Play animation can blend into the new animation over a number of frames (#106)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/model/animation.ts

@@ -388,6 +388,24 @@ export interface AnimationPlayer {
   /** Start order across the whole run: of two players setting one property, the later start wins. */
   order: number;
   bases?: Record<string, PropertyValue>;
+  blend?: AnimationBlend;
+}
+
+/**
+ * A crossfade into a player that just took over (#106): over `frames` of its own, a row that blends
+ * mixes from what the property held into the player's value. `from` is taken on the first tick of the
+ * blend; a property an earlier player writes in the same tick is mixed from that value instead, live.
+ */
+export interface AnimationBlend {
+  frames: number;
+  elapsed: number;
+  from?: Record<string, PropertyValue>;
+}
+
+/** How far a blend has come, 0…1; an animation without a frame rate has nothing to blend over. */
+export function blendShare(blend: AnimationBlend, fps: number): number {
+  if (!(blend.frames > 0) || !(fps > 0)) return 1;
+  return Math.min(1, (blend.elapsed * fps) / blend.frames);
 }
 
 interface PlayerHolder {
@@ -400,16 +418,21 @@ let startCount = 0;
  * Starts the animation on the object, or lets a paused one run on. One already running keeps its
  * frame, so a graph may ask for it every frame without holding it at its first frame – but it counts
  * as started last again: turning back to a walk that still runs has to show that walk.
+ * `blendFrames` crossfades into it – but only when it takes over: asked for again while it already
+ * leads on the object, it keeps the blend it has, so a graph calling every frame does not restart it.
  */
-export function playAnimation(holder: PlayerHolder, animationId: string): void {
+export function playAnimation(holder: PlayerHolder, animationId: string, blendFrames = 0): void {
   const players = (holder.animations ??= []);
   const known = players.find((p) => p.animationId === animationId);
+  const blend = blendFrames > 0 ? { frames: blendFrames, elapsed: 0 } : undefined;
   if (known) {
+    const leads = players.every((p) => p.order <= known.order);
     known.playing = true;
     known.order = ++startCount;
+    if (!leads) known.blend = blend;
     return;
   }
-  players.push({ animationId, time: 0, playing: true, order: ++startCount });
+  players.push({ animationId, time: 0, playing: true, order: ++startCount, ...(blend && { blend }) });
 }
 
 /** Ends it where it stands – the values stay; `''` ends every animation on the object. */