# Game Maker A browser-based tool "one step before a game engine": create **assets** (scenes, cameras, sprites), place them into scenes, tweak their properties in an inspector and press **Play** – all in one program, all drawn inside a single WebGL canvas. Live: · Widget playground: ## Concepts | Term | Meaning | |---|---| | **Asset** | Named thing with a type (`scene`, `camera`, `sprite`, `text`, `behavior`, `collider`, `container`, `texture`) and typed properties. Assets have **no position**. | | **Texture** | An uploaded image. Scenes and textures are the two types that cannot be placed: a texture has no position, it is *worn* by a sprite through the ordinary property `texture`. See *Textures* below. | | **Text** | An asset that draws a line of text (`text`, `fontSize`, `color`, `align`) – titles, HUD readouts, button labels. Its content is an ordinary property, so a `Properties` node of type Text with `text` switched to Set changes it while playing. | | **Enabled** | Every placeable asset has the switch `enabled` (default on). A disabled object is not drawn, does not collide and its behavior does not run – **and neither does anything parented below it**. Switching one instance off is a normal override, which makes a container a *screen*: switch it on to show it, off to hide it. From a graph, `Properties` with `enabled` switched to Set does it, and `Is enabled` asks (including the parents). | | **Scene object** | An asset placed into a scene: a link to the asset + a **position** (`x`, `y`, `z`) + sparse **overrides** of the asset's properties + an optional **parent** (another object of the scene). Scenes cannot be placed. | | **Container** | A reusable group ("prefab"): the asset holds a **template** of objects (asset link, local position, overrides, parent links) centred on its origin. Placing a container stores the container object **and nothing else** – its content is resolved from the template on every read, so a template edit reaches every instance at once. **Create container** turns the current selection into such an asset: the selected objects move into the template and the scene keeps one placed container at their centre (world positions unchanged). The template is edited in the **Container** tab (see *Editing a container*) or from an instance through **Apply to container**. | | **Selection** | One or more scene objects. Click selects, `Ctrl`/`Shift`-click adds/removes, dragging on empty scene space draws a rubber band (objects whose position lies inside are selected). Dragging a selected object moves the whole selection; `Delete` removes it; the inspector offers **Create container** and **Remove** for several objects. | | **Behavior** | Logic as an asset: a **node graph** plus the **variables** it declares. Placed like anything else (a diamond symbol in the scene); its parent is the **owner** the graph acts on. | | **Run in editor** | A switch only behaviors have (default off). With it on the behavior keeps running while the project is **not** playing – and unlike Play it then edits the *saved* model: what it moves or writes is a real change, autosaved and not on the undo stack. See *Running without Play*. | | **Collider** | A shape (box or circle) for collision detection, placed as a **child** of the object it belongs to (its *body*). Drawn as a green outline in edit mode only. | | **Override** | A per-object value that replaces the asset's value for one property. Only changed keys are stored; the inspector paints their label gold and offers a reset (↺) and an **apply** (↑) that writes the value into the asset instead. Both appear only where the value really differs from its base – an override carrying the asset's own value is none. | | **Default** | Every property has a schema default. Asset values that differ from it are marked gold in the inspector and can be reset (↺) the same way an override can. The asset's name in the **Assets** panel carries the same gold while any of them does (`assetDeviates`, `src/core/properties/deviation.ts` – the inspector marks the rows of that same schema, so the list cannot claim a deviation the inspector then fails to show). A behavior's variables are not in it: their default is declared on the variable itself, so the asset value cannot differ from it. | | **Instance** | An object resolved from a placed container's template. Its **values** can be edited – position, name, property overrides – and only what actually deviates is stored, as a sparse `instanceEdits` entry on the container object keyed by the template path. Everything else keeps following the template. Its **structure** cannot: it can not be removed, re-parented or given a collider, and nothing of the scene can be parented into it. ↺ ▾ resets to the nearest base, which inside a container is the **template as the containers in between leave it**; ↑ ▾ offers **Apply to container** (write the value into the template, where every other instance still at the old value picks it up) and/or **Apply to asset** (which drops the template's override as well). With one base left the menu collapses to a plain ↺ / ↑, with none the buttons disappear. Gold marks the same thing the buttons offer. | | **Unpack** | Writes a placed container's content into the scene as ordinary objects (hierarchy context menu, or the inspector's *Content* section). The ids survive it, so the selection does; from then on the structure is editable and the objects no longer follow the template. | | **Edit mode** | The Scene tab draws the project model; panels, grid, gizmos and drag-to-move are active. | | **Play mode** | The editor switches to a **runtime copy** of the project (same ids). A `World` is instantiated from its active scene (one entity per scene object, resolved properties as mutable state) and shown in the **Game** tab. Systems tick it and mirror their changes into the copy; every panel stays open and edits the copy, which the world picks up immediately. **Stop discards world and copy** – the saved model is never touched. | **The camera defines what is rendered.** A camera asset has a view size (`width` × `height`, default 640×360) and a `zoom`; the frame it covers in world units is `size / zoom`. The Scene tab draws that frame as an outline. The Game tab renders only that frame – fitted into the view with black letterboxing – and objects outside it are culled. Of several cameras the first one in hierarchy order that is **switched on** (`enabled`, its parents included) is the active one, so switching it off hands the view to the next; with none on the tab stays empty and says *No active camera*. Without playing the tab previews the scene through that same camera, so it shows what Play would show, minus the editor symbols. Play has no built-in behaviour (no input, no movement): everything that happens is defined by behavior graphs. **Toolbar and panels.