+/**
+ * Takes the frames of both tracks as the encoders hand them out and writes them in clusters, in the
+ * order of their time. The two tracks arrive apart, so a block waits until the other track has come
+ * as far, or until it is `INTERLEAVE_WAIT_MS` behind what that track already brought.
+ */
+export class MatroskaWriter {
+  private readonly waiting: [Block[], Block[]] = [[], []];
+  /** The latest time each track has brought, null before its first block. */
+  private readonly latest: [number | null, number | null] = [null, null];
+  private cluster: { ms: number; blocks: Uint8Array[]; bytes: number } | null = null;
+  private done: Uint8Array[] = [];
+  private written = 0;
+
+  /** `tracks` 1 is a recording without sound. */
+  constructor(private readonly tracks: 1 | 2) {}
+
+  /** Bytes of the clusters so far, the open one included: what the file is without its head. */
+  get bytes(): number {
+    return this.written + (this.cluster?.bytes ?? 0);
+  }
+
+  add(track: MatroskaTrack, timestampUs: number, key: boolean, data: Uint8Array): void {
+    const ms = Math.round(timestampUs / 1000);
+    this.waiting[track - 1]!.push({ track, ms, key, data });
+    const latest = this.latest[track - 1] ?? null;
+    this.latest[track - 1] = latest === null ? ms : Math.max(latest, ms);
+    this.interleave(false);
+  }
+
+  /** The clusters finished since the last call, in file order. */
+  take(): Uint8Array[] {
+    const done = this.done;
+    this.done = [];
+    return done;
+  }
+
+  /** Writes what still waits and closes the last cluster; the head goes in front of everything taken. */
+  finish(head: MatroskaHead): { head: Uint8Array; rest: Uint8Array[] } {
+    this.interleave(true);
+    this.closeCluster();
+    return { head: headOf(head, this.written), rest: this.take() };
+  }
+
+  private interleave(all: boolean): void {
+    for (;;) {
+      const next = this.nextBlock(all);
+      if (!next) return;
+      this.write(next);
+    }
+  }
+
+  /** The earliest waiting block, if it may be written yet. */
+  private nextBlock(all: boolean): Block | null {
+    const [first, second] = this.waiting;
+    const a = first[0];
+    const b = this.tracks === 2 ? second[0] : undefined;
+    const earliest = a && b ? (b.ms < a.ms ? b : a) : (a ?? b);
+    if (!earliest) return null;
+    if (!all && this.tracks === 2) {
+      const other = this.latest[earliest.track === 1 ? 1 : 0];
+      const own = this.latest[earliest.track - 1] ?? earliest.ms;
+      const caughtUp = other !== null && other >= earliest.ms;
+      if (!caughtUp && own - earliest.ms < INTERLEAVE_WAIT_MS) return null;
+    }
+    this.waiting[earliest.track - 1]!.shift();
+    return earliest;
+  }
+
+  private write(block: Block): void {
+    const open = this.cluster;
+    const startsCluster =
+      !open ||
+      (block.track === 1 && block.key && open.blocks.length > 0) ||
+      Math.abs(block.ms - open.ms) > CLUSTER_SPAN_MS ||
+      open.bytes > CLUSTER_MOST_BYTES;
+    if (startsCluster) {
+      this.closeCluster();
+      this.cluster = { ms: block.ms, blocks: [], bytes: 0 };
+    }
+    const cluster = this.cluster!;
+    const header = new Uint8Array(4);
+    header[0] = 0x80 | block.track;
+    new DataView(header.buffer).setInt16(1, block.ms - cluster.ms);
+    header[3] = block.key ? 0x80 : 0;
+    const element = concat([
+      idBytes(ID.simpleBlock),
+      sizeBytes(header.length + block.data.length),
+      header,
+      block.data,
+    ]);
+    cluster.blocks.push(element);
+    cluster.bytes += element.length;
+  }
+
+  private closeCluster(): void {
+    const cluster = this.cluster;
+    this.cluster = null;
+    if (!cluster || cluster.blocks.length === 0) return;
+    const body = [uint(ID.timestamp, Math.max(0, cluster.ms)), ...cluster.blocks];
+    const bytes = master(ID.cluster, body);
+    this.done.push(bytes);
+    this.written += bytes.length;
+  }
+}
+
+/** The EBML header, and the segment's own head with its size, info and tracks. */
+function headOf(head: MatroskaHead, clusterBytes: number): Uint8Array {
+  const ebml = master(ID.ebml, [
+    uint(ID.ebmlVersion, 1),
+    uint(ID.ebmlReadVersion, 1),
+    uint(ID.ebmlMaxIdLength, 4),
+    uint(ID.ebmlMaxSizeLength, 8),
+    text(ID.docType, 'matroska'),
+    uint(ID.docTypeVersion, 4),
+    uint(ID.docTypeReadVersion, 2),
+  ]);
+  const info = master(ID.info, [
+    uint(ID.timestampScale, TIMESTAMP_SCALE),
+    float(ID.duration, head.durationMs),
+    text(ID.muxingApp, head.app),
+    text(ID.writingApp, head.app),
+  ]);
+  const { video, audio } = head;
+  const entries = [
+    master(ID.trackEntry, [
+      uint(ID.trackNumber, 1),
+      uint(ID.trackUid, 1),
+      uint(ID.trackType, 1),
+      uint(ID.flagLacing, 0),
+      text(ID.codecId, video.codec),
+      ...(video.private ? [binary(ID.codecPrivate, video.private)] : []),
+      uint(ID.defaultDuration, Math.round(1e9 / video.fps)),
+      master(ID.video, [
+        uint(ID.pixelWidth, video.width),
+        uint(ID.pixelHeight, video.height),
+        master(ID.colour, [
+          uint(ID.matrixCoefficients, video.colour.matrix),
+          uint(ID.range, video.colour.fullRange ? 2 : 1),
+          uint(ID.transferCharacteristics, video.colour.transfer),
+          uint(ID.primaries, video.colour.primaries),
+        ]),
+      ]),
+    ]),
+  ];
+  if (audio)
+    entries.push(
+      master(ID.trackEntry, [
+        uint(ID.trackNumber, 2),
+        uint(ID.trackUid, 2),
+        uint(ID.trackType, 2),
+        uint(ID.flagLacing, 0),
+        text(ID.codecId, OPUS_CODEC),
+        ...(audio.private ? [binary(ID.codecPrivate, audio.private)] : []),
+        uint(ID.codecDelay, Math.round((audio.preSkip * 1e9) / OPUS_RATE)),
+        uint(ID.seekPreRoll, OPUS_PRE_ROLL_NS),
+        master(ID.audio, [float(ID.samplingFrequency, OPUS_RATE), uint(ID.channels, audio.channels)]),
+      ]),
+    );
+  const tracks = master(ID.tracks, entries);
+  const segmentSize = info.length + tracks.length + clusterBytes;
+  return concat([ebml, idBytes(ID.segment), sizeBytes(segmentSize, 8), info, tracks]);
+}
+
+/** An element id as it is written: its own marker bits are part of the number. */
+function idBytes(id: number): Uint8Array {
+  const length = id > 0xffffff ? 4 : id > 0xffff ? 3 : id > 0xff ? 2 : 1;
+  const out = new Uint8Array(length);
+  for (let i = 0; i < length; i++) out[i] = Math.floor(id / 256 ** (length - 1 - i)) % 256;
+  return out;
+}
+
+/** A size as an EBML variable-length integer, in the fewest bytes or in `length` of them. */
+function sizeBytes(size: number, length = sizeLength(size)): Uint8Array {
+  const out = new Uint8Array(length);
+  let rest = size;
+  for (let i = length - 1; i >= 0; i--) {
+    out[i] = rest % 256;
+    rest = Math.floor(rest / 256);
+  }
+  out[0]! |= 0x80 >> (length - 1);
+  return out;
+}
+
+/** All ones is "unknown", so a size needs one value more than its bits would hold. */
+function sizeLength(size: number): number {
+  let length = 1;
+  while (size >= 2 ** (7 * length) - 1) length++;
+  return length;
+}
+
+function element(id: number, data: Uint8Array): Uint8Array {
+  return concat([idBytes(id), sizeBytes(data.length), data]);
+}
+
+function master(id: number, children: readonly Uint8Array[]): Uint8Array {
+  return element(id, concat(children));
+}
+
+function uint(id: number, value: number): Uint8Array {
+  const bytes: number[] = [];
+  let rest = Math.max(0, Math.round(value));
+  do {
+    bytes.unshift(rest % 256);
+    rest = Math.floor(rest / 256);
+  } while (rest > 0);
+  return element(id, Uint8Array.from(bytes));
+}
+
+function float(id: number, value: number): Uint8Array {
+  const data = new Uint8Array(8);
+  new DataView(data.buffer).setFloat64(0, value);
+  return element(id, data);
+}
+
+function text(id: number, value: string): Uint8Array {
+  return element(id, new TextEncoder().encode(value));
+}
+
+function binary(id: number, value: Uint8Array): Uint8Array {
+  return element(id, value);
+}
+
+function concat(parts: readonly Uint8Array[]): Uint8Array {
+  const out = new Uint8Array(parts.reduce((sum, part) => sum + part.length, 0));
+  let at = 0;
+  for (const part of parts) {
+    out.set(part, at);
+    at += part.length;
+  }
+  return out;
+}

src/core/recording/recording.ts

@@ -0,0 +1,177 @@
+/**
+ * Recording the game view (#405): what `Start recording` asks for, and the rules a recording is made
+ * by – its size, its name, which H.264 it is, when a frame is taken and where the picture lands.
+ * The browser's side, encoders and canvas, is `viewport/GameRecorder.ts`.
+ */
+
+export type RecordingResolution = 'view' | '720p' | '1080p';
+
+export interface RecordingSettings {
+  /** Empty: named after the moment it starts (`recordingName`). */
+  name: string;
+  resolution: RecordingResolution;
+  fps: 30 | 60;
+}
+
+export const RECORDING_RESOLUTIONS: readonly { value: RecordingResolution; label: string }[] = [
+  { value: 'view', label: 'Game view' },
+  { value: '720p', label: '720p' },
+  { value: '1080p', label: '1080p' },
+];
+
+export const RECORDING_FPS = [
+  { value: '60', label: '60' },
+  { value: '30', label: '30' },
+];
+
+/** What a run hands its graphs to record with; each answer is a line for the log, '' for none. */
+export interface RecordingSink {
+  start(settings: RecordingSettings): string;
+  stop(): string;
+}
+
+/** The upload takes 2 GB (`MAX_VIDEO_BYTES`); a recording stops a little before, its head still to come. */
+export const RECORDING_MOST_BYTES = 2048 * 1024 * 1024 - 16 * 1024 * 1024;
+
+/** What `Game view` records at most: a 4K picture, larger ones are scaled down to it. */
+const MOST_WIDTH = 3840;
+const MOST_HEIGHT = 2160;
+/** Bits per pixel of a frame: 10 Mbit/s at 1080p60. Flat game graphics need less than film. */
+const BITS_PER_PIXEL = 0.08;
+const KEY_FRAME_SECONDS = 2;
+
+export function recordingSettingsOf(
+  name: string,
+  resolution: string,
+  fps: string | number,
+): RecordingSettings {
+  const known = RECORDING_RESOLUTIONS.find((r) => r.value === resolution);
+  return { name: name.trim(), resolution: known?.value ?? 'view', fps: Number(fps) === 30 ? 30 : 60 };
+}
+
+/** The name a recording is saved under: the one asked for, else `Recording 2026-10-11 16:42:05`. */
+export function recordingName(asked: string, at: Date): string {
+  if (asked.trim()) return asked.trim();
+  const two = (n: number) => String(n).padStart(2, '0');
+  const day = `${at.getFullYear()}-${two(at.getMonth() + 1)}-${two(at.getDate())}`;
+  return `Recording ${day} ${two(at.getHours())}:${two(at.getMinutes())}:${two(at.getSeconds())}`;
+}
+
+/**
+ * The size of the frames, even on both sides as the encoders want it. `shown` is the camera's
+ * picture as it lies on screen and `whole` all of it, both in device pixels: they differ only while
+ * a fixed scale cuts the picture to the view. `Game view` records what is shown at its size; 720p
+ * and 1080p hold the height and take the width from the camera.
+ */
+export function recordingSize(
+  resolution: RecordingResolution,
+  shown: { w: number; h: number },
+  whole: { w: number; h: number },
+): { w: number; h: number } {
+  if (resolution === 'view') {
+    const fit = Math.min(1, MOST_WIDTH / shown.w, MOST_HEIGHT / shown.h);
+    return { w: even(shown.w * fit), h: even(shown.h * fit) };
+  }
+  const h = resolution === '720p' ? 720 : 1080;
+  return { w: even((h * whole.w) / whole.h), h };
+}
+
+function even(value: number): number {
+  return Math.max(2, Math.round(value / 2) * 2);
+}
+
+/** H.264 levels by the frame size and macroblocks a second they allow (ITU-T H.264, table A-1). */
+const AVC_LEVELS: readonly { level: number; frame: number; rate: number }[] = [
+  { level: 0x1f, frame: 3600, rate: 108_000 },
+  { level: 0x20, frame: 5120, rate: 216_000 },
+  { level: 0x28, frame: 8192, rate: 245_760 },
+  { level: 0x2a, frame: 8704, rate: 522_240 },
+  { level: 0x32, frame: 22_080, rate: 589_824 },
+  { level: 0x33, frame: 36_864, rate: 983_040 },
+  { level: 0x34, frame: 36_864, rate: 2_073_600 },
+  { level: 0x3c, frame: 139_264, rate: 4_177_920 },
+];
+
+/** The codec string of H.264 High at the lowest level that takes these frames. */
+export function avcCodec(w: number, h: number, fps: number): string {
+  const frame = Math.ceil(w / 16) * Math.ceil(h / 16);
+  const found = AVC_LEVELS.find((l) => frame <= l.frame && frame * fps <= l.rate) ?? AVC_LEVELS.at(-1)!;
+  return `avc1.6400${found.level.toString(16).padStart(2, '0')}`;
+}
+
+/** VP9 profile 0 at 8 bits; level 4.1 takes 1080p60, 5.1 4K60. */
+export function vp9Codec(w: number, h: number): string {
+  return w * h <= 2_228_224 ? 'vp09.00.41.08' : 'vp09.00.51.08';
+}
+
+export function videoBitrate(w: number, h: number, fps: number): number {
+  return Math.round(w * h * fps * BITS_PER_PIXEL);
+}
+
+/** Whether the `index`th frame taken (from 0) starts a group, so a player can seek to it. */
+export function isKeyFrame(index: number, fps: number): boolean {
+  return index % (fps * KEY_FRAME_SECONDS) === 0;
+}
+
+/**
+ * The time of a recording in milliseconds, from its first frame: real time, minus what the editor
+ * held it paused. A game that pauses itself is still seen, so that is not taken out.
+ */
+export class RecordingClock {
+  private origin: number | null = null;
+  private pausedAt: number | null = null;
+  private paused = 0;
+
+  get started(): boolean {
+    return this.origin !== null;
+  }
+
+  start(now: number): void {
+    this.origin = now;
+  }
+
+  pause(now: number): void {
+    this.pausedAt ??= now;
+  }
+
+  resume(now: number): void {
+    if (this.pausedAt === null) return;
+    this.paused += now - this.pausedAt;
+    this.pausedAt = null;
+  }
+
+  at(now: number): number {
+    if (this.origin === null) return 0;
+    return Math.max(0, (this.pausedAt ?? now) - this.origin - this.paused);
+  }
+}
+
+/**
+ * Which drawn frames a recording takes: about `fps` a second whatever the screen draws. A frame a
+ * little early still counts, so 60 at 60 Hz takes every one however unevenly they come.
+ */
+export class FrameGate {
+  private next = 0;
+
+  constructor(private readonly fps: number) {}
+
+  take(ms: number): boolean {
+    const period = 1000 / this.fps;
+    if (ms < this.next - period / 4) return false;
+    this.next = Math.max(this.next + period, ms + period / 2);
+    return true;
+  }
+}
+
+/**
+ * Where the view's local CSS pixels land when `area` of it (a centre and a size) fills a frame of
+ * `size` pixels: the recording draws into a texture of that size, whatever the window's.
+ */
+export function pictureTransform(
+  area: { x: number; y: number; w: number; h: number },
+  size: { w: number; h: number },
+): { a: number; d: number; tx: number; ty: number } {
+  const sx = size.w / area.w;
+  const sy = size.h / area.h;
+  return { a: sx, d: sy, tx: size.w / 2 - area.x * sx, ty: size.h / 2 - area.y * sy };
+}

