modules/spl-meter/client.js

@@ -350,7 +350,18 @@ function spectrumView(root, moduleApi, scrolling) {
   root.innerHTML = '<canvas class="spl-canvas"></canvas>';
   const canvas = root.querySelector('.spl-canvas');
   const ctx = canvas.getContext('2d', { alpha: false });
-  let painted = false;          // spectrogram: has the background been laid down
+  /**
+   * The spectrogram's scrolling history, on its own surface.
+   *
+   * It used to scroll by copying the visible canvas onto itself, which also
+   * copied whatever had been drawn on top of it — so every frame dragged the
+   * frequency labels and gridlines one step further along and left a smear.
+   * Only measured pixels belong on a surface that scrolls; annotations are
+   * drawn fresh over the top of it, every frame, and never into it.
+   */
+  let history = null;
+  let historyCtx = null;
+  let painted = false;          // spectrogram: has the history been cleared yet
   let smoothed = null;          // RTA: displayed values, damped toward the real ones
   let smoothedFor = '';
   let axisBtn = null;
@@ -497,62 +508,99 @@ function spectrumView(root, moduleApi, scrolling) {
       // ── spectrogram ──
       const timeX = axis() === 'x';
       const colW = 2;
-      const gutter = showLabels ? axisPad : 0;
-      // Frequency occupies whichever axis time does not.
-      const freqExtent = timeX ? h - gutter : w - gutter;
-      const timeExtent = timeX ? w : h;
+      // A label along the bottom needs only its line height; one down the
+      // side has to fit "12.5k" beside the plot rather than on top of it.
+      const gutter = (showLabels || grid) ? (timeX ? 30 : axisPad) : 0;
+      // Labels belong beside the frequency axis: down the left when frequency
+      // runs vertically, along the bottom when it runs horizontally.
+      const plotX = timeX ? gutter : 0;
+      const plotW = timeX ? w - gutter : w;
+      const plotH = timeX ? h : h - gutter;
+      const freqExtent = timeX ? plotH : plotW;
+      const timeExtent = timeX ? plotW : plotH;
+      if (plotW < 4 || plotH < 4) return;
 
-      if (!painted) { ctx.fillStyle = bg; ctx.fillRect(0, 0, w, h); painted = true; }
-
-      // Source rect in device pixels, destination in CSS pixels: the context
-      // carries a devicePixelRatio transform but the canvas as an image
-      // source does not, and letting drawImage infer them scales the picture
-      // up and pushes every column off the canvas.
-      if (timeX) {
-        ctx.drawImage(canvas, 0, 0, canvas.width, canvas.height, -colW, 0, w, h);
-      } else {
-        ctx.drawImage(canvas, 0, 0, canvas.width, canvas.height, 0, -colW, w, h);
+      // The history surface is sized to the plot, in device pixels, and is
+      // thrown away whenever that geometry changes — a scroll buffer means
+      // nothing once it is being stretched into a different shape.
+      const dpr = window.devicePixelRatio || 1;
+      const needW = Math.max(1, Math.round(plotW * dpr));
+      const needH = Math.max(1, Math.round(plotH * dpr));
+      if (!history || history.width !== needW || history.height !== needH) {
+        history = document.createElement('canvas');
+        history.width = needW;
+        history.height = needH;
+        historyCtx = history.getContext('2d', { alpha: false });
+        historyCtx.setTransform(dpr, 0, 0, dpr, 0, 0);
+        painted = false;
       }
+      if (!painted) {
+        historyCtx.fillStyle = bg;
+        historyCtx.fillRect(0, 0, plotW, plotH);
+        painted = true;
+      }
+
+      // Scroll the history, then add the newest column. The source rectangle
+      // is in device pixels and the destination in CSS pixels, because the
+      // context carries the dpr transform but the canvas as an image source
+      // does not.
+      historyCtx.drawImage(history, 0, 0, history.width, history.height,
+                           timeX ? -colW : 0, timeX ? 0 : -colW, plotW, plotH);
 
