None of these methods will produce transparent pixels if the context was created with the
alpha: false parameter.
You can use the
clearRect method to clear any rectangular section of the canvas.
// Clear the entire canvas ctx.clearRect(0, 0, canvas.width, canvas.height);
clearRectis dependent on the transformation matrix.
To deal with this, it's possible to reset the transformation matrix before you clear the canvas.
ctx.save(); // Save the current context state ctx.setTransform(1, 0, 0, 1, 0, 0); // Reset the transformation matrix ctx.clearRect(0, 0, canvas.width, canvas.height); // Clear the canvas ctx.restore(); // Revert context state including // transformation matrix
ctx.restoreare only requiered if you wish to keep the canvas 2D context state. In some situations save and restore can be be slow and generally should be avoided if not required.
It's possible to write directly to the rendered image data using
putImageData. By creating new image data then assigning it to the canvas, you will clear the entire screen.
var imageData = ctx.createImageData(canvas.width, canvas.height); ctx.putImageData(imageData, 0, 0);
putImageData is not affected by any transformations applied to the context. It will write data directly to the rendered pixel region.
It's possible to clear complex shaped regions by changing the
// All pixels being drawn will be transparent ctx.globalCompositeOperation = 'destination-out'; // Clear a triangular section ctx.globalAlpha = 1; // ensure alpha is 1 ctx.fillStyle = '#000'; // ensure the current fillStyle does not have any transparency ctx.beginPath(); ctx.moveTo(10, 0); ctx.lineTo(0, 10); ctx.lineTo(20, 10); ctx.fill(); // Begin drawing normally again ctx.globalCompositeOperation = 'source-over';
Rather than use
clearRect which makes all pixels transparent you may want a background.
To clear with a gradient
// create the background gradient once var bgGrad = ctx.createLinearGradient(0,0,0,canvas.height); bgGrad.addColorStop(0,"#0FF"); bgGrad.addColorStop(1,"#08F"); // Every time you need to clear the canvas ctx.fillStyle = bgGrad; ctx.fillRect(0,0,canvas.width,canvas.height);
This is about half as quick
0.008ms as clearRect
0.004ms but the 4millions of a second should not negatively impact any realtime animation. (Times will vary considerably depending on device, resolution, browser, and browser configuration. Times are for comparison only)
Clear the canvas using compositing operation. This will clear the canvas independent of transforms but is not as fast as
ctx.globalCompositeOperation = 'copy';
anything drawn next will clear previous content.