compare: interpolate between frames for smooth point-trajectory motion

Default is smooth (lerp each point between adjacent frames); a motion:step
toggle preserves the snap-to-frame behaviour. Points missing in either
adjacent frame are hidden during the transition to avoid pop-in.

app/web/static/compare.js

@@ -21,6 +21,7 @@ const playBtn = document.getElementById('cc-play');
 const scrub = document.getElementById('cc-scrub');
 const speedSel = document.getElementById('cc-speed');
 const syncSel = document.getElementById('cc-sync');
+const motionSel = document.getElementById('cc-motion');
 const timeAEl = document.getElementById('cc-time').querySelector('[data-role="time-a"]');
 const timeBEl = document.getElementById('cc-time').querySelector('[data-role="time-b"]');
 
@@ -211,7 +212,43 @@ function createPanel({ slotId, panelEl, data }) {
     packedN = j;
     geo.attributes.position.needsUpdate = true;
     geo.setDrawRange(0, packedN);
-    // If there was a highlighted id, reapply it so the overlay follows.
+    applyHighlightForCurrentFrame();
+  }
+
+  // Pack an interpolated frame. uLocal is a continuous index in [0, T-1].
+  // Points missing in either adjacent frame are skipped for the duration of
+  // that transition (no connect-back to the last-known position).
+  function setFrameInterpolated(uLocal) {
+    const T = data.frames.length;
+    if (T === 0) return;
+    if (uLocal <= 0)        return setFrame(0);
+    if (uLocal >= T - 1)    return setFrame(T - 1);
+    const f0 = Math.floor(uLocal);
+    const f1 = f0 + 1;
+    const t  = uLocal - f0;
+    const fr0 = data.frames[f0], fr1 = data.frames[f1];
+    const x0 = fr0.x, y0 = fr0.y, x1 = fr1.x, y1 = fr1.y;
+    const ptIds = data.point_ids;
+    let j = 0;
+    for (let i = 0; i < x0.length; i++) {
+      const a = x0[i], b = x1[i], c = y0[i], d = y1[i];
+      if (a === null || a === undefined || Number.isNaN(a)
+          || b === null || b === undefined || Number.isNaN(b)
+          || c === null || c === undefined || Number.isNaN(c)
+          || d === null || d === undefined || Number.isNaN(d)) continue;
+      const xi = a + (b - a) * t;
+      const yi = c + (d - c) * t;
+      positions[j * 3 + 0] = xi;
+      positions[j * 3 + 1] = yi;
+      positions[j * 3 + 2] = 0;
+      packedX[j] = xi;
+      packedY[j] = yi;
+      packedId[j] = ptIds[i];
+      j++;
+    }
+    packedN = j;
+    geo.attributes.position.needsUpdate = true;
+    geo.setDrawRange(0, packedN);
     applyHighlightForCurrentFrame();
   }
 
@@ -301,6 +338,7 @@ function createPanel({ slotId, panelEl, data }) {
     panelEl,
     data,
     setFrame,
+    setFrameInterpolated,
     setBounds,
     setHighlight,
     resize,
@@ -400,13 +438,19 @@ async function main() {
 
   function applyU(u) {
     u = Math.max(0, Math.min(1, u));
+    const smooth = motionSel.value === 'smooth';
     for (const p of Object.values(panels)) {
       if (!p) continue;
       const T = framesOf(p);
       if (T <= 0) continue;
-      const idx = Math.max(0, Math.min(T - 1, Math.round(u * (T - 1))));
+      const uLocal = u * (T - 1);
+      if (smooth) {
+        p.setFrameInterpolated(uLocal);
+      } else {
+        const idx = Math.max(0, Math.min(T - 1, Math.round(uLocal)));
         p.setFrame(idx);
       }
+    }
     timeAEl.textContent = timeLabelFor(panels.a, u);
     timeBEl.textContent = timeLabelFor(panels.b, u);
   }
@@ -476,6 +520,10 @@ async function main() {
 
   speedSel.addEventListener('change', () => { lastTs = 0; });
 
+  motionSel.addEventListener('change', () => {
+    applyU(parseFloat(scrub.value) / SCRUB_MAX);
+  });
+
   // ---- linked hover -----------------------------------------------------
   function wireHover(pA, pB) {
     if (!pA) return;

app/web/static/style.css

@@ -1627,7 +1627,8 @@ button.submit:disabled { background: var(--faint); border-color: var(--faint); c
 .compare-controls .cc-time-b { color: var(--warm); }
 
 .compare-controls .cc-speed-wrap,
-.compare-controls .cc-sync-wrap {
+.compare-controls .cc-sync-wrap,
+.compare-controls .cc-motion-wrap {
   display: inline-flex;
   align-items: center;
   gap: 0.35rem;

app/web/templates/compare.html

@@ -4,7 +4,7 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width,initial-scale=1" />
   <title>embedding notebook &middot; compare</title>
-  <link rel="stylesheet" href="/static/style.css?v=21" />
+  <link rel="stylesheet" href="/static/style.css?v=22" />
   <script type="importmap">
   {
     "imports": {
@@ -91,6 +91,14 @@
       </select>
     </label>
 
+    <label class="cc-motion-wrap">
+      <span class="cc-lbl">motion</span>
+      <select class="cc-motion" id="cc-motion">
+        <option value="smooth" selected>smooth</option>
+        <option value="step">step</option>
+      </select>
+    </label>
+
     <label class="cc-sync-wrap">
       <span class="cc-lbl">axes</span>
       <select class="cc-sync" id="cc-sync">
@@ -103,6 +111,6 @@
 </section>
 
 <script src="/static/theme.js?v=11"></script>
-<script type="module" src="/static/compare.js?v=1"></script>
+<script type="module" src="/static/compare.js?v=2"></script>
 </body>
 </html>

app/web/templates/index.html

@@ -4,7 +4,7 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width,initial-scale=1" />
   <title>embedding notebook</title>
-  <link rel="stylesheet" href="/static/style.css?v=21" />
+  <link rel="stylesheet" href="/static/style.css?v=22" />
   <script src="https://unpkg.com/htmx.org@2.0.4"></script>
   <script type="importmap">
   {