src/editor/Editor.ts

@@ -76,6 +76,8 @@ import { HierarchyPanel } from './panels/HierarchyPanel';
 import { TilemapPanel } from './panels/TilemapPanel';
 import { Toolbar } from './panels/Toolbar';
 import { type PlayHost, PlaySession, type WorldSnapshot } from './PlaySession';
+import { type RecordingUpload, RecordingUploads } from './recordingUploads';
+import type { MadeRecording } from '../viewport/GameRecorder';
 import { ProjectSession } from './ProjectSession';
 import { ParticlesEditor } from './particles/ParticlesEditor';
 import { ShaderEditor } from './shader/ShaderEditor';
@@ -313,6 +315,7 @@ export class Editor implements PlayHost {
   /** Frame rate, refreshed twice a second; null until the first measurement. */
   fps: number | null = null;
   private readonly play: PlaySession;
+  private readonly recordings: RecordingUploads;
   private readonly editorRun: EditorRun;
   private readonly projects: ProjectSession;
   private readonly leftColumn: VStack;
@@ -400,6 +403,7 @@ export class Editor implements PlayHost {
     this.hierarchy = new HierarchyPanel(state);
     this.inspector = new Inspector(state);
     this.tilemapPanel = new TilemapPanel(state);
+    this.recordings = new RecordingUploads({ state, videos });
     this.play = new PlaySession(state, this);
     this.editorRun = new EditorRun(state, {
       // The game view is the one place where no key means anything to the editor, so that is where
@@ -1341,6 +1345,7 @@ export class Editor implements PlayHost {
     this.editorRun.tick(step);
     this.viewport.animate(sceneDraws);
     this.gameView.animate(gameDraws);
+    this.play.recordFrame();
     this.console.refresh();
     if (this.shows('animation')) this.animationEditor.tick(step);
     if (this.shows('tile')) this.tileEditor.tick(step);
@@ -1480,4 +1485,13 @@ export class Editor implements PlayHost {
       performance: this.play.performance(),
     };
   }
+
+  /** The recordings of the game view (#405) and what became of them on the way into the project. */
+  recordingInfo(): ReturnType<PlaySession['recordingInfo']> & { uploads: RecordingUpload[] } {
+    return { ...this.play.recordingInfo(), uploads: this.recordings.info() };
+  }
+
+  recordingSaved(recording: MadeRecording): void {
+    this.recordings.add(recording);
+  }
 }

src/editor/PlaySession.ts

@@ -14,8 +14,9 @@ import { gameClock } from '../core/time';
 import { loadPhysics, readyPhysics } from '../runtime/physics/engine';
 import { Input } from '../runtime/systems/Input';
 import type { World } from '../runtime/World';
-import { ui } from '../ui';
+import { onFrame, ui } from '../ui';
 import { soundLibrary } from '../viewport/SoundLibrary';
+import { GameRecorder, type MadeRecording } from '../viewport/GameRecorder';
 import { VIDEO_WAIT_MS, videoLibrary } from '../viewport/VideoLibrary';
 import { FrameMeter, type FrameTimes } from './frameMeter';
 import { saveArea } from './saves';
@@ -55,6 +56,8 @@ export interface PlayHost {
   showGame(): void;
   /** Leave the game view if it is the one on screen. */
   leaveGame(): void;
+  /** A recording of the game view is made (#405): it goes into the project. */
+  recordingSaved(recording: MadeRecording): void;
 }
 
 export class PlaySession {
@@ -74,11 +77,21 @@ export class PlaySession {
    * frames of a cut showed white without them.
    */
   private videoWait: number | null = null;
+  /** Records the game view for `Start recording` (#405); one for the editor, so it outlives a restart. */
+  private readonly recorder: GameRecorder;
 
   constructor(
     private readonly state: EditorState,
     private readonly host: PlayHost,
   ) {
+    this.recorder = new GameRecorder({
+      view: () => host.gameView,
+      log: (line) => {
+        this.world?.log.push(line);
+        console.warn('[recording]', line);
+      },
+      saved: (recording) => host.recordingSaved(recording),
+    });
     // An object picked during a run is looked at now: the hierarchy has to list it to show it picked.
     state.on('selection', () => {
       if (this.world && this.panels.holdsStructure) this.follow(this.panels.flush());
@@ -121,6 +134,7 @@ export class PlaySession {
     // on mid-note, rather than playing to their end over a still image.
     soundLibrary.setSuspended(this.state.paused);
     videoLibrary.setPaused(this.state.paused);
+    this.recorder.setPaused(this.state.paused);
     ui().focus.blur();
     this.state.setNotice(this.playNotice(), 'quiet');
   }
@@ -173,6 +187,7 @@ export class PlaySession {
       physics,
       clock: gameClock,
       saves: saveArea(runtime.id, 'play'),
+      recorder: this.recorder,
     };
     this.run = GameRun.start(runtime, scene.id, this.input, services);
     videoLibrary.startRun();
@@ -237,7 +252,12 @@ export class PlaySession {
       : 'No camera is switched on – nothing is rendered · Esc stops';
   }
 
-  stop(): void {
+  /** `restarting`: a `Restart game` plays on at once, and a recording goes on with it. */
+  stop(restarting = false): void {
+    if (!restarting) {
+      this.recorder.end();
+      this.recorder.setPaused(false);
+    }
     // The run's seconds end with it: the editor goes back to the picture of a freshly loaded scene.
     gameClock.reset();
     // And its sounds end with it, looping music included – nothing may outlive the world.
@@ -282,7 +302,7 @@ export class PlaySession {
       return;
     }
     if (request === 'restart') {
-      this.stop();
+      this.stop(true);
       this.play();
       return;
     }
@@ -302,6 +322,23 @@ export class PlaySession {
     this.follow(world.paused && !wasPaused ? this.panels.flush() : update);
   }
 
+  /**
+   * Called once everything that draws the game view this frame is queued: a recording takes the
+   * frame after all of it, behind another tab too.
+   */
+  recordFrame(): void {
+    if (!this.recorder.isRecording || !this.world) return;
+    onFrame('recording', () => {
+      if (!this.host.gameView.visible) this.host.gameView.redrawHidden();
+      this.recorder.capture();
+    });
+  }
+
+  /** What became of the recordings (#405, test hook). */
+  recordingInfo(): ReturnType<GameRecorder['info']> {
+    return this.recorder.info();
+  }
+
   /** Brings the panels up to date as far as `update` says; the game's own picture never waits. */
   private follow(update: PanelUpdate): void {
     if (update.structure) {

src/editor/assetImport.ts

@@ -73,7 +73,7 @@ async function importFile<U extends UploadedFile, D extends ImportDeps>(
  * what the asset is called. The server stores by content hash, so a re-imported file comes back
  * as the one the project already references; two assets on one file would disagree about it.
  */
-function adopt<U extends UploadedFile, D extends ImportDeps>(
+export function adopt<U extends UploadedFile, D extends ImportDeps>(
   kind: ImportKind<U, D>,
   uploaded: U,
   nameBase: string,

src/editor/debug/videoHooks.ts

@@ -36,5 +36,7 @@ export function videoHooks({ state, editor }: HookContext) {
     videoPickerRow: (file: string) => currentVideoPicker()?.rowGlobal(file) ?? null,
     /** What each player behind a sprite is doing: its time, whether it runs and is heard. */
     videos: () => videoLibrary.debugInfo(),
+    /** The recordings of the game view (#405): each take, and its way into the project. */
+    recording: () => editor.recordingInfo(),
   };
 }

src/editor/recordingUploads.ts

@@ -0,0 +1,84 @@
+import type { UploadedVideo } from '../core/store/VideoStore';
+import type { MadeRecording } from '../viewport/GameRecorder';
+import { adoptVideo, type VideoImportDeps } from './videoImport';
+
+/** How often the server is asked whether a recording is converted, as the video picker does. */
+const POLL_MS = 1000;
+
+type UploadState = 'uploading' | 'converting' | 'added' | 'elsewhere' | 'failed';
+
+/** What became of a recording on its way into the project (test hook). */
+export interface RecordingUpload {
+  name: string;
+  state: UploadState;
+  /** The name the server gave it, once it is up. */
+  file: string;
+  assetId: string;
+  bytes: number;
+  error: string;
+}
+
+/**
+ * Brings a recording of the game view into the project (#405): up to the server, which converts it
+ * like any video, then an asset named after the recording once it is ready. Only while the editor
+ * is not playing: a run plays a copy of the project that is thrown away, and so would the asset be.
+ */
+export class RecordingUploads {
+  private readonly uploads: RecordingUpload[] = [];
+
+  constructor(private readonly deps: VideoImportDeps) {}
+
+  add(recording: MadeRecording): void {
+    const upload: RecordingUpload = {
+      name: recording.name,
+      state: 'uploading',
+      file: '',
+      assetId: '',
+      bytes: recording.blob.size,
+      error: '',
+    };
+    this.uploads.push(upload);
+    void this.bring(recording, upload).catch((error: unknown) => {
+      upload.state = 'failed';
+      upload.error = error instanceof Error ? error.message : String(error);
+      this.deps.state.setNotice(`Could not save the recording "${recording.name}": ${upload.error}`, 'error');
+    });
+  }
+
+  info(): RecordingUpload[] {
+    return this.uploads.map((upload) => ({ ...upload }));
+  }
+
+  private async bring(recording: MadeRecording, upload: RecordingUpload): Promise<void> {
+    const { state, videos } = this.deps;
+    const projectId = state.project.id;
+    state.setNotice(`Uploading the recording "${recording.name}"…`);
+    let video: UploadedVideo = await videos.upload(
+      recording.blob,
+      'application/octet-stream',
+      `${recording.name}.mkv`,
+    );
+    upload.file = video.file;
+    upload.state = 'converting';
+    while (video.status !== 'ready') {
+      if (video.status === 'failed') throw new Error(video.error ?? 'the server could not convert it');
+      await wait(POLL_MS);
+      video = (await videos.list()).find((v) => v.file === upload.file) ?? video;
+    }
+    while (state.mode !== 'edit') await wait(POLL_MS);
+    if (state.project.id !== projectId) {
+      upload.state = 'elsewhere';
+      state.setNotice(`The recording "${recording.name}" is ready – the video picker has it.`);
+      return;
+    }
+    const outcome = adoptVideo(video, recording.name, this.deps);
+    if (!outcome.assetId) throw new Error(outcome.error ?? 'the project did not take it');
+    upload.state = 'added';
+    upload.assetId = outcome.assetId;
+    state.setNotice(`Added the recording "${recording.name}" to the project.`);
+  }
+}
+
+function wait(ms: number): Promise<void> {
+  return new Promise((resolve) => setTimeout(resolve, ms));
+}

src/editor/song/SongEditor.ts

@@ -26,7 +26,7 @@ import { findSound } from '../../core/model/sound';
 import { textOf } from '../../core/model/Properties';
 import { assetFxOf } from '../../core/properties/assetFx';
 import type { SoundStore } from '../../core/store/SoundStore';
-import { Button, Dialog, Dropdown, Label, Menu, ui, VStack, type Widget } from '../../ui';
+import { Button, Dialog, downloadBlob, Dropdown, Label, Menu, ui, VStack, type Widget } from '../../ui';
 import { Music } from '../../viewport/music';
 import { MASTER_MIX } from '../../core/music/mixer';
 import { soundLibrary } from '../../viewport/SoundLibrary';
@@ -596,10 +596,5 @@ export class SongEditor extends AssetTabEditor<SongAsset, SongTimeline> {
 