** The toolbar holds the **Project** menu (new project, switch project, *Delete this project…*), the project name (type to rename), the **Scene** picker for the active scene, Undo/Redo, Play/Pause, and on the right the notice line, the save status and the frame rate. The **Assets** panel's `+` creates an asset; right-clicking a row offers *Set as active scene* (double-click or `Enter` does the same), *Create sprite from texture*, *Create animation from texture* and *Delete…*. In **Scene objects** it offers *Select asset*, *Add collider*, *Create container*, *Unpack container*, *Unparent*, *Expand all* / *Collapse all* and *Remove*. **Editing while playing.** Play opens the Game tab but leaves the editor fully usable: the Scene tab shows the live positions (symbols, gizmos and dragging included), the hierarchy and inspector show and edit the running objects, the graph can be changed, undo/redo work on the play session's own history. All of it happens in the runtime copy: values a behavior changes appear as overrides (gold, ↺ resets to the asset value), objects a graph destroys vanish from the lists, objects a graph spawns (`Spawn` node) appear in them – selectable, editable, still only in the runtime copy –, and Esc/Stop brings back exactly what was saved. Autosave pauses while playing. **Pausing.** The toolbar's *Pause* button (or `Ctrl+Enter`) holds the running world: it is no longer ticked, so behaviors, timers and collisions stand still and the world clock stops with them. It is a sub-state of play mode, not a third mode – the runtime copy, the panels and the keyboard behave exactly as while playing, and everything the editor can do during play it can do during a hold. Keys and clicks that arrive while it is held are **dropped** rather than queued, so resuming does not fire a frame's worth of input at once. Stop clears the hold, so a paused game can never outlive its run. **Running without Play.** A behavior switched to **Run in editor** goes on running while the project is not playing (`src/editor/EditorRun.ts`, ticked by the Editor next to the play session). The switch is an ordinary property, so it can be set on the behavior asset and overridden per object. What it runs is a `World` over the **saved** model, and that is the whole difference to Play: a position or variable it changes is a real edit – it is autosaved, the panels show it, and **undo does not take it back**. Which is why the switch is off by default and why it belongs on behaviors that arrange or build content rather than on ones that simulate. **Input goes to the tab you are looking at.** While the **Scene** tab is on screen the keyboard and the mouse are the editor's – typing a name or walking the hierarchy with the arrow keys must not steer a game at the same time. In the **Game** tab, where no key means anything to the editor, they reach the run instead: `On key …`, `Key is down` and `On click` work there, and the tab says so in its top-left corner. Leaving the tab (or focusing a field in it) hands the keyboard back and lets every held key go, so nothing keeps walking with nobody pressing anything. What the run does *not* get: - **structural changes to the content of a placed container** – such content is resolved from the template, not stored (see *Containers*), and the editor run must not bake it out, since that is itself an edit. Its behaviors do run, and what they move or set is kept as an `instanceEdits` entry on the instance; but a `Destroy` of resolved content is dropped from the running world without being saved, and comes back the next time the world is rebuilt. - **a second life** – only the behaviors of the **loaded** scene run, and only in edit mode; Play stops the editor run, since that world runs every behavior anyway. An exception in a graph, or a `Stop game` node, ends the run and puts the reason in the notice line instead of repeating it every frame. ## Behaviors (node graphs) A behavior asset holds a Blueprint-style graph edited in the **Graph** tab of the centre area (the **Scene** tab shows the editable scene, the **Game** tab the running game). Ports come in two kinds: **exec** (white arrows, control flow) and **data** (coloured circles; `number`, `boolean`, `string`, `object`, `list`, `struct`, `any`). Event nodes start a pulse; the interpreter follows exec edges and pulls data inputs lazily from connected outputs. Unconnected data inputs use the inline value shown on the node and editable in the inspector. - **Variables** are the behavior's own property schema (`BehaviorAsset.variables`): they appear in the inspector, can be overridden per placed instance, and each running instance gets its own mutable copy. One node reads and writes them, `Variables`: its settings are a *list* of variables, and each one is an output that can be read at any time – no flow has to reach it – and is read at the moment it is asked, so a read after a write sees the new value. A variable switched to **Set** in the inspector is an input too, written when the flow arrives; the node has its flow pins only while at least one is. Unconnected and with nothing typed, it writes the zero of its kind (0, false, empty), and the node shows that value beside the name. All of them are written together, their inputs read first, so one node swaps two variables. `count = count + 1` is one `Variables` node feeding its own output through an `Add` back into its input. The ports are picked from the behavior's variables rather than named, they take their type from the declaration, and renaming a variable carries them (and their edges) along. `Properties` is the same node for the object it is handed (fixed input `object`). Its **Type** setting, above the list, decides what `+ Property` offers: *Sprite*, *Text*, *Collider*, *Camera* and *Container* offer their fields, *Behavior* the variables of the behavior picked below it (renaming such a variable carries the port along, in any graph), and *Any* offers the kinds (Number, Text, …) and leaves the name to be typed. A picked property keeps its field and default – an unconnected `color` is a colour field starting white, shown on the node as `#ffffff`, and a camera's `width` is its whole-pixel view width. Switching the type keeps the ports; one the new type lacks is marked. The node shows its type as a setting row (*Type: Sprite*, plus *Behavior: Game* under that type). Textures and scenes are no type here: they are not objects in a running game. A