Platformer: add physics-based runner jump arcs

src/renderer/effects/platformerScroll.test.ts

@@ -7,7 +7,8 @@ import {
   PlatformerScrollEffect,
   platformAt,
   runnerJumpOffset,
-  supportTopY
+  runnerTraversalY,
+  supportTopY,
 } from "./platformerScroll";
 
 const createGradient = () => ({ addColorStop: vi.fn() });
@@ -87,6 +88,27 @@ describe("platformerScroll helpers", () => {
     expect(down).toBe(0);
   });
 
+  it("runnerTraversalY adds a jump arc for upward steps", () => {
+    const start = runnerTraversalY(220, 188, 0, 0.2);
+    const preTakeoff = runnerTraversalY(220, 188, 0.04, 0.2);
+    const peak = runnerTraversalY(220, 188, 0.5, 0.2);
+    const end = runnerTraversalY(220, 188, 1, 0.2);
+
+    expect(start).toBe(220);
+    expect(preTakeoff).toBe(220);
+    expect(peak).toBeLessThan(194);
+    expect(end).toBe(188);
+  });
+
+  it("runnerTraversalY keeps footing briefly before dropping down", () => {
+    const early = runnerTraversalY(188, 220, 0.1, 0.2);
+    const late = runnerTraversalY(188, 220, 0.9, 0.2);
+
+    expect(early).toBe(188);
+    expect(late).toBeGreaterThan(210);
+    expect(late).toBeLessThanOrEqual(220);
+  });
+
   it("buildRunnerSprite creates a colorful mascot silhouette with animated limbs", () => {
     const earlyFrame = buildRunnerSprite(100, 160, 16, 0, 0.2);
     const laterFrame = buildRunnerSprite(100, 160, 16, 0.2, 0.2);

src/renderer/effects/platformerScroll.ts

@@ -116,6 +116,28 @@ export function runnerJumpOffset(prevSupportY: number, currentSupportY: number,
   return Math.floor(arc * (lift * 0.85 + extra));
 }
 
+export function runnerTraversalY(prevSupportY: number, currentSupportY: number, colProgress: number, audioAmount: number): number {
+  const progress = clamp(colProgress, 0, 1);
+  const heightDelta = prevSupportY - currentSupportY;
+
+  if (heightDelta > 0) {
+    const jumpWindowStart = 0.08;
+    const jumpWindowEnd = 0.96;
+    const jumpProgress = smoothstep((progress - jumpWindowStart) / (jumpWindowEnd - jumpWindowStart));
+    const arcLift = Math.sin(jumpProgress * Math.PI) * Math.max(6, heightDelta * 0.9 + audioAmount * 4);
+    const baseY = prevSupportY + (currentSupportY - prevSupportY) * jumpProgress;
+    return baseY - arcLift;
+  }
+
+  if (heightDelta < 0) {
+    const ledgeHold = 0.22;
+    const fallProgress = progress <= ledgeHold ? 0 : smoothstep((progress - ledgeHold) / (1 - ledgeHold));
+    return prevSupportY + (currentSupportY - prevSupportY) * fallProgress;
+  }
+
+  return prevSupportY;
+}
+
 export function buildRunnerSprite(baseX: number, footY: number, tileSize: number, time: number, audioAmount: number): RunnerSprite {
   const unit = Math.max(1, Math.round(tileSize / 16));
   const x = baseX - 4 * unit;
@@ -406,10 +428,9 @@ export class PlatformerScrollEffect implements Effect {
       frontPulse
     );
 
-    const supportY = Math.floor(prevSupportY + (currentSupportY - prevSupportY) * smoothstep(colProgress));
-    const hop = runnerJumpOffset(prevSupportY, currentSupportY, colProgress, audioAmount);
+    const supportY = runnerTraversalY(prevSupportY, currentSupportY, colProgress, audioAmount);
     const runBob = Math.floor((Math.sin(time * 14) * 1.5 + audioAmount * 1.5) * 0.5);
-    const runnerFootY = supportY - hop - runBob - 1;
+    const runnerFootY = Math.floor(supportY) - runBob - 1;
     const runnerSprite = buildRunnerSprite(runnerBaseX, runnerFootY, tileSize, time, audioAmount);
 
     ctx.fillStyle = runnerSprite.shadow.color;