 /** Hands the take to the browser as a file, for when the server would not take it. */
 function downloadWav(wav: Uint8Array<ArrayBuffer>, name: string): void {
-  const url = URL.createObjectURL(new Blob([wav], { type: 'audio/wav' }));
+  downloadBlob(new Blob([wav], { type: 'audio/wav' }), `${name}.wav`);
-  const link = document.createElement('a');
-  link.href = url;
-  link.download = `${name}.wav`;
-  link.click();
-  setTimeout(() => URL.revokeObjectURL(url), 60_000);
 }

src/editor/video/VideoEditor.ts

@@ -1,5 +1,6 @@
 import type { Asset, VideoAsset } from '../../core/model/Asset';
-import { Button } from '../../ui';
+import { Button, downloadFile } from '../../ui';
+import { videoFileUrl } from '../../viewport/VideoLibrary';
 import { AssetTabEditor } from '../AssetTabEditor';
 import { TOOLBAR_ICON_SVG } from '../panels/toolbarIcons';
 import type { EditorState } from '../state';
@@ -15,6 +16,8 @@ const INFO_WIDTH = 220;
 export class VideoEditor extends AssetTabEditor<VideoAsset, VideoCanvas> {
   /** Public for the check scripts. */
   readonly playButton: Button;
+  /** Saves the MP4 the server made (#405): a recording of the game view is had this way. */
+  readonly downloadButton: Button;
 
   constructor(state: EditorState) {
     super(state, { infoWidth: INFO_WIDTH });
@@ -24,10 +27,14 @@ export class VideoEditor extends AssetTabEditor<VideoAsset, VideoCanvas> {
       tooltip: 'Play the picked clip, else the whole video (Space)',
       onClick: () => this.togglePlay(),
     });
+    this.downloadButton = new Button('Download', {
+      tooltip: 'Save the video as an MP4 file',
+      onClick: () => this.download(),
+    });
     const canvas = new VideoCanvas(state);
     canvas.onTogglePlay = () => this.togglePlay();
     canvas.onStopped = () => this.syncPlayButton();
-    this.mount(canvas, [this.playButton]);
+    this.mount(canvas, [this.playButton, this.downloadButton]);
     state.on('load', () => this.canvas.stop());
     state.on('selection', () => this.refresh());
   }
@@ -50,6 +57,11 @@ export class VideoEditor extends AssetTabEditor<VideoAsset, VideoCanvas> {
     this.syncPlayButton();
   }
 
+  private download(): void {
+    const asset = this.asset();
+    if (asset?.file) downloadFile(videoFileUrl(asset.file), `${asset.name}.mp4`);
+  }
+
   /** Only called while the tab is visible. */
   tick(): void {
     this.canvas.tick();
@@ -72,6 +84,7 @@ export class VideoEditor extends AssetTabEditor<VideoAsset, VideoCanvas> {
 
   protected override syncExtras(asset: VideoAsset | null): void {
     this.fitButton.setEnabled(Boolean(asset?.file));
+    this.downloadButton.setEnabled(Boolean(asset?.file));
     this.syncPlayButton();
   }
 

src/editor/videoImport.ts

@@ -1,6 +1,6 @@
 import { CreateVideoCommand } from '../core/commands/assetCommands';
 import type { UploadedVideo, VideoStore } from '../core/store/VideoStore';
-import { type ImportKind, type ImportOutcome, importStored } from './assetImport';
+import { adopt, type ImportKind, type ImportOutcome, importStored } from './assetImport';
 import type { EditorState } from './state';
 
 /** What the file dialog offers and a drop accepts; ffmpeg on the server has the last word. */
@@ -44,6 +44,11 @@ export function importStoredVideo(video: UploadedVideo, deps: VideoImportDeps):
   return importStored(VIDEO_KIND, video, deps);
 }
 
+/** Makes an asset named `name` of a video the server has converted, without opening it (#405). */
+export function adoptVideo(video: UploadedVideo, name: string, deps: VideoImportDeps): ImportOutcome {
+  return adopt(VIDEO_KIND, video, name, name, deps);
+}
+
 /**
  * Sends the files up one after the other and says how it went; what is done already is made an
  * asset at once. The rest is converted on the server – `waiting` gets their names, for the picker

src/editor/viewport/GameViewport.ts

@@ -2,6 +2,7 @@
  * The scene seen through its active camera at the chosen scale, letterboxed: the running world while
  * playing, the model as a still preview otherwise.
  */
+import type { Container } from 'pixi.js';
 import { isSymbolType } from '../../core/model/AssetTypes';
 import type { EditorState } from '../state';
 import type { World } from '../../runtime/World';
@@ -44,6 +45,8 @@ export class GameViewport extends WorldViewport<EditorState> {
   /** The model collected once per redraw of the preview: view, cameras and items all read it. */
   private redrawFrame: Frame | null = null;
   private redrawing = false;
+  /** Set while a recording draws the view behind another tab (#405). */
+  private forced = false;
 
   constructor(state: EditorState) {
     super(state);
@@ -119,7 +122,7 @@ export class GameViewport extends WorldViewport<EditorState> {
    * otherwise run on every selection change behind another tab. Showing the tab redraws it.
    */
   override redraw(): void {
-    if (!this.visible) return;
+    if (!this.visible && !this.forced) return;
     const start = performance.now();
     this.redrawing = true;
     try {
@@ -131,6 +134,16 @@ export class GameViewport extends WorldViewport<EditorState> {
     this.drawTime += performance.now() - start;
   }
 
+  /** A redraw while the tab is hidden: a recording goes on behind another tab (#405). */
+  redrawHidden(): void {
+    this.forced = true;
+    try {
+      this.redraw();
+    } finally {
+      this.forced = false;
+    }
+  }
+
   get scaleMode(): GameScale {
     return this.state.gameScale;
   }
@@ -207,6 +220,10 @@ export class GameViewport extends WorldViewport<EditorState> {
     return Theme.colors.background;
   }
 
+  protected override chrome(): Container[] {
+    return [this.hint, this.editorRunCaption, this.fullscreenButton, this.performanceOverlay];
+  }
+
   protected override beforeItems(view: View | null): void {
     this.onScaleShown?.(view ? this.factor : null);
     this.hint.visible = view === null;

src/player/GameScreen.ts

@@ -1,4 +1,4 @@
-import type { TextStyle } from 'pixi.js';
+import type { Container, TextStyle } from 'pixi.js';
 import type { World } from '../runtime/World';
 import { Theme, textStyle, ui, crispText } from '../ui';
 import { type ActiveCamera, cameraOfWorld, overlaysOfWorld } from '../viewport/activeCamera';
@@ -119,6 +119,10 @@ export class GameScreen extends WorldViewport {
     return Theme.colors.background;
   }
 
+  protected override chrome(): Container[] {
+    return [this.caption, this.hint, this.fullscreenButton];
+  }
+
   protected override beforeItems(view: View | null): void {
     this.caption.visible = this.running === null;
     this.caption.position.set(this.w / 2, this.h / 2);

src/player/Player.ts

@@ -5,7 +5,8 @@ import { GameRun } from '../runtime/GameRun';
 import { loadPhysics, readyPhysics } from '../runtime/physics/engine';
 import { behaviorError } from '../runtime/systems/BehaviorSystem';
 import { Input } from '../runtime/systems/Input';
-import { requestRender } from '../ui';
+import { downloadBlob, requestRender } from '../ui';
+import { GameRecorder } from '../viewport/GameRecorder';
 import { soundLibrary } from '../viewport/SoundLibrary';
 import { VIDEO_WAIT_MS, videoLibrary } from '../viewport/VideoLibrary';
 import type { GameScreen } from './GameScreen';
@@ -20,6 +21,8 @@ export class Player {
   private readonly input = new Input();
   /** Until when a run waits at its start for its driven videos (#404); see `PlaySession`. */
   private videoWait: number | null = null;
+  /** `Start recording` (#405): a released game has no project to add to, so the file is downloaded. */
+  private readonly recorder: GameRecorder;
 
   constructor(
     private readonly project: Project,
@@ -27,6 +30,14 @@ export class Player {
     private readonly saves: SaveArea,
   ) {
     screen.onStart = () => this.start();
+    this.recorder = new GameRecorder({
+      view: () => screen,
+      log: (line) => {
+        this.run?.world.log.push(line);
+        console.warn('[recording]', line);
+      },
+      saved: (recording) => downloadBlob(recording.blob, `${recording.name}.mkv`),
+    });
   }
 
   get isRunning(): boolean {
@@ -59,6 +70,7 @@ export class Player {
       physics,
       clock: gameClock,
       saves: this.saves,
+      recorder: this.recorder,
     });
     videoLibrary.startRun();
     this.input.attach();
@@ -67,9 +79,11 @@ export class Player {
     if (this.run.world.stopRequested) this.stop();
   }
 
-  stop(): void {
+  /** `restarting`: a `Restart game` plays on at once, and a recording goes on with it. */
+  stop(restarting = false): void {
     const run = this.run;
     if (!run) return;
+    if (!restarting) this.recorder.end();
     this.run = null;
     this.videoWait = null;
     this.input.detach();
@@ -98,14 +112,17 @@ export class Player {
     const request = run.tick(step, timeStep(dt, gameClock.scale, false));
     if (request === 'stop') this.stop();
     else if (request === 'restart') {
-      this.stop();
+      this.stop(true);
       this.start();
     } else if (request === 'scene') {
       this.screen.setWorld(run.world);
       this.waitForVideos();
       if (run.world.stopRequested) this.stop();
     }
-    if (this.run) this.screen.redraw();
+    if (this.run) {
+      this.screen.redraw();
+      this.recorder.capture();
+    }
     requestRender();
   }
 

src/runtime/GameRun.ts

@@ -6,6 +6,7 @@ import type { SaveArea } from '../core/graph/savedValues';
 import type { SceneAsset } from '../core/model/Asset';
 import { findScene, type Project } from '../core/model/Project';
 import { mixOf } from '../core/music/mixer';
+import type { RecordingSink } from '../core/recording/recording';
 import type { AudioSink } from './systems/Audio';
 import { addGameSystems, type BehaviorSystem, type TimeScaled } from './systems/BehaviorSystem';
 import type { Input } from './systems/Input';
@@ -18,6 +19,8 @@ export interface RunServices {
   physics: Rapier;
   clock: TimeScaled;
   saves: SaveArea;
+  /** Outlives a `Load scene` and a restart, so a recording goes on through both (#405). */
+  recorder?: RecordingSink;
 }
 
 /** What a scene holds of objects, which is all a run writes into it. */

src/runtime/systems/BehaviorSystem.ts

@@ -55,6 +55,7 @@ import { CollisionSystem } from './CollisionSystem';
 import type { CollisionSource } from './solids';
 import { AnimationSystem } from './AnimationSystem';
 import type { Input } from './Input';
+import type { RecordingSink } from '../../core/recording/recording';
 import type { System } from './System';
 
 export type { TimeScaled } from './behaviorContext';
@@ -115,6 +116,8 @@ export interface GameSystemOptions {
   clock?: TimeScaled | null;
   /** Where persistent Variables assets are read from at the start and written to as they change (#166). */
   saves?: SaveArea | null;
+  /** Where `Start recording` records to (#405): only a played game has one. */
+  recorder?: RecordingSink | null;
 }
 
 /** Collisions and animations are ticked first, so behaviors see this frame's events. */
@@ -130,11 +133,12 @@ export function addGameSystems(
     physics = null,
     clock = null,
     saves = null,
+    recorder = null,
   }: GameSystemOptions = {},
 ): BehaviorSystem {
   const collisions = physics ? new PhysicsSystem(physics) : new CollisionSystem();
   const animations = new AnimationSystem();
-  const sources = { collisions, animations, audio, clock, saves };
+  const sources = { collisions, animations, audio, clock, saves, recorder };
   const behaviors = new BehaviorSystem(project, input, sources, shouldRun);
   if (carried) behaviors.carryOver(carried);
   if (kept) {
@@ -155,6 +159,7 @@ export interface EventSources {
   audio?: AudioSink | null;
   clock?: TimeScaled | null;
   saves?: SaveArea | null;
+  recorder?: RecordingSink | null;
 }
 
