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To draw freehand on a canvas, let the Canvas 2D API paint pixels and let Pointer Events supply the input. Create a <canvas> and get its context with getContext("2d"). On pointerdown, remember where the pointer started. On each pointermove, draw a short line from the last point to the new one. On pointerup or pointercancel, forget that pointer. Because Pointer Events treat mouse, pen and fingertip alike, one set of handlers covers desktop and mobile. The sections below build this up and cover the details that usually break it: blurry or distorted lines, touch scrolling, multi-touch, and accessibility.
A complete working sketch pad
This is the smallest version that behaves well on mouse, pen and touch. The sections after it explain each decision.
<canvas id="pad" width="800" height="500"
style="touch-action: none; border: 1px solid #888">
Your browser does not support canvas.
</canvas>
<script>
const canvas = document.getElementById("pad");
const ctx = canvas.getContext("2d");
if (!ctx) throw new Error("2D canvas context unavailable");
ctx.lineWidth = 4;
ctx.lineCap = "round";
ctx.lineJoin = "round";
ctx.strokeStyle = "#111";
// One entry per active pointer: pointerId -> last point
const active = new Map();
function toCanvas(e) {
const r = canvas.getBoundingClientRect();
return {
x: (e.clientX - r.left) * (canvas.width / r.width),
y: (e.clientY - r.top) * (canvas.height / r.height)
};
}
canvas.addEventListener("pointerdown", function (e) {
canvas.setPointerCapture(e.pointerId);
const p = toCanvas(e);
active.set(e.pointerId, p);
// Make a single tap leave a dot
ctx.beginPath();
ctx.arc(p.x, p.y, ctx.lineWidth / 2, 0, Math.PI * 2);
ctx.fillStyle = ctx.strokeStyle;
ctx.fill();
});
canvas.addEventListener("pointermove", function (e) {
const last = active.get(e.pointerId);
if (!last) return; // this pointer is not drawing
const p = toCanvas(e);
ctx.beginPath();
ctx.moveTo(last.x, last.y);
ctx.lineTo(p.x, p.y);
ctx.stroke();
active.set(e.pointerId, p);
});
function end(e) { active.delete(e.pointerId); }
canvas.addEventListener("pointerup", end);
canvas.addEventListener("pointercancel", end);
</script>How it works, step by step
1. Set the canvas’s intrinsic size
The width and height attributes (or properties) define the canvas’s coordinate space, which is the size of its bitmap. Resizing only with CSS stretches that bitmap, so lines can look distorted or soft. Maximum usable dimensions vary by browser and environment, so don’t assume very large canvases will work everywhere. The toCanvas helper in the sketch converts screen coordinates into bitmap coordinates, so drawing stays correct even when CSS displays the canvas at a different size from its attributes.
2. Get the 2D context and check it
canvas.getContext("2d") returns the object that owns every path and stroke method. It can return null if the context type isn’t supported, or if the canvas has already been put into a different context mode (for example, after a "webgl" request). Check the result before using it, as the sketch does.
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3. Draw segments, not whole paths
Pointer movement arrives as a sequence of discrete samples. Drawing a short segment from the previous sample to the current one on each pointermove gives you an incremental stroke without redrawing anything. Setting lineCap and lineJoin to "round" hides the seams between segments.
4. Track each pointer by pointerId
Every active pointer has its own pointerId. Keeping a map from ID to last point means two fingers draw two independent strokes instead of one line jumping between them. This is the reason to avoid a single global lastX/lastY pair. The same Pointer Events model also lets a mouse, a pen and touches coexist without separate code paths.
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5. End strokes on both pointerup and pointercancel
Browsers fire pointercancel when they take over a gesture (a scroll or pinch, for instance) or the input is otherwise interrupted. If you only handle pointerup, the cancelled pointer stays in your map and can produce a stray line later. Delete the state for that ID in both handlers.
6. Stop touch from scrolling the page
By default, a finger dragged across a canvas scrolls or zooms the page, and the browser may cancel your pointer stream to do so. MDN’s drawing example sets touch-action: none on the canvas to prevent that. Apply it deliberately: it disables default touch panning and zooming on that element, so a full-screen canvas can trap users who need to scroll past it. Leave some non-canvas page area, or provide a mode toggle, when the canvas fills the viewport.
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The setPointerCapture call in the sketch is a complementary measure: it keeps delivering events to the canvas even if the pointer slides outside it mid-stroke, so a stroke doesn’t silently stop at the edge.
Pointer Events vs. separate mouse and touch handlers
| Approach | Device coverage | Concurrent inputs | Trade-off |
|---|---|---|---|
| Pointer Events | Mouse, pen and fingertip through one event model | Multiple pointers, distinguished by pointerId |
One code path; you still need touch-action for gesture control |
Separate mouse* and touch* handlers |
Needs a handler set per device class | Touch lists for touch; mouse handled separately | More code and more room for inconsistent behavior |
For a new drawing surface, Pointer Events is the simpler choice.
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Common failures and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Lines are offset from the cursor | CSS size differs from the canvas attributes and coordinates aren’t scaled | Convert with getBoundingClientRect() and the width/height ratio, as in toCanvas |
| Drawing looks stretched or fuzzy | Canvas sized only with CSS | Set intrinsic width and height; if you resize the canvas, remember that doing so clears it |
| Page scrolls while drawing on a phone | Default touch behavior is active | Add touch-action: none to the canvas |
| Strokes stop abruptly on touch | The browser cancelled the pointer for a gesture | Handle pointercancel and set touch-action |
| Stray line connects two strokes | State from an ended pointer was kept | Delete the pointer’s entry on pointerup and pointercancel |
getContext returns null |
Unsupported context, or the canvas is already in another context mode | Use one context type per canvas and check for null |
Sharper lines on high-density screens
If lines look soft on a high-density display, a common technique is to make the bitmap larger than its CSS size and scale the context. This is general canvas practice rather than something Pointer Events require. Set canvas.width = cssWidth * devicePixelRatio (same for height), set the CSS width and height to the original values, and keep using the ratio-aware toCanvas conversion. Because the conversion already maps CSS pixels to bitmap pixels, you only need to scale line widths to taste.
Accessibility
A canvas is a bitmap. Assistive technology cannot see the shapes you draw, so strokes are invisible to it. Put fallback content inside the <canvas> element, and give a drawing tool real HTML controls (buttons for undo, clear, colour and save, with proper labels) rather than painting them onto the canvas. If the drawing carries significant information, provide that information as text or other accessible markup alongside it. Pointer-only input also excludes keyboard users, so decide whether your use case needs an alternative way to perform the same action.
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When to consider OffscreenCanvas
A simple sketch pad doesn’t need it. If you render something heavy, such as large filters, many layers or replaying thousands of strokes, and the main thread stutters, OffscreenCanvas lets a worker do the rendering while the interface stays responsive. It adds architectural complexity (messaging, transferring the canvas), so adopt it only once you’ve seen a real responsiveness problem. Check current browser support for your target environments before depending on it.
Where to read more
The behaviors above are documented by MDN Web Docs in its pages on the Canvas API, the <canvas> element, CanvasRenderingContext2D, Pointer Events (including its drawing app example and multi-touch interaction guide) and HTMLCanvasElement. Browser-specific compatibility should be verified for your deployment. No stylus is required: mouse and fingertip work with the same code, and a pen simply adds another pointer type.
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