variable is a **number, a yes/no, a text, a colour, a texture, a list or a struct**; a list declares **nothing** about its items – each one is a number, a text, a yes/no, a colour, a texture or a struct and says so itself, so one list may hold several kinds at once. A list is compared by its items, never by identity – otherwise every reload would look like an override. There is no list *of objects*: a list is stored in the project file and an object is a live handle that no reload survives. A set of objects is a list of their names, walked with `Repeat` and `Object by name`. A list property is shown as a button saying how many items it holds; clicking it opens the **List** tab, which gives each item a row with the same widget the inspector would use for that kind – a struct item is its fields as rows under each other. The inspector stays visible beside the tab, so what the list belongs to is still on screen. Which list the tab shows is a `ListSlot` (asset property, object override or node setting, plus the `field` of a struct that holds one) on the editor state, because no tab may reach into the `Editor` to switch itself on. An **unconnected `list` input** of a node is the same button and the same tab: its inline value lives in the very same params bag a setting does, and the slot stops resolving the moment an edge feeds the port. What an item holds is **its own** and is chosen there: the **kind mark** in front of a row (`#`, `Aa`, `✓`, `◧`, `▦`, `⊞`) is the picker – a click opens the menu, with no chevron, since the mark has to stay as narrow as its column – and `+ Item ▾` asks for the kind of the one it appends. Because the kind lives in the value, every such change is an ordinary **value edit**: one command, one undo step, and no neighbouring item touched. A **struct item** carries its own shape the same way: its fields are renamed, retyped, added (`+ Field ▾`) and removed on its own rows, which grows that one item and not the list. Two kinds need saying, because JSON cannot tell them from the value beside them: a **colour** is a number and a **texture** is an id, which is a text. Such a *tagged* kind wears its name as the one key of a small object – `{"$color": 16711680}`, `{"$texture": "walk-up"}` (the picker a sprite's `texture` property has – a whole picture, one frame of a sheet, or an animation). A field name cannot start with `$` (`isValidFieldName`), so a tag can never collide with a struct item of one field. Either reads as what it wraps on a data edge (`Item at`), the graph having neither colours nor pictures of its own, and `Set item` leaves the slot the kind it was. A **texture is a kind wherever a kind is picked** – a variable, a struct field, a list item, an Any port of `Properties`, a field of `Pack` – and the `Texture` node hands one into the graph, as `Number` and `Text` do a number and a text. **Every kind is one row of one table** (`core/properties/kinds.ts`): its word and mark, what it travels as on an edge, its empty value, how a value is read as it, whether it is tagged and where it may be picked. The kind lists, the `+ …` menus, the tags, the port fields and the literal nodes all follow from the row, so a new kind is that row plus, if it names something in the project, its picker. Declared (a variable, a struct field of one, a port) it needs no tag: the declaration says what it is, and the value is the id – the way a colour field is a number. A declared port of kind colour or texture carries the field that edits it and travels as a number or a text, since the graph has neither; a port declared as the number a colour travels as still names the property (`isPortOf`). A **struct** variable is the other way round: few fields, so they fit under each other right in the inspector. Its head row renames it and adds fields (`+ Field`), each field row below carries its own name, its value and a remove – the port editor `Pack` already has, one level down and with a value column – and a field may itself be a **list**, which is the one count button that opens the tab one level deeper. Adding, renaming or removing a field reaches the declaration, the behavior's own value **and every instance override** in one command, so no stored value keeps a shape its declaration no longer promises. A placed instance draws every variable, whatever its kind, with the very same rows (`buildVariable`), minus the renaming, the kind chooser and the `+ Field` / `×` buttons: it may override the values, not declare them. A struct's ↺ / ↑ sit on its head row, because an override covers the whole bundle rather than a single field. All of it sits on **one column grid** with the rest of the inspector: mark (28 px), gap and name (90 px) are exactly as wide as a plain row's label (124 px), a nested field's name gives up what its indent takes, and names are cut with `…` rather than pushing their value out of line. - **Owner** = parent: a placed behavior is usually parented to the object it controls (inspector → Parent, or drag it onto that object in the hierarchy; dashed line in the viewport). The `Owner` node returns it; `Self` returns the behavior itself; `Parent` walks up from any object. - **Auto-repeat on `On key down`**: two settings on the node, *Repeat after (ms)* and *Repeat interval (ms)* – milliseconds, so both stay whole numbers. A delay of 0 leaves the node as it was – one fire per press; with a delay set, a key that stays held fires again after it and then every interval (0 = every frame). That is the sideways slide of a Tetris piece, or a held arrow walking a menu. The browser's own key repeat is deliberately not used: its delay and rate are an operating-system setting, not a game's. Both rows stay off the node body while the delay is 0, so a plain key node keeps its single line. - **Node set** (`src/core/graph/nodes/`, one file per node): events `On start`, `On update`, `On key down/up/held`, `On timer`, `On collision enter/stay/exit`, `On click` (the mouse pressed on the owner – anywhere when it has none), `On animation finished`, `Start` · flow `Branch`, `Sequence`, `Repeat`, `Run behavior`, `End` · data `Self`, `Owner`, `Parent`, `Object by name`, `Name of`, `Is asset` (is this object an instance of asset X – how you tell what you collided with), `Variables`, `Properties`, `Get position` (local and world), `Is enabled`, literals, `Add/Subtract/Multiply/Divide`, `Min/Max/Abs/Sqrt/Clamp`, `Floor/Ceil/Round`, `Modulo`, `Random`, `Time`, `To text`, `To number`, `Join text`, `Substring`, `Find text`, `Equals`, lists `List` (a constant), `Fill` (one value repeated – how an empty board is made), `Count`, `Item at`, `Set item` (a *new* list, the