 /** A behavior's `runWhilePaused` switch: whether it is ticked while Pause game holds the world. */
@@ -198,6 +203,7 @@ export class BehaviorSystem implements System {
   private readonly audio: AudioSink;
   private readonly clock: TimeScaled | null;
   private readonly saves: SaveArea | null;
+  private readonly recorder: RecordingSink | null;
   /** What was last written to `saves`, so a frame that changed nothing writes nothing. */
   private lastSaved = '';
 
@@ -216,6 +222,7 @@ export class BehaviorSystem implements System {
     this.audio = sources.audio ?? SILENT_AUDIO;
     this.clock = sources.clock ?? null;
     this.saves = sources.saves ?? null;
+    this.recorder = sources.recorder ?? null;
   }
 
   /** The values `global` declarations share this run – read-only to callers, rebuilt by `start`. */
@@ -750,6 +757,7 @@ export class BehaviorSystem implements System {
       collisions: this.collisions,
       audio: this.audio,
       clock: this.clock,
+      recorder: this.recorder,
       shared: this.sharedState,
       stateChanged: (machineId, from, to) => this.pending.push({ machineId, from, to, source: instance }),
     });

src/runtime/systems/behaviorContext.ts

@@ -42,6 +42,7 @@ import type { World } from '../World';
 import type { AudioSink, NoteTiming } from './Audio';
 import type { Instance } from './behaviorPulses';
 import type { Input } from './Input';
+import type { RecordingSink } from '../../core/recording/recording';
 import { bodyOf, type CollisionSource, layerHolderOf, NOT_BLOCKED } from './solids';
 
 export interface TimeScaled {
@@ -55,6 +56,8 @@ export interface ContextSources {
   collisions: CollisionSource | null;
   audio: AudioSink;
   clock: TimeScaled | null;
+  /** Where a recording of the game view goes (#405); only a played game has one. */
+  recorder: RecordingSink | null;
   shared: SharedState;
   /** A graph moved one of this instance's machines; its events follow between exec flows. */
   stateChanged(machineId: string, from: string, to: string): void;
@@ -265,9 +268,20 @@ export function behaviorContext(
     setPaused: (paused) => {
       world.paused = paused;
     },
+    startRecording: (settings) => tell(world, run.recorder?.start(settings) ?? NO_RECORDER),
+    stopRecording: () => tell(world, run.recorder?.stop() ?? NO_RECORDER),
   };
 }
 
+const NO_RECORDER = 'Recording works only while the game is played, not in the editor run';
+
+/** A line of the host for the log, as the `Log` node writes one; nothing for an empty one. */
+function tell(world: World, line: string): void {
+  if (!line) return;
+  world.log.push(line);
+  console.warn('[recording]', line);
+}
+
 /**
  * What the graph asked for, as the sink understands it: the asset's own volume scaled by the
  * node's, and its loop flag unless the node overrules it either way. The asset is what a sound

src/ui/download.ts

@@ -0,0 +1,14 @@
+/** Hands a file at `href` to the browser to save as `fileName`; the page stays where it is. */
+export function downloadFile(href: string, fileName: string): void {
+  const link = document.createElement('a');
+  link.href = href;
+  link.download = fileName;
+  link.click();
+}
+
+/** Hands bytes made in the page to the browser as a file. */
+export function downloadBlob(blob: Blob, fileName: string): void {
+  const url = URL.createObjectURL(blob);
+  downloadFile(url, fileName);
+  setTimeout(() => URL.revokeObjectURL(url), 60_000);
+}

src/ui/index.ts

@@ -44,6 +44,7 @@ export {
   type GridOptions,
 } from './draw';
 export { clamp, centreY, formatBytes, formatNumber, NUMBER_DECIMALS, snapToStep, zoomFactor } from './math';
+export { downloadBlob, downloadFile } from './download';
 export { activeDrag, type DragPayload, endDrag, onDragChange, startDrag } from './drag';
 export { Label, truncate } from './widgets/Label';
 export { Button, type ButtonOptions } from './widgets/Button';