       const bandExtent = freqExtent / values.length;
       for (let i = 0; i < values.length; i++) {
-        ctx.fillStyle = heat(norm(values[i], payload.calibrated));
+        historyCtx.fillStyle = heat(norm(values[i], payload.calibrated));
         if (timeX) {
           // Low frequencies at the bottom, as every spectrogram has them.
-          ctx.fillRect(timeExtent - colW, freqExtent - (i + 1) * bandExtent,
+          historyCtx.fillRect(timeExtent - colW, freqExtent - (i + 1) * bandExtent,
                               colW, Math.ceil(bandExtent) + 1);
         } else {
           // Low frequencies at the left.
-          ctx.fillRect(gutter + i * bandExtent, timeExtent - colW,
+          historyCtx.fillRect(i * bandExtent, timeExtent - colW,
                               Math.ceil(bandExtent) + 1, colW);
         }
       }
 
-      if (grid || showLabels) {
+      // Repaint the visible canvas from scratch every frame: history first,
+      // annotations over it. Nothing drawn here can ever reach the scroll.
+      ctx.fillStyle = bg; ctx.fillRect(0, 0, w, h);
+      ctx.drawImage(history, 0, 0, history.width, history.height,
+                    plotX, 0, plotW, plotH);
+
+      if (!grid && !showLabels) return;
+
+      const span = Math.log2(freqs[freqs.length - 1] / freqs[0]);
       const posOf = (f) => {
-          const t = Math.log2(f / freqs[0]) / Math.log2(freqs[freqs.length - 1] / freqs[0]);
-          return timeX ? freqExtent - t * freqExtent : gutter + t * freqExtent;
+        const t = Math.log2(f / freqs[0]) / span;
+        return timeX ? plotH - t * plotH : plotX + t * plotW;
       };
-        const labels = chooseLabels(freqs, posOf, timeX ? freqExtent : w, timeX ? 16 : 34);
-        ctx.fillStyle = bg;
-        if (timeX) ctx.fillRect(0, h - gutter, w, gutter);
-        else ctx.fillRect(0, 0, gutter, h);
-        ctx.fillStyle = muted; ctx.font = '9px system-ui';
-        ctx.strokeStyle = border; ctx.globalAlpha = grid ? 0.35 : 0; ctx.beginPath();
+      const labels = chooseLabels(freqs, posOf, timeX ? plotH : w, timeX ? 14 : 34);
+
+      if (grid) {
+        ctx.strokeStyle = border; ctx.lineWidth = 1; ctx.globalAlpha = 0.35;
+        ctx.beginPath();
         for (const l of labels) {
           if (timeX) {
             const y = Math.round(l.pos) + 0.5;
-            ctx.moveTo(0, y); ctx.lineTo(w, y);
+            ctx.moveTo(plotX, y); ctx.lineTo(w, y);
           } else {
             const x = Math.round(l.pos) + 0.5;
-            ctx.moveTo(x, 0); ctx.lineTo(x, h);
+            ctx.moveTo(x, 0); ctx.lineTo(x, plotH);
           }
         }
         ctx.stroke(); ctx.globalAlpha = 1;
-        ctx.textAlign = timeX ? 'left' : 'center';
-        for (const l of showLabels ? labels : []) {
-          if (timeX) ctx.fillText(hzLabel(l.f), 2, Math.min(h - gutter - 2, l.pos + 3));
-          else ctx.fillText(hzLabel(l.f), l.pos, h - 3);
+      }
+
+      if (showLabels) {
+        ctx.fillStyle = muted; ctx.font = '9px system-ui';
+        ctx.textAlign = timeX ? 'right' : 'center';
+        for (const l of labels) {
+          if (timeX) {
+            ctx.fillText(hzLabel(l.f), gutter - 2,
+                         Math.max(7, Math.min(h - 2, l.pos + 3)));
+          } else {
+            ctx.fillText(hzLabel(l.f), l.pos, h - 3);
+          }
         }
       }
     },

modules/spl-meter/module.json

@@ -1,7 +1,7 @@
 {
   "id": "spl-meter",
   "name": "SPL Meter",
-  "version": "0.4.0",
+  "version": "0.4.1",
   "proddash": ">=1.5.0",
   "description": "Calibrated sound level, Leq and RTA from an audio input on this machine",
   "permissions": [