one that came in is left alone), `Sub list`, `Join lists`, `Index of`, structs `Pack`/`Expand`, `Length`, `Distance`, `Normalize`, `Axis input` (four keys → a normalized direction, so diagonals are not faster), `Compare`, `And/Or/Not`, `Key is down`, `Mouse position`, `Mouse button`, `Is hovered`, `Is touching` · actions `Move` (dx, dy, scale, dt – connect an event's dt for per-second movement), `Set position`, `Spawn` (asset setting + x/y/z, optional parent → outputs the new object), `Destroy`, `Log`, `Stop game`, `Restart game`. A node type is one file exporting a `NodeDef` (ports, settings, `run`), listed in `nodes/index.ts`; palette, ports, validation and inspector fields are generated from it. A port may declare a `default` for its inline value (e.g. `Move.scale = 1`), shown on the node and in the inspector. - **Ports that are not fixed by the type**: a `NodeDef` may say `declaresPorts` (the node carries its own list, edited in the inspector) or `derivePorts` (the node works them out from its settings and from the graph around it). `resolveDef(node, graph, lookup)` is what applies both, so the canvas and the inspector see a definition that fits *this* node; a node whose ports are derived reads them back off that definition, which is why `run` is handed it. `Sequence` keeps one free output below the last connected one – connecting the spare grows it, unplugging the bottom edge shrinks it again (keys `a`, `b`, `c`, … up to 26). `Run behavior` takes its ports from the behavior it names, and a behavior's `Start`/`End` node declares them. Pointing a call at a different behavior **takes the edges of the ports that go away** and gives them back on undo: an edge on a port that no longer exists draws as nothing but survives every save, and would come back to life once the port returns. - **Structs**: several named values travelling as one, so a bundle needs one edge instead of one per part. `Pack` declares its fields as inputs, `Expand` reads them back out as outputs, a behavior port may be declared `struct` – which is how a bundle crosses a call – and a **variable** may be one, which is how a bundle is stored. A `Pack`/`Expand` can take its fields from a variable that already has the shape (**From variable**: a struct variable, or the first struct item a list holds – a list declares nothing, so its value is the only source) instead of having them typed again; that adds the fields it does not have yet as one undo step and leaves the ones it has alone. Writing a struct variable keeps the fields it declares: what arrives fills the ones it names, the rest stay put (`mergeStruct`), so one node can set part of a bundle without erasing it – "the same piece, one row lower" is a `Pack` of that one field. Depth is bounded by the *value*, not by convention: a list item is a primitive, a tagged colour or a struct of flat ones (`listItems.ts`), a struct field says what it holds (`StructFieldKind`, primitives plus `list`), and the list a *field* holds takes flat items only. Forbidding just that one combination is what keeps the shapes finite: `list → struct → primitive` and `struct → list → primitive` are the two deepest a stored value goes – exactly what `asStoredValue` lets through an untyped slot – so the deep compare and the editor stay finite. The List tab **offers only the kinds its slot can hold** (`ListAccess.kinds`): on a list of its own all five, on a list a struct field holds the four flat ones. Offering the struct there would add an item the next write drops – which reads as the menu entry doing nothing. A live entity is an object on a data edge too, so it carries a `Symbol` brand (`ENTITY_HANDLE`) that no struct field name can impersonate; nothing may hand a property value to `String()` (`textOf` instead), or a struct would read as `[object Object]`. A struct port has no inline value (its parts come from a `Pack`), and compatibility is per *kind*, not per shape – any struct fits any struct port. - **Loops**: `Repeat` runs its `body` output a number of times, counting `index` up from 0, and continues at `done`. Data nodes are cached for the length of one event pulse, but a loop round drops that cache, so an expression built on `index` really changes with it. That is what makes a graph able to work through a list: the Tetris example draws its 200 board cells that way, `Object by name` taking the name the round put together (`"c" + index`). Two guards, not one: a pulse may take 100 000 steps (a loop legitimately runs long), but exec edges may only nest 1000 deep – that is what catches a flow feeding back into itself before the call stack does. - **A behavior as a node** (nesting): a behavior that has a **Start** and an **End** node can be run as a step of another behavior's graph. Every input declared on the start node becomes an input port of the calling node, every output declared on the end node one of its outputs; the node is drawn under the behavior's own name, and the palette lists it there too, so a call is one pick. Declaring a port keeps both sides in step – renaming carries the typed-in value along, removing takes the edges hanging on it in *every* graph that calls the behavior. A call is a **call, not an inlining**: the nested graph runs in its own pulse cache with a fresh copy of its variable defaults, so values travel in and out through the ports and nothing else. That makes a call repeatable and keeps a nested behavior free of hidden state – for state, use a behavior object in the scene (see *Cross-object state* below). A behavior that would run itself raises an error instead of looping; there is one start and one end node per behavior. Three ways in and out of a call: **drag** a behavior out of the asset panel onto the graph and it lands as a call where you let go; **double-click** a call node to open the graph it runs. The way down stays above the graph as a **breadcrumb** (`Game › Move › Other`): click a crumb to go back there; the first one returns to where you started and the bar goes away. Opening a behavior any other way – the asset panel, the scene, the picker – ends the trail. And a call that cannot do anything **says so on the node** – no behavior picked, the behavior deleted, or no start node in it. The last one is the one worth naming: without a start node there is no way in, so the call is skipped and the flow simply carries on at its `exec` output. A missing *end* node is not a defect – a call is always synchronous, "done" is the nested flow running out, and the end node only decides what comes back. - **Cross-object state**: a behavior's variables *are* the properties of its object, so another graph reads and writes