make a cute little canvas to draw on and hook it up to some ojs

.quarto/_freeze/site_libs/quarto-listing/quarto-listing.js

@@ -2,8 +2,7 @@ const kProgressiveAttr = "data-src";
 let categoriesLoaded = false;
 
 window.quartoListingCategory = (category) => {
-  // category is URI encoded in EJS template for UTF-8 support
-  category = decodeURIComponent(atob(category));
+  category = atob(category);
   if (categoriesLoaded) {
     activateCategory(category);
     setCategoryHash(category);

posts/2025-05-25-Brachistochrone/index.qmd

@@ -0,0 +1,114 @@
+---
+title: "Brachistochrone"
+description: |
+  Lets create a double pendulum in Observable JS!
+date: 2025-05-09
+categories:
+  - Observable JS
+  - Code
+  - Math
+---
+
+This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text 
+
+:::{.column-screen}
+```{ojs}
+//| echo: false
+
+
+viewof canvas = {
+  const height = 500;
+  const canvas = html`<canvas width=${width} height="${height}" style="border: 1px solid #ccc; cursor: crosshair;"></canvas>`;
+  const ctx = canvas.getContext('2d');
+
+  let isDrawing = false;
+  let lastX = 0;
+  let lastY = 0;
+  let pathLength = 0;
+  let pathHistory = [{time: Date.now(), length: 0}];
+
+  // Drawing settings
+  ctx.strokeStyle = '#000';
+  ctx.lineWidth = 2;
+  ctx.lineCap = 'round';
+  ctx.lineJoin = 'round';
+
+  canvas.addEventListener('mousedown', (e) => {
+    if (!isDrawing) {
+      ctx.clearRect(0, 0, canvas.width, canvas.height);
+      ctx.arc(300, 100, 5, 0, 2 * Math.PI);
+      ctx.fillStyle = 'red';
+      ctx.fill();
+      ctx.arc(600, 300, 5, 0, 2 * Math.PI);
+      ctx.fillStyle = 'red';
+      ctx.fill();
+      pathLength = 0;
+      pathHistory = [{time: Date.now(), length: 0}];
+      canvas.value = {length: pathLength, history: pathHistory};
+      canvas.dispatchEvent(new CustomEvent("input"));
+    }
+    isDrawing = true;
+    [lastX, lastY] = [e.offsetX, e.offsetY];
+  });
+
+  canvas.addEventListener('mousemove', (e) => {
+    if (!isDrawing) return;
+
+    // Calculate distance
+    const distance = Math.sqrt(
+      Math.pow(e.offsetX - lastX, 2) + 
+      Math.pow(e.offsetY - lastY, 2)
+    );
+    const angle = Math.atan2(e.offsetY - lastY, e.offsetX - lastX)
+    pathLength += distance;
+
+    // Draw
+    ctx.beginPath();
+    ctx.moveTo(lastX, lastY);
+    ctx.lineTo(e.offsetX, e.offsetY);
+    ctx.stroke();
+    [lastX, lastY] = [e.offsetX, e.offsetY];
+
+    // Update value and emit event
+    pathHistory.push({time: Date.now(), length: pathLength});
+    canvas.value = {length: pathLength, history: pathHistory};
+    canvas.dispatchEvent(new CustomEvent("input"));
+  });
+
+  canvas.addEventListener('mouseup', () => isDrawing = false);
+  canvas.addEventListener('mouseout', () => isDrawing = false);
+
+  // Initial value
+  canvas.value = {length: 0, history: pathHistory};
+  return canvas;
+}
+
+```
+
+
+```{ojs}
+//| echo: false
+md`Current path length: **${canvas.length?.toFixed(0) || 0} pixels**`
+```
+
+```{ojs}
+//| echo: false
+import {Plot} from "@observablehq/plot"
+
+pathChart = Plot.plot({
+  width: width,
+  height: 200,
+  y: {label: "Path Length (pixels)"},
+  x: {label: "Time", type: "time"},
+  marks: [
+    Plot.line(canvas.history || [], {
+      x: "time", 
+      y: "length",
+      stroke: "steelblue",
+      strokeWidth: 2
+    })
+  ]
+})
+```
+
+This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text This is a bunch of text