src/viewport/GameRecorder.ts

@@ -0,0 +1,532 @@
+import { Matrix, type RenderTexture } from 'pixi.js';
+import { preSkipOf } from '../core/music/ogg';
+import { AVC_CODEC, colourOf, MatroskaWriter, VP9_CODEC } from '../core/recording/matroska';
+import {
+  avcCodec,
+  FrameGate,
+  isKeyFrame,
+  pictureTransform,
+  RECORDING_MOST_BYTES,
+  RecordingClock,
+  recordingName,
+  type RecordingSettings,
+  type RecordingSink,
+  recordingSize,
+  videoBitrate,
+  vp9Codec,
+} from '../core/recording/recording';
+import { ui } from '../ui';
+import { PictureReader } from './PictureReader';
+import { soundLibrary } from './SoundLibrary';
+import type { PictureArea } from './WorldViewport';
+
+const OPUS_BITRATE = 160_000;
+/** Frames the video encoder may have waiting before the next is dropped instead of piling up. */
+const MOST_QUEUED = 6;
+/** Encoded bytes held as loose pieces before they are folded into a Blob, out of the page's heap. */
+const FOLD_BYTES = 16 * 1024 * 1024;
+/** How long a stop waits for the last sound the worklet holds; a suspended context sends none. */
+const LAST_SOUND_MS = 300;
+/** How long a stop waits for the frames still on their way back from the GPU. */
+const LAST_FRAMES_MS = 500;
+
+/** The view a recording takes its frames from (`WorldViewport`). */
+export interface RecordingView {
+  picture(): { shown: PictureArea; whole: PictureArea } | null;
+  drawPicture(transform: Matrix, target: RenderTexture): void;
+}
+
+/** A recording once it is made. */
+export interface MadeRecording {
+  name: string;
+  blob: Blob;
+  durationMs: number;
+  width: number;
+  height: number;
+}
+
+/** Where the recorder looks, and what it tells. */
+export interface RecordingHost {
+  view(): RecordingView | null;
+  /** A line for the log of the run that plays. */
+  log(line: string): void;
+  saved(recording: MadeRecording): void;
+}
+
+/**
+ * `waiting` for a picture to take its size from: `On start` runs before the view is handed the
+ * world, and a game may switch its camera on later.
+ */
+type TakeState = 'waiting' | 'starting' | 'recording' | 'finishing' | 'done' | 'failed';
+
+/** What a check reads of a recording (test hook). */
+export interface TakeInfo {
+  name: string;
+  state: TakeState;
+  width: number;
+  height: number;
+  fps: number;
+  codec: string;
+  sound: boolean;
+  frames: number;
+  dropped: number;
+  soundFrames: number;
+  /** From the first frame to the end of the last, in ms. */
+  durationMs: number;
+  /** What the video encoder said its colour is, null while it said nothing. */
+  colour: VideoColorSpaceInit | null;
+  bytes: number;
+  failure: string | null;
+}
+
+/**
+ * Records the game view (#405): `Start recording` and `Stop recording` reach it through the run, the
+ * host hands it the view and draws a frame for it each tick (`capture`). One recording at a time; it
+ * outlives the world, so a `Load scene` or a restart goes on in the same file.
+ */
+export class GameRecorder implements RecordingSink {
+  private take: Take | null = null;
+  private paused = false;
+  /** Every recording of this page, newest last (test hook). */
+  private readonly takes: Take[] = [];
+
+  constructor(private readonly host: RecordingHost) {}
+
+  get isRecording(): boolean {
+    return this.take !== null;
+  }
+
+  start(settings: RecordingSettings): string {
+    if (this.take)
+      return `Start recording is ignored: "${this.take.name}" is recording already – Stop recording ends it`;
+    const take = new Take(recordingName(settings.name, new Date()), settings, this.host, () => {
+      if (this.take === take) this.take = null;
+    });
+    this.take = take;
+    this.takes.push(take);
+    if (this.paused) take.setPaused(true);
+    return '';
+  }
+
+  stop(): string {
+    if (!this.take) return 'Stop recording: nothing is recording';
+    this.take.finish('');
+    this.take = null;
+    return '';
+  }
+
+  /** The editor holds the run: the recording holds with it, and its time leaves the pause out. */
+  setPaused(paused: boolean): void {
+    this.paused = paused;
+    this.take?.setPaused(paused);
+  }
+
+  /** The view has drawn this frame: the recording takes it, if it is due. */
+  capture(): void {
+    const view = this.take && this.host.view();
+    if (view) this.take!.capture(view);
+  }
+
+  /** The run is over: a recording still going is saved, not lost. */
+  end(): void {
+    this.stop();
+  }
+
+  info(): { recording: boolean; takes: TakeInfo[] } {
+    return { recording: this.isRecording, takes: this.takes.map((take) => take.info()) };
+  }
+}
+
+/** One recording, from its encoders to its file. */
+class Take {
+  private state: TakeState = 'waiting';
+  private size = { w: 0, h: 0 };
+  private readonly clock = new RecordingClock();
+  private readonly gate: FrameGate;
+  private video: VideoEncoder | null = null;
+  private audio: AudioEncoder | null = null;
+  private tap: { node: AudioWorkletNode; rate: number; release(): void } | null = null;
+  private writer: MatroskaWriter | null = null;
+  private codec = '';
+  private videoHead: Uint8Array | null = null;
+  private colour: VideoColorSpaceInit | null = null;
+  private audioHead: Uint8Array | null = null;
+  private reader: PictureReader | null = null;
+  private toldNoCamera = false;
+  private folded: Blob[] = [];
+  private loose: Uint8Array[] = [];
+  private looseBytes = 0;
+  private frames = 0;
+  private dropped = 0;
+  private lastMs = 0;
+  private soundFrames = 0;
+  /** The recording's time of the first sample heard, in ms; null before it. */
+  private soundStart: number | null = null;
+  private soundEnded: (() => void) | null = null;
+  private stopAsked = false;
+  private paused = false;
+  private failure: string | null = null;
+
+  constructor(
+    readonly name: string,
+    private readonly settings: RecordingSettings,
+    private readonly host: RecordingHost,
+    private readonly gone: () => void,
+  ) {
+    this.gate = new FrameGate(settings.fps);
+  }
+
+  /**
+   * Takes the size from the first picture there is and sets up the encoders; until they are ready
+   * frames are not taken, and a stop asked meanwhile ends it after.
+   */
+  private open(picture: { shown: PictureArea; whole: PictureArea }): void {
+    const ratio = ui().app.renderer.resolution;
+    const device = (area: PictureArea) => ({ w: area.w * ratio, h: area.h * ratio });
+    this.size = recordingSize(this.settings.resolution, device(picture.shown), device(picture.whole));
+    this.state = 'starting';
+    try {
+      this.reader = new PictureReader(this.size.w, this.size.h);
+    } catch (error) {
+      this.fail(error instanceof Error ? error.message : String(error));
+      return;
+    }
+    void this.openEncoders().then(
+      () => {
+        if (this.state !== 'starting') return;
+        this.state = 'recording';
+        if (this.stopAsked) this.finish('');
+      },
+      (error: unknown) => this.fail(error instanceof Error ? error.message : String(error)),
+    );
+  }
+
+  private async openEncoders(): Promise<void> {
+    if (typeof VideoEncoder === 'undefined')
+      throw new Error('this browser cannot encode video – recording needs Chrome');
+    const { w, h } = this.size;
+    const fps = this.settings.fps;
+    const base = { width: w, height: h, framerate: fps, bitrate: videoBitrate(w, h, fps) };
+    const configs: VideoEncoderConfig[] = [
+      { ...base, codec: avcCodec(w, h, fps), avc: { format: 'avc' }, latencyMode: 'realtime' },
+      { ...base, codec: vp9Codec(w, h), latencyMode: 'realtime' },
+    ];
+    let config: VideoEncoderConfig | null = null;
+    for (const candidate of configs)
+      if (
+        await VideoEncoder.isConfigSupported(candidate).then(
+          (s) => s.supported === true,
+          () => false,
+        )
+      ) {
+        config = candidate;
+        break;
+      }
+    if (!config) throw new Error(`this browser cannot encode ${w}x${h} at ${fps} fps`);
+    this.codec = config.codec;
+    await this.openSound();
+    const writer = new MatroskaWriter(this.audio ? 2 : 1);
+    this.writer = writer;
+    const video = new VideoEncoder({
+      output: (chunk, meta) => {
+        const head = meta?.decoderConfig?.description;
+        if (head && !this.videoHead) this.videoHead = copyOf(head);
+        this.colour ??= meta?.decoderConfig?.colorSpace ?? null;
+        writer.add(1, chunk.timestamp, chunk.type === 'key', chunkBytes(chunk));
+        this.collect();
+      },
+      error: (error) => this.fail(error.message),
+    });
+    video.configure(config);
+    this.video = video;
+  }
+
+  /** The master behind the limiter, as Opus; without it the recording goes on silent and says so. */
+  private async openSound(): Promise<void> {
+    const tap = await soundLibrary.outputTap().catch(() => null);
+    const config: AudioEncoderConfig = {
+      codec: 'opus',
+      sampleRate: tap?.rate ?? 48000,
+      numberOfChannels: 2,
+      bitrate: OPUS_BITRATE,
+    };
+    const supported =
+      tap !== null &&
+      typeof AudioEncoder !== 'undefined' &&
+      (await AudioEncoder.isConfigSupported(config).then(
+        (s) => s.supported === true,
+        () => false,
+      ));
+    if (!tap || !supported) {
+      tap?.release();
+      this.host.log(`Recording "${this.name}" has no sound: this browser cannot record it`);
+      return;
+    }
+    const audio = new AudioEncoder({
+      output: (chunk, meta) => {
+        const head = meta?.decoderConfig?.description;
+        if (head && !this.audioHead) this.audioHead = copyOf(head);
+        this.writer?.add(2, chunk.timestamp, true, chunkBytes(chunk));
+        this.collect();
+      },
+      error: (error) => this.fail(error.message),
+    });
+    audio.configure(config);
+    this.audio = audio;
+    this.tap = tap;
+    tap.node.port.onmessage = (event: MessageEvent<Float32Array[]>) => this.heard(event.data);
+  }
+
+  setPaused(paused: boolean): void {
+    if (paused === this.paused) return;
+    this.paused = paused;
+    const now = performance.now();
+    if (paused) this.clock.pause(now);
+    else this.clock.resume(now);
+  }
+
+  capture(view: RecordingView): void {
+    if (this.state === 'waiting') {
+      const picture = view.picture();
+      if (picture) this.open(picture);
+      else if (!this.toldNoCamera) {
+        this.toldNoCamera = true;
+        this.host.log(
+          `Recording "${this.name}" waits: no camera is switched on, so there is nothing to record`,
+        );
+      }
+      return;
+    }
+    const video = this.video;
+    const reader = this.reader;
+    if (this.state !== 'recording' || !video || !reader) return;
+    reader.collect((bytes, ms) => this.encode(bytes, ms));
+    // A frame taken may have reached the most a file holds and ended the recording.
+    if (!this.taking || this.paused) return;
+    const now = performance.now();
+    if (!this.clock.started) this.clock.start(now);
+    const ms = this.clock.at(now);
+    if (!this.gate.take(ms)) return;
+    const picture = view.picture();
+    if (!picture || reader.busy || video.encodeQueueSize >= MOST_QUEUED) {
+      this.dropped++;
+      return;
+    }
+    const m = pictureTransform(
+      this.settings.resolution === 'view' ? picture.shown : picture.whole,
+      this.size,
+    );
+    view.drawPicture(new Matrix(m.a, 0, 0, m.d, m.tx, m.ty), reader.texture);
+    reader.read(ms);
+  }
+
+  private get taking(): boolean {
+    return this.state === 'recording';
+  }
+
+  /** A frame back from the GPU, into the video encoder. */
+  private encode(bytes: Uint8Array, ms: number): void {
+    const video = this.video;
+    if (!video || video.state !== 'configured') return;
+    const frame = new VideoFrame(bytes, {
+      format: 'RGBX',
+      codedWidth: this.size.w,
+      codedHeight: this.size.h,
+      timestamp: Math.round(ms * 1000),
+      duration: Math.round(1e6 / this.settings.fps),
+    });
+    video.encode(frame, { keyFrame: isKeyFrame(this.frames, this.settings.fps) });
+    frame.close();
+    this.frames++;
+    this.lastMs = ms;
+    if (this.taking && (this.writer?.bytes ?? 0) > RECORDING_MOST_BYTES)
+      this.finish(`Recording "${this.name}" reached 2 GB, the most the server takes, and was stopped`);
+  }
+
+  /** A piece of the master from the worklet; an empty one is the last. */
+  private heard(sides: Float32Array[]): void {
+    const audio = this.audio;
+    const frames = sides[0]?.length ?? 0;
+    if (frames === 0) {
+      this.soundEnded?.();
+      return;
+    }
+    // Before the first frame there is no time to put it at. Pieces keep coming during a pause only
+    // as long as they were on their way, so they belong before it and are kept.
+    if (!audio || !this.tap || this.state !== 'recording' || !this.clock.started) return;
+    const rate = this.tap.rate;
+    this.soundStart ??= Math.max(0, this.clock.at(performance.now()) - (frames / rate) * 1000);
+    const planar = new Float32Array(frames * 2);
+    planar.set(sides[0]!, 0);
+    planar.set(sides[1] ?? sides[0]!, frames);
+    const data = new AudioData({
+      format: 'f32-planar',
+      sampleRate: rate,
+      numberOfFrames: frames,
+      numberOfChannels: 2,
+      timestamp: Math.round(this.soundStart * 1000 + (this.soundFrames / rate) * 1e6),
+      data: planar,
+    });
+    audio.encode(data);
+    data.close();
+    this.soundFrames += frames;
+  }
+
+  /** Ends the recording and saves what it has; `why` goes to the log when it is not the node's stop. */
+  finish(why: string): void {
+    if (why) this.host.log(why);
+    if (this.state === 'waiting') {
+      this.state = 'done';
+      this.gone();
+      this.host.log(`Recording "${this.name}" ended before its first frame, so nothing was saved`);
+      return;
+    }
+    if (this.state === 'starting') {
+      this.stopAsked = true;
+      return;
+    }
+    if (this.state !== 'recording') return;
+    this.state = 'finishing';
+    this.gone();
+    void this.save().catch((error: unknown) =>
+      this.fail(error instanceof Error ? error.message : String(error)),
+    );
+  }
+
+  private async save(): Promise<void> {
+    await Promise.all([this.lastFrames(), this.lastSound()]);
+    if (this.video?.state === 'configured') await this.video.flush();
+    if (this.audio?.state === 'configured') await this.audio.flush();
+    this.close();
+    const writer = this.writer;
+    if (!writer || this.frames === 0) {
+      this.state = 'done';
+      this.host.log(`Recording "${this.name}" ended before its first frame, so nothing was saved`);
+      return;
+    }
+    const frame = 1000 / this.settings.fps;
+    const { head, rest } = writer.finish({
+      video: {
+        codec: this.codec.startsWith('avc1') ? AVC_CODEC : VP9_CODEC,
+        width: this.size.w,
+        height: this.size.h,
+        private: this.videoHead,
+        fps: this.settings.fps,
+        colour: colourOf(this.colour),
+      },
+      audio: this.audio
+        ? {
+            channels: 2,
+            private: this.audioHead,
+            preSkip: preSkipOf(this.audioHead ?? new Uint8Array()) ?? 0,
+          }
+        : null,
+      durationMs: this.lastMs + frame,
+      app: 'game-maker',
+    });
+    this.collect();
+    const blob = new Blob([head, ...this.folded, ...this.loose, ...rest] as BlobPart[], {
+      type: 'video/x-matroska',
+    });
+    this.folded = [];
+    this.loose = [];
+    this.state = 'done';
+    this.host.saved({
+      name: this.name,
+      blob,
+      durationMs: this.lastMs + frame,
+      width: this.size.w,
+      height: this.size.h,
+    });
+  }
+
+  /** Takes the frames still on their way back from the GPU, a moment at most. */
+  private async lastFrames(): Promise<void> {
+    const until = performance.now() + LAST_FRAMES_MS;
+    for (;;) {
+      this.reader?.collect((bytes, ms) => this.encode(bytes, ms));
+      if (!this.reader?.pending || performance.now() > until) return;
+      await new Promise((resolve) => setTimeout(resolve, 4));
+    }
+  }
+
+  /** Asks the worklet for what it still holds and waits for it, a moment at most. */
+  private lastSound(): Promise<void> {
+    const tap = this.tap;
+    if (!tap) return Promise.resolve();
+    return new Promise<void>((resolve) => {
+      const timer = setTimeout(done, LAST_SOUND_MS);
+      function done(): void {
+        clearTimeout(timer);
+        resolve();
+      }
+      this.soundEnded = done;
+      tap.node.port.postMessage('stop');
+    }).finally(() => {
+      tap.release();
+      this.tap = null;
+    });
+  }
+
+  /** Moves the clusters the writer finished into the file, folding them into Blobs as they pile up. */
+  private collect(): void {
+    for (const part of this.writer?.take() ?? []) {
+      this.loose.push(part);
+      this.looseBytes += part.length;
+    }
+    if (this.looseBytes < FOLD_BYTES) return;
+    this.folded.push(new Blob(this.loose as BlobPart[]));
+    this.loose = [];
+    this.looseBytes = 0;
+  }
+
+  private close(): void {
+    for (const encoder of [this.video, this.audio])
+      if (encoder && encoder.state !== 'closed') encoder.close();
+    this.reader?.destroy();
+    this.reader = null;
+  }
+
+  private fail(reason: string): void {
+    if (this.state === 'done' || this.state === 'failed') return;
+    this.failure = reason;
+    this.state = 'failed';
+    this.tap?.release();
+    this.tap = null;
+    this.close();
+    this.gone();
+    this.host.log(`Recording "${this.name}" failed: ${reason}`);
+  }
+
+  info(): TakeInfo {
+    return {
+      name: this.name,
+      state: this.state,
+      width: this.size.w,
+      height: this.size.h,
+      fps: this.settings.fps,
+      codec: this.codec,
+      sound: this.audio !== null,
+      frames: this.frames,
+      dropped: this.dropped,
+      soundFrames: this.soundFrames,
+      durationMs: this.frames > 0 ? Math.round(this.lastMs + 1000 / this.settings.fps) : 0,
+      colour: this.colour,
+      bytes: this.writer?.bytes ?? 0,
+      failure: this.failure,
+    };
+  }
+}
+
+function chunkBytes(chunk: EncodedVideoChunk | EncodedAudioChunk): Uint8Array {
+  const data = new Uint8Array(chunk.byteLength);
+  chunk.copyTo(data);
+  return data;
+}
+
+function copyOf(source: AllowSharedBufferSource): Uint8Array {
+  const view = ArrayBuffer.isView(source)
+    ? new Uint8Array(source.buffer, source.byteOffset, source.byteLength)
+    : new Uint8Array(source);
+  return view.slice();
+}

src/viewport/PictureReader.ts

@@ -0,0 +1,102 @@
+import { RenderTexture, type WebGLRenderer } from 'pixi.js';
+import { ui } from '../ui';
+
+/** Frames on their way back from the GPU at once; while all are, a frame due is dropped. */
+const MOST_IN_FLIGHT = 3;
+
+interface InFlight {
+  buffer: WebGLBuffer;
+  fence: WebGLSync;
+  ms: number;
+}
+
+/**
+ * Hands a recording its frames without drawing into the canvas on screen (#405): the low latency
+ * canvas shows what is drawn into it at once, so a frame drawn there flickered in its corner. The
+ * view is drawn into `texture` instead and read back through pixel buffers, so the main thread does
+ * not wait for the GPU: a frame comes out of `collect` a tick or two after it was drawn.
+ */
+export class PictureReader {
+  readonly texture: RenderTexture;
+  private readonly gl: WebGL2RenderingContext;
+  private readonly free: WebGLBuffer[] = [];
+  private readonly inFlight: InFlight[] = [];
+  private readonly bytes: Uint8Array;
+
+  constructor(
+    readonly w: number,
+    readonly h: number,
+  ) {
+    const renderer = ui().app.renderer as WebGLRenderer;
+    const gl = renderer.gl as WebGL2RenderingContext | undefined;
+    if (!gl || typeof gl.fenceSync !== 'function') throw new Error('recording needs WebGL 2');
+    this.gl = gl;
+    this.texture = RenderTexture.create({ width: w, height: h, resolution: 1, antialias: true });
+    for (let i = 0; i < MOST_IN_FLIGHT; i++) {
+      const buffer = gl.createBuffer();
+      gl.bindBuffer(gl.PIXEL_PACK_BUFFER, buffer);
+      gl.bufferData(gl.PIXEL_PACK_BUFFER, w * h * 4, gl.STREAM_READ);
+      this.free.push(buffer);
+    }
+    gl.bindBuffer(gl.PIXEL_PACK_BUFFER, null);
+    this.bytes = new Uint8Array(w * h * 4);
+  }
+
+  /** Whether a frame drawn now would have no buffer to go back in. */
+  get busy(): boolean {
+    return this.free.length === 0;
+  }
+
+  get pending(): number {
+    return this.inFlight.length;
+  }
+
+  /** Starts reading back what `texture` holds now, the frame of recording time `ms`. */
+  read(ms: number): void {
+    const buffer = this.free.pop();
+    if (!buffer) return;
+    const gl = this.gl;
+    const renderer = ui().app.renderer as WebGLRenderer;
+    const target = renderer.renderTarget.getRenderTarget(this.texture);
+    // Resolves the multisampled picture into the texture; a second time is only a copy.
+    renderer.renderTarget.adaptor.finishRenderPass(target);
+    const gpu = renderer.renderTarget.getGpuRenderTarget(target);
+    // Through the adaptor, so Pixi knows which framebuffer is bound.
+    renderer.renderTarget.adaptor.bindFramebuffer(gpu.resolveTargetFramebuffer);
+    gl.bindBuffer(gl.PIXEL_PACK_BUFFER, buffer);
+    gl.readPixels(0, 0, this.w, this.h, gl.RGBA, gl.UNSIGNED_BYTE, 0);
+    gl.bindBuffer(gl.PIXEL_PACK_BUFFER, null);
+    const fence = gl.fenceSync(gl.SYNC_GPU_COMMANDS_COMPLETE, 0)!;
+    gl.flush();
+    this.inFlight.push({ buffer, fence, ms });
+  }
+
+  /**
+   * Hands over each frame the GPU has finished, oldest first, its rows top first as Pixi draws a
+   * texture. The bytes are reused for the next frame, so `take` copies what it keeps.
+   */
+  collect(take: (bytes: Uint8Array, ms: number) => void): void {
+    const gl = this.gl;
+    while (this.inFlight.length > 0) {
+      const next = this.inFlight[0]!;
+      if (gl.clientWaitSync(next.fence, 0, 0) === gl.TIMEOUT_EXPIRED) return;
+      gl.deleteSync(next.fence);
+      gl.bindBuffer(gl.PIXEL_PACK_BUFFER, next.buffer);
+      gl.getBufferSubData(gl.PIXEL_PACK_BUFFER, 0, this.bytes);
+      gl.bindBuffer(gl.PIXEL_PACK_BUFFER, null);
+      this.inFlight.shift();
+      this.free.push(next.buffer);
+      take(this.bytes, next.ms);
+    }
+  }
+
+  destroy(): void {
+    const gl = this.gl;
+    for (const { buffer, fence } of this.inFlight.splice(0)) {
+      gl.deleteSync(fence);
+      gl.deleteBuffer(buffer);
+    }
+    for (const buffer of this.free.splice(0)) gl.deleteBuffer(buffer);
+    this.texture.destroy(true);
+  }
+}

src/viewport/SoundLibrary.ts

@@ -614,6 +614,25 @@ export class SoundLibrary implements AudioSink {
     return readMeter((this.guardMeter ??= this.tapMeter(guard.limiter)));
   }
 