them with `Object by name` → `Properties` of type *Behavior*. That is how the example game keeps one coin counter for the whole run: picking a coin up is one node, `coins` fed through `Add` back into itself. - **Mouse**: the Game tab feeds the pointer to the world in world units. `On click` fires for the behavior whose owner was hit – colliders decide when there are any, otherwise the sprites and texts of the owner and everything below it, so a button made of a box plus a label works as one. A switched-off screen cannot swallow clicks. A behavior **without an owner** has no shape that could be hit, so it takes every click inside the camera frame instead (the `world x`/`world y` outputs say where) – that is the "click anywhere" listener. - **Idiomatic movement** (the demo's "Mover", five nodes): `On update → Move(Owner, dx = Axis input.x, dy = Axis input.y, scale = speed, dt = On update.dt)`, `speed` being a `Variables` node. - **Idiomatic platformer** (the example game's "Player"): `On update` adds `gravity · dt` to a `vy` variable and moves the owner by `(axis.x · speed, vy)` with *Collide with solids* on; `blocked y` while falling means standing, so `vy` goes back to 0 and `onGround` becomes true; `On key down (Space)` sets `vy = −jumpSpeed` when `onGround`. - **Editing**: right-click the canvas for the searchable palette, drag port → port to connect (types are checked; a data input holds one link and is replaced), drag nodes, right-click a node or port to delete/disconnect, `Delete` removes the selection, wheel zooms. Everything is undoable. - **Mouse and selection work as in the scene view.** The **middle button pans** from anywhere, a node under the pointer included (shift-drag on empty canvas does too); the left button never pans. Dragging on empty canvas pulls a **rubber band** instead, which selects every node it touches – nodes are large rectangles, so touching counts, unlike the scene view's test of an object's position. Ctrl/Shift-click adds or removes single nodes, dragging any node of a selection moves all of them, and `Delete` removes all of them – one undo step either way. - **Dragging an edge tells you where it would land** before you let go: the port it would connect to wears a white ring and the line snaps to it. Dropping on a node's *body* rather than a port takes that node's first free compatible port – or, when none is free, the last compatible one, whose link is then replaced. A drop into empty space opens the palette filtered to the node types this edge could reach at all, and the picked node arrives already wired; node and link go in as **one** undo step (`core/graph/dropTarget.ts` decides all of this, the canvas only draws it). The dangling edge stays on screen while that palette is up, so the connection being made is still visible while its node is chosen. - **A link can be hidden**, so a connection across the whole graph does not have to be drawn as a line across the whole graph. Right-click a port → *Hide link*; the curve is replaced by a short stub at each end, ending in a dot captioned with the node at the *other* end. The connection is untouched – hiding is drawing only, and `folded` on the edge is the entire change. The curve comes back, dimmed, **while either of its two nodes is selected**, and always while a pulse is travelling it, so the live trace never loses its thread. Clicking a dot selects the node at the far end, which both answers "where does this go?" and lends the curve back; right-clicking it offers *Show link* and *Disconnect* (`editor/graph/folding.ts` holds the visibility rule, the canvas only draws it). **A port's hidden links share one mark** – drawn per link, several of them would stack stub, dot and caption on the very same pixels. The mark then grows a little, is captioned `Add +2`, and one click selects *every* far end at once, which is also what lends all of its curves back; the menu counts what it acts on (*Show 3 links*), and does it as one undo step. - **A node can be collapsed** to its title bar: the chevron in front of the title (it leaves the selection alone, like the hierarchy's), or right-click → *Collapse node* / *Expand node*, which acts on the whole selection as one undo step. Every link of one side then meets at a single point on the title bar, and its hidden links share one mark per side. It narrows to its title (never wider than an open node, where a long title is cut). A collapsed node has no ports to grab, so a link dropped on it goes nowhere – expand it to wire it. Like hiding a link, this is drawing only: `collapsed` on the node is saved with the project. A collapsed `Variables` or `Properties` node, titled just that while open, is titled by what it does, since its ports are out of sight: *Set 2 variables* when more than one is written, else *Get 2 variables* when more than one is only read, else *Set speed* / *Get speed*, and *No variables* when it has none. - **Header bar** (the same one the Texture and Container tabs have): the behavior picker, **Fit** (which zooms out far enough to hold every node, measuring each by its own drawn height – `editor/framing.ts`), and the size of the graph as "N nodes · M links". - **Snap** (header checkbox, off by default): a drag drops each node it carries on the nearest 24 px grid line – the same grid the background draws. It takes effect on the next drag only, so switching it on never shuffles a graph that is already laid out, and a node nobody drags never moves. Every node of a selection snaps **for itself**, so dragging a group is how you line one up; the offset is a decision about rectangles and lives with the others in `editor/framing.ts`. - **Selection-following, not selection-bound**: selecting a behavior asset, a placed behavior or one of its nodes shows that graph; a selection about anything else (a sprite, nothing at all) leaves the tab on the behavior you were editing, because checking a sprite's position must not empty your work. Only a behavior that no longer exists is dropped, and then the tab falls back to another one – the picker is how you switch on purpose. - **Live view**: Play always switches to the Game tab; switch to Graph while playing and the exec edges and nodes that just ran light up – the same canvas draws editor and game, so this costs nothing extra. Each fire of an exec edge also sends a bead along it (0.3 s, at most one per 0.15 s and edge): a key press is a single bead, `On update` an even chain. Beads run on game time, so a paused game holds them where they are. Every data output shows, in the same colour, the value it last carried (`0.017 dt`, `Pause box object`), refreshed ten times a second. With several objects running the same behavior it is the one that produced last; in a loop, the last round. An output nothing has asked for yet shows nothing. The graph stays editable meanwhile: the interpreter reads the runtime copy's graph on every pulse, so an edit takes effect on the next frame and is discarded with the run. A new `On start` node does not fire for a behavior that has already started. - **Runtime**: `BehaviorSystem` creates one `GraphInterpreter` per behavior entity; graphs only touch world entities. A runaway pulse throws (see *Loops* for the two limits) and stops play with a notice. **Placing an asset: drag it in.