+  /**
+   * A node that hears what leaves the output, limiter and all (#405): a recording of the game view
+   * takes its sound there, in pieces of both sides (`record-tap`). Null without a live output.
+   */
+  async outputTap(): Promise<{ node: AudioWorkletNode; rate: number; release(): void } | null> {
+    const context = this.audio();
+    if (!(context instanceof AudioContext) || !(await this.fxWorklet())) return null;
+    const guard = this.guard;
+    if (!guard) return null;
+    const node = new AudioWorkletNode(context, 'record-tap', {
+      numberOfOutputs: 0,
+      channelCount: 2,
+      channelCountMode: 'explicit',
+    });
+    const source = guard.limiter ?? guard.input;
+    source.connect(node);
+    return { node, rate: context.sampleRate, release: () => source.disconnect(node) };
+  }
+
   /** Lets every level meter go. */
   unmeter(): void {
     for (const bus of this.buses.values()) this.dropMeter(bus);

src/viewport/VideoLibrary.ts

@@ -27,6 +27,11 @@ import { soundLibrary } from './SoundLibrary';
 import { followVideo, readSpeed, type SpeedReading } from './videoFollow';
 
 const VIDEO_BASE = '/game-maker/data/videos';
+
+/** Where the MP4 of a video asset's `file` is served. */
+export function videoFileUrl(file: string): string {
+  return `${VIDEO_BASE}/${file}`;
+}
 /** How close to its end a clip counts as there: half a frame at 30 fps. */
 const END_SLACK = 1 / 60;
 /** A player nothing has asked for in this long is let go: no sprite wears it any more. */
@@ -281,7 +286,7 @@ export class VideoLibrary {
     const at = drive ? drivenTimeOf(play, time, NaN) : play.start;
     const player: Player = {
       play,
-      element: openVideo(`${VIDEO_BASE}/${play.file}`, at, (texture) => {
+      element: openVideo(videoFileUrl(play.file), at, (texture) => {
         if (player.gone) return texture.destroy(true);
         player.texture = texture;
         if (drive) this.follow(player, drive);

src/viewport/WorldViewport.ts

@@ -1,4 +1,12 @@
-import { Container, type FederatedPointerEvent, Graphics, type Mesh, Texture } from 'pixi.js';
+import {
+  Container,
+  type FederatedPointerEvent,
+  Graphics,
+  type Matrix,
+  type Mesh,
+  type RenderTexture,
+  Texture,
+} from 'pixi.js';
 import { isSymbolType } from '../core/model/AssetTypes';
 import { type Extent, scaleOf } from '../core/model/matrix';
 import type { Vec2 } from '../core/model/Properties';
@@ -51,6 +59,14 @@ export interface Frame {
   background: number;
 }
 
+/** A part of the view by its centre and size, in local CSS pixels. */
+export interface PictureArea {
+  x: number;
+  y: number;
+  w: number;
+  h: number;
+}
+
 export abstract class WorldViewport<S extends ViewSource = ViewSource> extends Widget {
   protected readonly bg = new Graphics();
   protected readonly content = new Container();
@@ -512,6 +528,49 @@ export abstract class WorldViewport<S extends ViewSource = ViewSource> extends W
     return (shading && shadedPicture(shading, shaders, ppu, true)?.grid) || view.grid;
   }
 
+  /**
+   * The camera's picture as the view lays it out (#405), in local CSS pixels by its centre: the part
+   * on screen, and all of it, which is larger only while a fixed scale cuts it to the view. Null
+   * without a camera.
+   */
+  picture(): { shown: PictureArea; whole: PictureArea } | null {
+    const view = this.currentView();
+    const camera = view && this.mainCamera();
+    if (!view || !camera) return null;
+    const { rect } = view;
+    const frame = cameraFrame(camera.props);
+    const x = rect.x + rect.w / 2;
+    const y = rect.y + rect.h / 2;
+    return {
+      shown: { x, y, w: rect.w, h: rect.h },
+      whole: { x, y, w: frame.w * view.scale, h: frame.h * view.scale },
+    };
+  }
+
+  /**
+   * Draws the view into `target` mapped by `transform`, without what lies over the picture
+   * (`chrome`): a recording reads it from there (#405). Never into the canvas on screen, which shows
+   * at once what is drawn into it while its context is low latency. Drawn even while its tab is
+   * hidden, so a recording goes on behind another tab.
+   */
+  drawPicture(transform: Matrix, target: RenderTexture): void {
+    const shown = this.chrome().filter((part) => part.visible);
+    for (const part of shown) part.visible = false;
+    const visible = this.visible;
+    this.visible = true;
+    try {
+      ui().app.renderer.render({ container: this, target, transform, clear: true, clearColor: [0, 0, 0, 1] });
+    } finally {
+      this.visible = visible;
+      for (const part of shown) part.visible = true;
+    }
+  }
+
+  /** What the view lays over the game's picture and a recording leaves out. */
+  protected chrome(): Container[] {
+    return [];
+  }
+
   /** Whether the game view is at a fixed scale rather than fit: the only one overlays snap at. */
   protected fixedScale(): boolean {
     return false;

src/viewport/fxWorklet.ts

@@ -391,3 +391,52 @@ class ChannelProbe extends AudioWorkletProcessor {
 }
 
 registerProcessor('channel-probe', ChannelProbe);
+
+/** Frames a recording's sound is handed over in: about a tenth of a second. */
+const RECORD_BLOCK = 4096;
+
+/**
+ * Hands what reaches it to the page, both sides, `RECORD_BLOCK` frames at a time (#405): the master
+ * as it leaves the limiter, for a recording of the game view. Told to stop, it sends what it holds
+ * and an empty piece after it, the sign that nothing more comes.
+ */
+class RecordTap extends Stoppable {
+  private readonly sides = [new Float32Array(RECORD_BLOCK), new Float32Array(RECORD_BLOCK)];
+  private filled = 0;
+
+  constructor() {
+    super();
+    this.port.onmessage = () => {
+      this.send();
+      this.port.postMessage([new Float32Array(0), new Float32Array(0)]);
+      this.stop();
+    };
+  }
+
+  process(inputs: Float32Array[][]): boolean {
+    if (this.stopped) return false;
+    const input = inputs[0] ?? [];
+    const left = input[0];
+    const frames = left?.length ?? 128;
+    this.sides.forEach((side, c) => {
+      const data = input[c] ?? left;
+      if (data) side.set(data, this.filled);
+      else side.fill(0, this.filled, this.filled + frames);
+    });
+    this.filled += frames;
+    if (this.filled + frames > RECORD_BLOCK) this.send();
+    return true;
+  }
+
+  private send(): void {
+    if (this.filled === 0) return;
+    const out = this.sides.map((side) => side.slice(0, this.filled));
+    this.filled = 0;
+    this.port.postMessage(
+      out,
+      out.map((side) => side.buffer),
+    );
+  }
+}
+
+registerProcessor('record-tap', RecordTap);

tests/core/matroska.test.ts

@@ -0,0 +1,202 @@
+import { describe, expect, it } from 'vitest';
+import { AVC_CODEC, colourOf, type MatroskaHead, MatroskaWriter } from '../../src/core/recording/matroska';
+
+/** One element as read back: its id in hex, its bytes, and what it holds where it is a master. */
+interface Read {
+  id: string;
+  data: Uint8Array;
+  children: Read[];
+}
+
+const MASTERS = new Set([
+  '1a45dfa3',
+  '18538067',
+  '1549a966',
+  '1654ae6b',
+  'ae',
+  'e0',
+  '55b0',
+  'e1',
+  '1f43b675',
+]);
+
+function vint(bytes: Uint8Array, at: number, keepMarker: boolean): { value: number; length: number } {
+  const first = bytes[at]!;
+  let length = 1;
+  while (length <= 8 && !(first & (0x80 >> (length - 1)))) length++;
+  let value = keepMarker ? first : first & (0xff >> length);
+  for (let i = 1; i < length; i++) value = value * 256 + bytes[at + i]!;
+  return { value, length };
+}
+
+function readAll(bytes: Uint8Array, from = 0, to = bytes.length): Read[] {
+  const out: Read[] = [];
+  let at = from;
+  while (at < to) {
+    const id = vint(bytes, at, true);
+    const size = vint(bytes, at + id.length, false);
+    const start = at + id.length + size.length;
+    const data = bytes.subarray(start, start + size.value);
+    const hex = id.value.toString(16);
+    out.push({ id: hex, data, children: MASTERS.has(hex) ? readAll(bytes, start, start + size.value) : [] });
+    at = start + size.value;
+  }
+  return out;
+}
+
+const find = (list: Read[], id: string) => list.find((e) => e.id === id);
+const uintOf = (e: Read | undefined) => (e ? e.data.reduce((v, b) => v * 256 + b, 0) : NaN);
+const textOf = (e: Read | undefined) => new TextDecoder().decode(e?.data);
+
+/** Track, time relative to its cluster, key flag and payload of each block, with the cluster's time. */
+function blocksOf(
+  file: Uint8Array,
+): { cluster: number; track: number; ms: number; key: boolean; data: number[] }[] {
+  const segment = find(readAll(file), '18538067')!;
+  return segment.children
+    .filter((e) => e.id === '1f43b675')
+    .flatMap((cluster) => {
+      const start = uintOf(find(cluster.children, 'e7'));
+      return cluster.children
+        .filter((e) => e.id === 'a3')
+        .map((block) => ({
+          cluster: start,
+          track: block.data[0]! & 0x7f,
+          ms: start + new DataView(block.data.buffer, block.data.byteOffset + 1, 2).getInt16(0),
+          key: (block.data[3]! & 0x80) !== 0,
+          data: [...block.data.subarray(4)],
+        }));
+    });
+}
+
+const HEAD: MatroskaHead = {
+  video: {
+    codec: AVC_CODEC,
+    width: 1280,
+    height: 720,
+    private: Uint8Array.from([1, 100, 0, 31]),
+    fps: 30,
+    colour: colourOf({ matrix: 'bt709', transfer: 'iec61966-2-1', primaries: 'bt709', fullRange: false }),
+  },
+  audio: { channels: 2, private: new TextEncoder().encode('OpusHead'), preSkip: 312 },
+  durationMs: 1234.5,
+  app: 'game-maker',
+};
+
+/** The whole file: the head, then the clusters handed out before (`earlier`) and at the end. */
+function fileOf(writer: MatroskaWriter, head: MatroskaHead = HEAD, earlier: Uint8Array[] = []): Uint8Array {
+  const taken = writer.take();
+  const { head: bytes, rest } = writer.finish(head);
+  const parts = [bytes, ...earlier, ...taken, ...rest];
+  const out = new Uint8Array(parts.reduce((n, p) => n + p.length, 0));
+  let at = 0;
+  for (const part of parts) {
+    out.set(part, at);
+    at += part.length;
+  }
+  return out;
+}
+
+describe('MatroskaWriter (#405)', () => {
+  it('writes a head that says what the tracks are and how long the file is', () => {
+    const writer = new MatroskaWriter(2);
+    writer.add(1, 0, true, Uint8Array.from([9]));
+    const file = fileOf(writer);
+    const [ebml, segment] = readAll(file);
+    expect(textOf(find(ebml!.children, '4282'))).toBe('matroska');
+    // The segment's size covers exactly the rest of the file.
+    expect(segment!.data.byteOffset + segment!.data.length).toBe(file.length);
+    const info = find(segment!.children, '1549a966')!;
+    expect(uintOf(find(info.children, '2ad7b1'))).toBe(1_000_000);
+    const duration = find(info.children, '4489')!;
+    expect(new DataView(duration.data.buffer, duration.data.byteOffset, 8).getFloat64(0)).toBe(1234.5);
+    const [video, audio] = find(segment!.children, '1654ae6b')!.children;
+    expect(textOf(find(video!.children, '86'))).toBe('V_MPEG4/ISO/AVC');
+    expect([...find(video!.children, '63a2')!.data]).toEqual([1, 100, 0, 31]);
+    const picture = find(video!.children, 'e0')!;
+    expect([uintOf(find(picture.children, 'b0')), uintOf(find(picture.children, 'ba'))]).toEqual([1280, 720]);
+    // A frame lasts a 30th of a second, in nanoseconds, so nothing has to guess the rate.
+    expect(uintOf(find(video!.children, '23e383'))).toBe(33_333_333);
+    // BT.709 pixels in the broadcast range, sRGB's transfer.
+    const colour = find(picture.children, '55b0')!.children;
+    expect(['55b1', '55b9', '55ba', '55bb'].map((id) => uintOf(find(colour, id)))).toEqual([1, 1, 13, 1]);
+    expect(textOf(find(audio!.children, '86'))).toBe('A_OPUS');
+    expect(textOf(find(audio!.children, '63a2'))).toBe('OpusHead');
+    // 312 samples at 48 kHz, in nanoseconds.
+    expect(uintOf(find(audio!.children, '56aa'))).toBe(6_500_000);
+    expect(uintOf(find(find(audio!.children, 'e1')!.children, '9f'))).toBe(2);
+  });
+
+  it('takes the colour from what the encoder says, BT.709 where it says nothing', () => {
+    expect(colourOf(null)).toEqual({ matrix: 1, transfer: 1, primaries: 1, fullRange: false });
+    expect(colourOf({ matrix: 'smpte170m', fullRange: true })).toEqual({
+      matrix: 6,
+      transfer: 1,
+      primaries: 1,
+      fullRange: true,
+    });
+  });
+
+  it('writes the blocks of both tracks in the order of their time, whatever order they come in', () => {
+    const writer = new MatroskaWriter(2);
+    writer.add(1, 0, true, Uint8Array.from([1]));
+    writer.add(1, 16_667, false, Uint8Array.from([2]));
+    writer.add(1, 33_333, false, Uint8Array.from([3]));
+    writer.add(2, 0, true, Uint8Array.from([10]));
+    writer.add(2, 20_000, true, Uint8Array.from([11]));
+    writer.add(2, 40_000, true, Uint8Array.from([12]));
+    const blocks = blocksOf(fileOf(writer));
+    expect(blocks.map((b) => [b.track, b.ms])).toEqual([
+      [1, 0],
+      [2, 0],
+      [1, 17],
+      [2, 20],
+      [1, 33],
+      [2, 40],
+    ]);
+    expect(blocks.map((b) => b.data[0])).toEqual([1, 10, 2, 11, 3, 12]);
+  });
+
+  it('hands out a cluster once the next key frame starts another', () => {
+    const writer = new MatroskaWriter(1);
+    writer.add(1, 0, true, Uint8Array.from([1]));
+    writer.add(1, 1_000_000, false, Uint8Array.from([2]));
+    expect(writer.take()).toEqual([]);
+    writer.add(1, 2_000_000, true, Uint8Array.from([3]));
+    const first = writer.take();
+    expect(first).toHaveLength(1);
+    const blocks = blocksOf(fileOf(writer, HEAD, first));
+    expect(blocks.map((b) => [b.cluster, b.ms, b.key])).toEqual([
+      [0, 0, true],
+      [0, 1000, false],
+      [2000, 2000, true],
+    ]);
+  });
+
+  it('starts a new cluster before a block would be too far from its start to be told', () => {
+    const writer = new MatroskaWriter(1);
+    writer.add(1, 0, true, Uint8Array.from([1]));
+    writer.add(1, 40_000_000, false, Uint8Array.from([2]));
+    const blocks = blocksOf(fileOf(writer));
+    expect(blocks.map((b) => [b.cluster, b.ms])).toEqual([
+      [0, 0],
+      [40_000, 40_000],
+    ]);
+  });
+
+  it('does not hold the video back for ever when no sound comes', () => {
+    const writer = new MatroskaWriter(2);
+    for (let i = 0; i < 90; i++) writer.add(1, i * 33_333, i % 30 === 0, Uint8Array.from([i]));
+    // Three seconds of video, no sound: what is more than a second behind has gone into clusters.
+    expect(writer.take().length).toBeGreaterThan(0);
+  });
+
+  it('counts its bytes, the open cluster included, and leaves out a missing sound track', () => {
+    const writer = new MatroskaWriter(1);
+    writer.add(1, 0, true, new Uint8Array(100));
+    expect(writer.bytes).toBeGreaterThan(100);
+    const file = fileOf(writer, { ...HEAD, audio: null });
+    const segment = find(readAll(file), '18538067')!;
+    expect(find(segment.children, '1654ae6b')!.children).toHaveLength(1);
+  });
+});

tests/core/node-catalog.test.ts

@@ -2393,6 +2393,21 @@ const CASES: Record<string, Case[]> = {
   ],
   StopGame: [{ name: 'ends play and the flow', next: null, after: (ctx) => expect(ctx.stopped).toBe(true) }],
   RestartGame: [{ name: 'ends the flow – the world restarts before the next tick', next: null }],
+  StartRecording: [
+    {
+      name: 'asks for a recording and flows on',
+      inputs: { name: 'Take', resolution: '1080p', fps: '60' },
+      next: 'exec',
+      after: (ctx) => expect(ctx.recordings).toEqual([{ name: 'Take', resolution: '1080p', fps: 60 }]),
+    },
+  ],
+  StopRecording: [
+    {
+      name: 'asks to stop and flows on',
+      next: 'exec',
+      after: (ctx) => expect(ctx.recordings).toEqual(['stop']),
+    },
+  ],
   PauseGame: [
     { name: 'pauses the game and flows on', next: 'exec', after: (ctx) => expect(ctx.paused).toBe(true) },
   ],

tests/core/recording.test.ts

@@ -0,0 +1,132 @@
+import { describe, expect, it } from 'vitest';
+import {
+  avcCodec,
+  FrameGate,
+  isKeyFrame,
+  pictureTransform,
+  RecordingClock,
+  recordingName,
+  recordingSettingsOf,
+  recordingSize,
+} from '../../src/core/recording/recording';
+import { runDef, fakeContext } from '../helpers';
+
+describe('recording settings (#405)', () => {
+  it('names a recording after its start when no name is given', () => {
+    const at = new Date(2026, 9, 11, 16, 42, 5);
+    expect(recordingName('', at)).toBe('Recording 2026-10-11 16:42:05');
+    expect(recordingName('  ', at)).toBe('Recording 2026-10-11 16:42:05');
+    expect(recordingName(' Boss fight ', at)).toBe('Boss fight');
+  });
+
+  it('reads what the node hands over, an unknown choice as the default', () => {
+    expect(recordingSettingsOf(' Run ', '1080p', '30')).toEqual({
+      name: 'Run',
+      resolution: '1080p',
+      fps: 30,
+    });
+    expect(recordingSettingsOf('', 'huge', 25)).toEqual({ name: '', resolution: 'view', fps: 60 });
+  });
+
+  it('records the game view at its size on screen, even on both sides', () => {
+    expect(recordingSize('view', { w: 1279, h: 721 }, { w: 1279, h: 721 })).toEqual({ w: 1280, h: 722 });
+    // A 5K screen is held to 4K.
+    expect(recordingSize('view', { w: 5120, h: 2880 }, { w: 5120, h: 2880 })).toEqual({ w: 3840, h: 2160 });
+  });
+
+  it('holds the height of 720p and 1080p and takes the width from the camera', () => {
+    expect(recordingSize('1080p', { w: 800, h: 450 }, { w: 800, h: 450 })).toEqual({ w: 1920, h: 1080 });
+    expect(recordingSize('720p', { w: 400, h: 400 }, { w: 400, h: 400 })).toEqual({ w: 720, h: 720 });
+    // 4:3 at 720p, and the camera's whole picture rules where the screen cuts it.
+    expect(recordingSize('720p', { w: 300, h: 400 }, { w: 640, h: 480 })).toEqual({ w: 960, h: 720 });
+  });
+
+  it('picks the lowest H.264 level that takes the frames', () => {
+    expect(avcCodec(1280, 720, 30)).toBe('avc1.64001f');
+    expect(avcCodec(1920, 1080, 30)).toBe('avc1.640028');
+    expect(avcCodec(1920, 1080, 60)).toBe('avc1.64002a');
+    expect(avcCodec(2560, 1440, 60)).toBe('avc1.640033');
+    expect(avcCodec(3840, 2160, 60)).toBe('avc1.640034');
+  });
+
+  it('starts a group every two seconds', () => {
+    expect([0, 1, 119, 120, 240].map((i) => isKeyFrame(i, 60))).toEqual([true, false, false, true, true]);
+    expect(isKeyFrame(60, 30)).toBe(true);
+  });
+});
+
+describe('RecordingClock', () => {
+  it('counts from the start and leaves the editor pause out', () => {
+    const clock = new RecordingClock();
+    expect(clock.at(500)).toBe(0);
+    clock.start(1000);
+    expect(clock.at(1500)).toBe(500);
+    clock.pause(2000);
+    expect(clock.at(5000)).toBe(1000);
+    clock.resume(6000);
+    expect(clock.at(6500)).toBe(1500);
+    // Pausing twice and resuming once still takes out the whole pause.
+    clock.pause(7000);
+    clock.pause(7500);
+    clock.resume(8000);
+    expect(clock.at(8000)).toBe(2000);
+  });
+});
+
+describe('FrameGate', () => {
+  const taken = (fps: number, frameMs: number, seconds: number, jitter = 0) => {
+    const gate = new FrameGate(fps);
+    let count = 0;
+    for (let i = 0; i * frameMs < seconds * 1000; i++) {
+      const wobble = jitter ? ((i * 7919) % 11) / 10 - 0.5 : 0;
+      if (gate.take(Math.max(0, i * frameMs + wobble * jitter))) count++;
+    }
+    return count;
+  };
+
+  it('takes every frame at 60 on a 60 Hz screen, however unevenly they come', () => {
+    expect(taken(60, 1000 / 60, 10)).toBe(600);
+    expect(taken(60, 1000 / 60, 10, 6)).toBe(600);
+  });
+
+  it('takes every other at 30', () => {
+    expect(taken(30, 1000 / 60, 10)).toBe(300);
+  });
+
+  it('takes about 60 a second from a 144 Hz screen', () => {
+    expect(Math.abs(taken(60, 1000 / 144, 10) - 600)).toBeLessThanOrEqual(6);
+  });
+});
+
+describe('pictureTransform', () => {
+  it('maps the picture onto the whole frame', () => {
+    // A picture of 640x360 CSS pixels centred at (400, 300), recorded at 1080p.
+    const area = { x: 400, y: 300, w: 640, h: 360 };
+    const m = pictureTransform(area, { w: 1920, h: 1080 });
+    const at = (x: number, y: number) => ({ x: m.a * x + m.tx, y: m.d * y + m.ty });
+    expect(at(80, 120)).toEqual({ x: 0, y: 0 });
+    expect(at(720, 480)).toEqual({ x: 1920, y: 1080 });
+    expect(at(400, 300)).toEqual({ x: 960, y: 540 });
+  });
+});
+
+describe('the recording nodes', () => {
+  it('Start recording hands its settings over and flows on', () => {
+    const ctx = fakeContext();
+    const result = runDef('StartRecording', ctx, { name: 'Intro', resolution: '720p', fps: '30' }, {});
+    expect(result.next).toBe('exec');
+    expect(ctx.recordings).toEqual([{ name: 'Intro', resolution: '720p', fps: 30 }]);
+  });
+
+  it('Start recording asks for the game view at 60 by default', () => {
+    const ctx = fakeContext();
+    runDef('StartRecording', ctx, {}, {});
+    expect(ctx.recordings).toEqual([{ name: '', resolution: 'view', fps: 60 }]);
+  });
+
+  it('Stop recording asks for the stop and flows on', () => {
+    const ctx = fakeContext();
+    expect(runDef('StopRecording', ctx, {}, {}).next).toBe('exec');
+    expect(ctx.recordings).toEqual(['stop']);
+  });
+});

tests/helpers.ts

@@ -32,6 +32,7 @@ import type { Position, PropertyBag, Vec2 } from '../src/core/model/Properties';
 import { createEmptyProject, findScene, type Project, ROUTING_ID } from '../src/core/model/Project';
 import { settleRouting } from '../src/core/music/projectRouting';
 import { MASTER_BUS } from '../src/core/properties/buses';
+import type { RecordingSettings } from '../src/core/recording/recording';
 import type { SceneObject } from '../src/core/model/SceneObject';
 import { deserializeProject, serializeProject } from '../src/core/store/serialize';
 import type { AudioSink } from '../src/runtime/systems/Audio';
@@ -291,6 +292,8 @@ export type FakeContext = NodeContext & {
   /** The time scale and the pause the context holds, as the game nodes left them. */
   scale: number;
   paused: boolean;
+  /** What the recording nodes asked for (#405), in order: the settings of a start, `'stop'` for a stop. */
+  recordings: (RecordingSettings | 'stop')[];
 };
 