** Drag a row out of the **Assets** panel and drop it on the view – the Scene tab places into the active scene, the Container tab into the open template, so what you see is what you fill. While the pointer is over a view, an outline shows the footprint the object will have and a crosshair the origin it snaps to; it turns **red** when the drop would be refused, which today means a container that would end up inside itself (`placementRejection`, `src/core/model/placement.ts` – the same function the command runs, so the preview cannot promise a placement the drop then denies). Picking a container up selects it, but the Container tab does *not* follow that while a drag is in flight: the target must not swap out from under the pointer. Dragging is the only way to place: a double-click on a scene makes it the active one, on anything else it does nothing. **Editor symbols are always on top.** The viewport draws scene content (sprites) in an `objects` layer and editor symbols (camera icon, labels) in an `overlay` layer above it, so a sprite can never hide a symbol regardless of z or hierarchy order. A viewport label keeps a constant size on screen however far you zoom, and both kinds of them – the object names of the Scene tab and the frame names of the Texture tab – read that size from one token (`Theme.viewportLabelSize`); the two `viewportLabel` styles differ in colour only. **Z-order is part of the position.** `position.z` is the draw order of a scene object: higher `z` is drawn on top; objects with equal `z` keep their hierarchy (tree) order. It is edited in the inspector next to X/Y, undoable like any move, and applies in edit and play mode alike. **Hierarchy (parenting).** Every scene object may have a parent in the same scene (`SceneObject.parentId`). Positions – including `z` – are **local to the parent**; the world position is the sum along the chain (`src/core/model/hierarchy.ts`). Children move with their parent and are removed with it (also when the parent's asset is deleted); undo brings the whole subtree back. Re-parenting keeps the world position by recomputing the local one, and cycles are rejected. In the **Scene objects** panel children are indented below their parent; drag a row onto another row to parent it, onto the empty space below the rows (or right-click → Unparent) to move it to the top level; the inspector offers the same as a **Parent** menu. Dragging into the **gap** above or below a row – its outer quarter, marked with a line instead of an outline – orders the object among *that* row's siblings rather than putting it inside, and re-parents it on the way when the gap belongs to another parent. Since equal `z` falls back to this order, that is also how two objects at the same depth are put in front of each other. A drag that lands where it started changes nothing and costs no undo step. A row's **name turns gold** when the object deviates from what it follows – its container template, or its asset – and also when anything below it does, so a folded subtree still shows that something inside was changed. It is the same gold the inspector paints an overridden label in. Rows with children **fold**: click the chevron (or press ←/→, or double-click the row) to open and close a subtree, right-click → **Expand all** / **Collapse all** for the whole scene. A scene opens fully collapsed – which rows are open is view state of the panel, never stored in the project – and selecting an object anywhere (viewport, `Add collider`, `Create container`) unfolds the way down to it and scrolls it into view, so a selection is never hidden. In play mode entities carry the same `parent` link: moving a parent entity moves its subtree, `Destroy` removes it. `Get position` exposes both the local and the world coordinates. **Editor offset (scene view only).** Every scene object additionally carries an `editorOffset` – an x/y shift of where it is *drawn, selected and dragged* in the Scene tab, and nothing else. The game, collisions and the graph never see it; it is there to pull apart objects that sit on top of each other, such as the stacked title/win/lose screens of a game. It cascades like a position (offsetting a container offsets its content) and is edited in the inspector below X/Y/Z, undoable, with ↺ back to 0. It belongs to the placed object, so `Create container` and placing a container never carry it into the template. ## Colliders and collisions A **collider** is an asset (`shape` box/circle, `width`/`height` or `radius`) placed like any other object – typically as a child of the sprite it belongs to, so its position is the offset from that *body* and it moves with it. Right-click an object in **Scene objects** → *Add collider* to get one with one click: a box collider sized like the sprite, parented at its centre. Colliders can be reused across objects, overridden per instance and stacked (hitbox + hurtbox on one sprite; colliders of the same body never collide with each other). They are editor symbols: green outlines in edit mode, invisible in play mode. Play runs a `CollisionSystem` (`src/runtime/systems/CollisionSystem.ts`) before the behaviors: every frame it tests all collider pairs (world positions applied; touching edges do not count) and sorts them into *enter*, *stay* and *exit*. Detection never pushes anything apart. A behavior receives `On collision enter/stay/exit` for the colliders of its **owner** (its parent; itself when top-level): the outputs are `other` (the other body), `other collider` and `collider` (the own one). `Is touching(a, b)` answers the same question on demand – an object stands for its collider children, a collider for itself. The geometry lives in `src/core/properties/collider.ts` so editor and runtime agree on it. **Blocking** is opt-in per movement: the `Move` node has a **Collide with solids** setting (default off). When enabled it moves an object one axis at a time and, after each axis, pushes its *solid* colliders back to just touch any solid collider they ran into – so the