 /** A NodeContext double: self/owner handles, `speed = 100`, dt 0.5, moves applied directly, `log` and `stop` recorded. */
@@ -411,6 +414,9 @@ export function fakeContext(overrides: Partial<NodeContext> = {}): FakeContext {
     setPaused: (paused) => {
       ctx.paused = paused;
     },
+    recordings: [],
+    startRecording: (settings) => ctx.recordings.push(settings),
+    stopRecording: () => ctx.recordings.push('stop'),
     ...overrides,
   };
   return ctx;

tools/e2e/60-release.mjs

@@ -13,7 +13,7 @@ const BACKGROUND = '#22252c';
 
 /**
  * A small game of its own: a red crate in the middle of the camera that the arrow keys move, a
- * `Log` on start, and S stopping the game.
+ * `Log` and a recording on start, and S stopping the game.
  */
 function game(id) {
   const mover = {
@@ -37,10 +37,15 @@ function game(id) {
     nodes: [
       node('c-start', 'OnStart', 0, 0),
       node('c-log', 'Log', 260, 0, { message: 'released game started' }),
+      node('c-record', 'StartRecording', 520, 0, { name: 'Released 60', fps: '30' }),
       node('c-key', 'OnKeyDown', 0, 160, { key: 's' }),
       node('c-stop', 'StopGame', 260, 160),
     ],
-    edges: [edge('c1', 'c-start', 'exec', 'c-log', 'exec'), edge('c2', 'c-key', 'exec', 'c-stop', 'exec')],
+    edges: [
+      edge('c1', 'c-start', 'exec', 'c-log', 'exec'),
+      edge('c2', 'c-key', 'exec', 'c-stop', 'exec'),
+      edge('c3', 'c-log', 'exec', 'c-record', 'exec'),
+    ],
   };
   return {
     formatVersion: 2,
@@ -226,6 +231,9 @@ export async function release(t) {
     );
 
     // 60d. A click starts it in the start scene, zoomed to Auto: a whole number of pixels per unit.
+    // What the page hands the browser to download is caught here instead (60j).
+    await t.evaluate(`window.__downloads = [];
+      HTMLAnchorElement.prototype.click = function () { window.__downloads.push({ name: this.download, href: this.href }); };`);
     const size = { w: await json('innerWidth'), h: await json('innerHeight') };
     await click(size.w / 2, size.h / 2);
     await waitUntil('__gameMakerPlay.running()', { label: 'the game runs' });
@@ -286,6 +294,24 @@ export async function release(t) {
       JSON.stringify({ moved, screen: await player('screen()') }),
     );
 
+    // 60j (#405). The game recorded itself from its start: ending the run saved the recording, and a
+    // released game, with no project to put it in, hands it to the browser as a Matroska file.
+    await waitUntil('window.__downloads.length > 0', { label: 'the recording is downloaded' });
+    const downloads = await json('window.__downloads');
+    const file = JSON.parse(
+      await t.evaluate(`fetch(${JSON.stringify(downloads[0]?.href ?? '')})
+        .then((r) => r.arrayBuffer())
+        .then((b) => JSON.stringify({ bytes: b.byteLength, head: [...new Uint8Array(b.slice(0, 4))] }))`),
+    );
+    check(
+      'a released game downloads its recording as an MKV file once the run ends',
+      downloads.length === 1 &&
+        downloads[0].name === 'Released 60.mkv' &&
+        file.bytes > 1000 &&
+        JSON.stringify(file.head) === JSON.stringify([0x1a, 0x45, 0xdf, 0xa3]),
+      JSON.stringify({ downloads: downloads.map((d) => d.name), file }),
+    );
+
     // 60h (#267). A click on the corner button makes the page fullscreen and does not reach the
     // game – it would start it again; a second click leaves fullscreen.
     const corner = (await player('fullscreen()')).button;

tools/e2e/67-recording.mjs

@@ -0,0 +1,183 @@
+import { MAKE_OBJECT } from './lib/scaffold.mjs';
+
+/**
+ * Recording the game view (#405). A behavior starts a recording at 1080p and 30 fps when the run
+ * starts, and V stops it: the server converts the file like any upload, and once the run is over a
+ * video asset of that name is in the project, 1920×1080 with sound, its picture the red sprite that
+ * covers the camera. G starts another at the size of the game view; a second G is ignored and says
+ * so in the log, and Esc ends the run, which saves that one too, named after when it started.
+ *
+ * Everything it adds carries an `rc67` id or is one of the two video assets, and goes again at the end.
+ */
+const CONTROL = 'rc67-control';
+const COVER = 'rc67-cover';
+const NAME = 'rc67 take';
+
+const node = (id, type, x, y, params = {}) => ({ id: `rc67-${id}`, type, x, y, params });
+const edge = (id, from, to) => ({
+  id: `rc67-${id}`,
+  from: { node: `rc67-${from}`, port: 'exec' },
+  to: { node: `rc67-${to}`, port: 'exec' },
+});
+
+export async function recording(t) {
+  const { check, sleep, json, evaluate, mutate, press, play, stop, showTab, waitUntil } = t;
+  await showTab('scene');
+
+  const graph = {
+    nodes: [
+      node('start', 'OnStart', 0, 0),
+      node('record', 'StartRecording', 300, 0, { name: NAME, resolution: '1080p', fps: '30' }),
+      node('v', 'OnKeyDown', 0, 200, { key: 'v' }),
+      node('stop', 'StopRecording', 300, 200),
+      node('g', 'OnKeyDown', 0, 400, { key: 'g' }),
+      node('again', 'StartRecording', 300, 400),
+    ],
+    edges: [edge('e1', 'start', 'record'), edge('e2', 'v', 'stop'), edge('e3', 'g', 'again')],
+  };
+  // Red over the whole camera picture, blue over its right half and green over its lower left
+  // quarter, in front of what earlier steps left there: a frame drawn at the wrong place, or read
+  // back upside down, shows the wrong colour.
+  await mutate(`
+    const p = __gm.state.project;
+    const scene = __gm.state.activeScene;
+    const cam = scene.objects.find((o) => o.id === 'o3').position;
+    p.assets.push({ id: '${CONTROL}', type: 'behavior', name: 'Recorder 67', props: { enabled: true, runInEditor: false }, variables: [], graph: ${JSON.stringify(graph)} });
+    ${MAKE_OBJECT}
+    const sprite = p.assets.find((a) => a.type === 'sprite');
+    const cover = make('${COVER}', sprite.id, '', 0, 'Cover 67');
+    Object.assign(cover.position, { x: cam.x, y: cam.y, z: 1000 });
+    cover.overrides = { width: 900, height: 600, color: 0xff0000, texture: '' };
+    const right = make('rc67-right', sprite.id, '', 0, 'Right 67');
+    Object.assign(right.position, { x: cam.x + 225, y: cam.y, z: 1001 });
+    right.overrides = { width: 450, height: 600, color: 0x0000ff, texture: '' };
+    const low = make('rc67-low', sprite.id, '', 0, 'Low 67');
+    Object.assign(low.position, { x: cam.x - 225, y: cam.y + 150, z: 1001 });
+    low.overrides = { width: 450, height: 300, color: 0x00ff00, texture: '' };
+    const logic = make('rc67-logic', '${CONTROL}', '', 0, 'Recorder 67');
+    Object.assign(logic.position, { x: cam.x + 400, y: cam.y });
+    scene.objects.push(cover, right, low, logic);
+  `);
+  const recordings = () => json('__gm.recording()');
+
+  // 67a. Recorded from the start of the run, at 1080p and 30 fps, with the master's sound. The window
+  //      is smaller than 1080p: the frame is drawn into a texture of its own (`PictureReader`).
+  await play();
+  await sleep(1200);
+  await press('v');
+  await waitUntil(`__gm.recording().takes[0]?.state === 'done'`, { label: 'the first take is saved' });
+  const first = (await recordings()).takes[0];
+  check(
+    'Start recording records the game view at the size asked, with sound, and Stop recording saves it',
+    first.name === NAME &&
+      first.width === 1920 &&
+      first.height === 1080 &&
+      first.fps === 30 &&
+      first.sound &&
+      first.soundFrames > 0 &&
+      first.frames >= 25 &&
+      first.frames <= 45 &&
+      first.failure === null,
+    JSON.stringify(first),
+  );
+
+  // 67b. A second start while one records is ignored, and says so in the log.
+  await press('g');
+  await sleep(300);
+  await press('g');
+  await sleep(900);
+  const log = (await json('__gm.world()'))?.log ?? [];
+  const during = await recordings();
+  check(
+    'a second Start recording while one records is ignored and says so in the log',
+    during.takes.length === 2 &&
+      log.some((line) => line.includes('Start recording is ignored') && line.includes(during.takes[1].name)),
+    JSON.stringify({ log, takes: during.takes.map((take) => take.name) }),
+  );
+  await stop();
+
+  // 67c. Ending the run saved the second take; both became video assets once the server had them.
+  await waitUntil(
+    `__gm.recording().uploads.length === 2 && __gm.recording().uploads.every((u) => u.state === 'added' || u.state === 'failed')`,
+    {
+      timeout: 60_000,
+      label: 'both recordings are in the project',
+    },
+  );
+  const { takes, uploads } = await recordings();
+  const second = takes[1];
+  const assets = await json(`__gm.state.project.assets
+    .filter((a) => a.type === 'video' && ${JSON.stringify(uploads.map((u) => u.assetId))}.includes(a.id))
+    .map((a) => ({ id: a.id, name: a.name, file: a.file, width: a.width, height: a.height, duration: a.duration, fps: a.fps, audio: a.audio }))`);
+  const named = assets.find((a) => a.name === NAME);
+  check(
+    'the recording becomes a video asset of its name: 1920×1080, its length, with sound',
+    named !== undefined &&
+      named.width === 1920 &&
+      named.height === 1080 &&
+      named.audio === true &&
+      Math.abs(named.duration - first.durationMs / 1000) < 0.1 &&
+      named.fps === 30,
+    JSON.stringify({ assets, uploads }),
+  );
+  const timed = assets.find((a) => a.name !== NAME);
+  check(
+    'ending the run saves a recording still going, named after when it started, at the game view size',
+    second.state === 'done' &&
+      /^Recording \d{4}-\d\d-\d\d \d\d:\d\d:\d\d$/.test(second.name) &&
+      timed?.name === second.name &&
+      timed.width === second.width &&
+      timed.height === second.height &&
+      second.width % 2 === 0 &&
+      second.fps === 60,
+    JSON.stringify({ second, timed }),
+  );
+
+  // 67d. The picture is the camera's: red at the top left, green below it and blue on the right,
+  //      none of what the view lays over it, the right way up, and as red as it was drawn; every
+  //      4th pixel is read.
+  //      From the MP4 as Chrome decodes it: the poster is no witness, ffmpeg cuts it with BT.601
+  //      whatever the file says (a red of 255 comes out 229).
+  const colorsOf = (file) =>
+    evaluate(`new Promise((resolve) => {
+    const video = document.createElement('video');
+    video.muted = true;
+    video.onerror = () => resolve('null');
+    video.src = '/game-maker/data/videos/${file}';
+    video.play().then(() => video.requestVideoFrameCallback(() => {
+      video.pause();
+      const canvas = document.createElement('canvas');
+      canvas.width = video.videoWidth;
+      canvas.height = video.videoHeight;
+      const pen = canvas.getContext('2d');
+      pen.drawImage(video, 0, 0);
+      const { data } = pen.getImageData(0, 0, canvas.width, canvas.height);
+      const off = [];
+      for (let y = 2; y < canvas.height; y += 4)
+        for (let x = 2; x < canvas.width; x += 4) {
+          const i = (y * canvas.width + x) * 4;
+          const edge = canvas.width / 2;
+          const middle = canvas.height / 2;
+          if (Math.abs(x - edge) < canvas.width / 100 || Math.abs(y - middle) < canvas.height / 100) continue;
+          const hot = x >= edge ? 2 : y >= middle ? 1 : 0;
+          const rgb = [data[i], data[i + 1], data[i + 2]];
+          if (rgb.some((v, c) => (c === hot ? v < 245 : v > 12))) off.push([x, y, ...rgb]);
+        }
+      resolve(JSON.stringify({ w: canvas.width, h: canvas.height, off: off.length, first: off.slice(0, 5) }));
+    }), () => resolve('null'));
+  })`);
+  const colors = `[${await colorsOf(named?.file ?? '')},${await colorsOf(timed?.file ?? '')}]`;
+  const red = JSON.parse(colors);
+  check(
+    'the recorded picture is the camera picture, the right way up, as red as it was drawn',
+    red.every((frame) => frame !== null && frame.w > 0 && frame.off === 0),
+    colors,
+  );
+
+  await mutate(`
+    const p = __gm.state.project;
+    const made = ${JSON.stringify(assets.map((a) => a.id))};
+    p.assets = p.assets.filter((a) => a.id !== '${CONTROL}' && !made.includes(a.id));
+    const scene = __gm.state.activeScene;
+    scene.objects = scene.objects.filter((o) => !o.id.startsWith('rc67-'));`);
+}

tools/e2e/index.mjs

@@ -69,6 +69,7 @@ import { renderLayers } from './63-render-layers.mjs';
 import { pattern } from './64-pattern.mjs';
 import { song } from './65-song.mjs';
 import { video } from './66-video.mjs';
+import { recording } from './67-recording.mjs';
 
 export const STEPS = [
   hierarchy,
@@ -137,4 +138,5 @@ export const STEPS = [
   pattern,
   song,
   video,
+  recording,
 ];