other axis still moves and the object slides along walls. Its `blocked x` / `blocked y` outputs tell which axes hit (always `false` with the setting off). A collider with **Trigger (no blocking)** enabled (`isTrigger`, drawn amber instead of green) still produces collision events but never blocks. An overlap that already exists is never resolved (no jolt when objects start stacked or inside a wall) but also never deepened, so such an object can always leave. A move longer than the thinnest collider involved is split into sub-steps – a fast object cannot jump across a thin wall and end up on the other side. Colliders of the moving object and of anything parented under it travel with it and cannot block it. `Move` without the setting and `Set position` ignore colliders entirely. ## Textures A sprite is a coloured box until it is given a picture. The picture is a **texture asset**, and the sprite points at it through the ordinary property `texture` — which is the whole design: because it is an ordinary property, instance overrides, **Apply to container**, **Reset to asset**, undo and `Properties` work on it without a single line written for them. One coin instance can wear a different picture than its nine siblings, and swapping a texture at runtime is a graph node that already exists. **Getting an image in.** **Dropping image files onto the canvas** or **Ctrl+V** pasting one uploads it and makes the asset in one go; several files at once are imported one after another, and a file that fails does not stop the rest. **+ → Texture** asks a different question and so opens a different dialog: not the machine's file system, but **every picture the server already holds**, as a grid of thumbnails. Files are never collected (below), so a picture one project dropped is still there for the next to pick up, and uploading the same bytes twice buys nothing. Picking one and pressing **Use image** makes a texture asset on that file; **Upload…** is the way in for a picture the server does not have yet, and lands in the same import as a drop. A tile captioned with an asset name is one this project already wears – picking it selects that asset rather than making a second one. **Where it goes.** The image is uploaded as raw bytes to `POST ?r=textures` and stored as `data/textures/{sha256}.{ext}` — the file is named after its own content. That name buys four things: the same picture uploaded twice is one file, a stored file can be cached forever (a name cannot come to mean different bytes), the project document holds a short string instead of megabytes of base64, and deleting a texture asset is undoable for free because nothing on disk is removed. The price is that files outlive the projects that referenced them. **Colour tints the picture.** `color` multiplies the texture, so it defaults to **white** (= no tint) and a picture shows its own colours. Tinting is worth having: one white coin picture becomes ten differently coloured coins, per instance, for nothing. Without a texture `color` is simply the fill of the box, as before. **`filter`** is the texture's only property: *Smooth* (default) or *Pixel art*, which is the difference between crisp and mush when a small image is scaled up. Switching it needs a `style.update()` on the texture source, not just the assignment: Pixi's `scaleMode` setter writes the filters but leaves the style's cached resource id in place, and that id is the key the GPU sampler is looked up under — so without it the picture only changes on a reload. ### Frames — cutting a sheet up The **Texture tab** shows one texture with its **frames** on top: named rectangles in image pixels. Drag on empty picture to cut one, drag it or its eight handles to adjust it, and edit the exact numbers in the inspector; `Delete` removes the selected frame. **Slice…** cuts a regular grid at once — you give columns and rows (not a cell size: everyone knows how many tiles their sheet has, and deriving the cell size from the picture cannot disagree with it), with optional offset and spacing for padded sheets, and the dialog says how many frames of what size the numbers would produce before it commits. A sprite then wears **a frame instead of the whole picture** — but through the *same* property. `texture` holds either a texture id (the whole image) or a frame id, because frame ids come from the same generator and are unique across the document. That is what keeps one picker, one property and one set of instance overrides, and what makes every sprite written before frames existed still mean exactly what it meant. The picker lists each sheet followed by its frames as "Sheet / Frame", and it is searchable, because one sliced sheet adds an entry per cell. Frames are intrinsic data on the texture, like a scene's objects and a behavior's graph — not properties, since nothing about them is overridable per instance. All edits go through commands, so cutting, dragging, slicing and re-slicing are undoable, and one drag is **one** undo step. Deleting a frame leaves the sprites pointing at it: a dangling reference draws the plain colour and comes back by itself on undo. Deleting the whole texture counts the sprites wearing *any* of its frames, not just the ones naming the texture directly. **Fitting the sprite to the picture.** The *Width* and *Height* rows of a sprite carry a ↔ / ↕ button offering two things: **Fit to 64 × 48** sets that one side from the other and the picture's proportions — the frame's rectangle when it wears a frame, the whole image otherwise — and **Set to 64 × 48 px** puts both sides at the picture's own pixel size. Like ↺ and ↑ the button exists only when it would do something, and follows the same rule for how many: no offer means no button, one is a direct click, both make it the menu form ↔ ▾. So a sprite without a picture, or one already in proportion *and* at pixel size, carries nothing. Either action is an ordinary property write, so on an object it becomes an override, and setting both sides is still one undo step. The proportions are compared with a relative tolerance rather than exactly — of the 40 000 picture sizes up to 200 × 200, 3135 do not survive the round trip through a division, and an exact test would leave the button standing after its own click. The pixel size is compared exactly, because those two numbers are written as they are. **A sprite straight from a texture.** Right-click a texture in the asset list → *Create sprite from texture* makes a sprite asset that wears the whole picture at its pixel size, named after the texture (numbered like any name clash, so `Coin` gives `Coin 2`) and selected. It is the same as creating a sprite, picking the texture and clicking *Set to W × H px*, but in one undo step. Rendering a frame is a texture matrix on the fill, not a Pixi sub-texture — a Graphics fill ignores a `Texture`'s `frame` (measured, see `viewport/textureFill.ts`). One consequence is worth having: a sheet stays **one** GPU upload however many frames of it are on screen. **Nothing ever breaks on a missing picture.** While an image loads, and for a reference that dangles because the texture was deleted, the sprite draws its plain colour — exactly what it looked like before textures existed. So there is no placeholder, no flicker and no error path. Deleting a texture warns how many sprites reference it (a texture is never *placed*, so counting placed instances would report "unused" for a picture a dozen sprites are showing). ### Animations An **animation asset** is a list of pictures played one after another. Each entry is what a sprite's `texture` can hold – a whole texture or one of its frames – so an animation may mix frames of several sheets, and a picture listed twice is held twice as long; there is no per-frame duration beyond that. Its two properties are **Frames per second** (default 10) and **Playback**: *Loop*, *Once* (stops on the last frame) or *Ping-pong* (back and forth, without showing the turning frames twice). **A sprite wears it through `texture`** – the same property, the same picker (animations are listed first, marked "(animation)") and the same instance overrides as a texture or a frame. The sprite stays what is drawn, sized, tinted and collided with; the animation is only a picture that changes over time. *Set to W × H px* takes the size of the first frame. The fastest way in is right-clicking a texture → **Create animation from texture**, which plays its frames in order (the whole picture when it has none), named after the texture, in one undo step. **The Animation tab** plays the animation above a strip of its frames. `▶` / `❚❚` pauses on the frame on screen, and resumes from the selected one. Clicking a tile pauses on it, dragging a tile into another gap reorders the frames, `Delete` removes the selected one, right-click offers *Duplicate frame* and *Remove frame*, and **+ Frames ▾** inserts after the selection: each sheet's frames at once, or single pictures. The selected tile belongs to the tab, not to the editor's selection, so the inspector keeps showing the animation's two properties meanwhile. The preview fits each frame on its own until the wheel zooms it about the pointer; from then on every frame is drawn at that one scale, centred, so frames of different sizes show their real proportion. The middle button (or `Shift`+drag) pans, **Fit** returns to fitting, and over the strip the wheel scrolls the tiles instead. **It runs in play only.** The Scene and Container tabs show the first frame: the edit mode has no clock and redraws on changes only. The `AnimationSystem` gives every sprite its own clock, kept on the entity and not in `props` (the world writes changed props back as overrides). The one rule that makes it usable from a graph: **a different animation starts from its first frame, the same one set again runs on** – so `On update → Properties (Sprite, texture on Set)` may choose Run or Idle every frame without freezing the animation on frame one. The system ticks before the behaviors, so **`On animation finished`** fires in the frame a *Once* animation reaches its end, exactly once, with the sprite as `object`. Like collisions it is heard by the behaviors of the sprite's owner subtree – the sprite itself, or a sprite placed next to the behavior below the same object – and by a behavior without an owner for every sprite; its *Animation* setting narrows it to one. There is no node to restart an animation that is already running; that and a state machine are left for when a game needs them. Deleting a texture counts the animations that play its frames besides the sprites wearing it; deleting an animation counts the sprites wearing it. Either leaves dangling references that draw the plain colour and come back on undo. **Limits.** nginx caps request bodies at 1 MB, so the editor shrinks images over 2048 px per side and re-encodes to WebP when they are still too big — but an image that already fits is uploaded byte for byte, because re-encoding pixel art would destroy it. Anything still too large is refused with a clear message instead of a broken upload. The server accepts PNG, JPEG, WebP and GIF, deriving the type from the content rather than from anything the client says, and rejects images over 8192 px per side. ## Editing a container The **Container tab** shows one container's template with the *same* viewport the Scene tab uses – `ContainerCanvas` subclasses `SceneViewport` and swaps nothing but which asset holds the objects, so picking, rubber band, dragging, gizmos, labels and parent lines are not written twice. It adds a cross at the **origin**, because that is the point the container is held by when placed: where the content sits relative to it is the thing being authored. A template has no background of its own – painting one would invent the scene it is not in. The **Scene objects** panel lists whatever the visible tab edits, so in the Container tab it lists the template, with the same drag-to-parent, context menu and folding as a scene; the inspector, `Delete` and `Ctrl+G` work on template objects like on placed ones. None of that is a special case: a scene asset and a container asset are both `ObjectHolder`s, and every object command takes a `holderId` that is either one. **Header bar**: the container picker (searchable), **Fit** (frames the content, origin always included) and the size of the template as "N objects". Placing needs no button of its own: an asset dragged from the panel onto this view goes into the template, and a container dragged onto its own template is refused while it hovers rather than at the drop. Like the Graph tab it follows the selection without being bound to it – the container asset, one of its placed copies or one of its template objects shows it, anything else leaves the tab on what you were editing, and only a deleted container makes it fall back to another one. Selecting a template *object* also switches to the tab, the rule a graph node and a texture frame already follow: it exists in no other view. Selecting the asset itself does not. Placing a container anywhere creates **one** object. Its content is never copied: `resolveObjects` (`src/core/model/instance.ts`) weaves the template in whenever the holder is read, one level per nesting step, and the resolved objects get derived ids – `~