javascript - High CPU usage with canvas and requestAnimationFrame - Stack Overflow

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I am facing high CPU usage (30 to 40%) when calling recursively requestAnimationFrame, does anyone has good strategies to lower it down?

Simple example:

var canvas = document.createElement('canvas');
canvas.width = 100;
canvas.height = 20;

var canvasContext = canvas.getContext('2d');
document.body.appendChild(canvas)

var rafId;
function drawLoop(time) {
  canvasContext.clearRect(0, 0, 100, 20);
  canvasContext.fillRect(0, 0, Math.random() * 100 * 1.4, 20);
  rafID = window.requestAnimationFrame(drawLoop);
}

drawLoop();

I am facing high CPU usage (30 to 40%) when calling recursively requestAnimationFrame, does anyone has good strategies to lower it down?

Simple example:

var canvas = document.createElement('canvas');
canvas.width = 100;
canvas.height = 20;

var canvasContext = canvas.getContext('2d');
document.body.appendChild(canvas)

var rafId;
function drawLoop(time) {
  canvasContext.clearRect(0, 0, 100, 20);
  canvasContext.fillRect(0, 0, Math.random() * 100 * 1.4, 20);
  rafID = window.requestAnimationFrame(drawLoop);
}

drawLoop();

• javascript
• html5-canvas
• cpu-usage
• requestanimationframe

Share Improve this question Follow edited Apr 4, 2016 at 4:50 user1693593 asked Feb 8, 2015 at 3:50 LaurentLaurent 98733 gold badges1313 silver badges2727 bronze badges 7

• requestAnimationFrame() is supposed to run as fast as possible, and thus use a lot of CPU. if you need fewer updates, use a timer to target a reasonable FPS rate instead. – dandavis Commented Feb 8, 2015 at 3:54
• 1 @dandavis: As far as I know requestAnimationFrame() is usually capped to 60 FPS and not "as fast as possible": developer.mozilla/en-US/docs/Web/API/… - with 60 FPS or more you should have plenty of time to for little updates on <canvas> without maxing out CPU – Mikko Ohtamaa Commented Feb 8, 2015 at 4:28
• 1 @Laurent - try different browsers and devices. Your drawing code is so simple that it should only overpower very weak puter unless there are some issues (no HW acceleration, etc) – Mikko Ohtamaa Commented Feb 8, 2015 at 4:31
• 1 My 2-cents: Your resource usage is a bit high for modern devices. But, an older single-core, no-GPU, low memory mobile device with system+browser overhead plus rAF might pull as much as that. Anecdotally, I've seen weaker mobile devices get crushed by html canvas--but not with the simple code you have presented. – markE Commented Feb 8, 2015 at 4:49
• @MikkoOhtamaa: yes, good point. I meant as fast as the device can go, which if the refresh rate is 60, is 60. i could have been more clear, but if the CPU is getting flooded, then OP is unlikely to be actually seeing 60 updates a second, and scheduling around, say 20FPS, would lower the CPU use... afaik, there is no way to adjust RAF's 'target FPS' yourself. – dandavis Commented Feb 8, 2015 at 5:10

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1 Answer 1

Sorted by: Reset to default 3

I cannot get this example to do anything to my CPU worth mentioning, but I did manage to get it down by employing these two methods. My CPU was running at about 4-5% running your snippet, by running save / restore on the context that shaved off 2%.Unsure why - because we haven't made any transformations. The latter example just uses the old hacker way of doing this by resetting the canvas.width - this wipes the entire canvas context each time - and should be more expensive - however it got this example down to 1.4%

var canvas = document.createElement('canvas');
canvas.width = 100;
canvas.height = 20;

var canvasContext = canvas.getContext('2d');
document.body.appendChild(canvas)

var rafId;
function drawLoop(time) {
  canvasContext.save();
  canvasContext.clearRect(0, 0, 100, 20);
  canvasContext.fillRect(0, 0, Math.random() * 100 * 1.4, 20);
  canvasContext.restore();
  rafID = window.requestAnimationFrame(drawLoop);
}

drawLoop();

var canvas = document.createElement('canvas');
canvas.width = 100;
canvas.height = 20;

var canvasContext = canvas.getContext('2d');
document.body.appendChild(canvas)

var rafId;
function drawLoop(time) {
  canvas.width = canvas.width;
  canvasContext.fillRect(0, 0, Math.random() * 100 * 1.4, 20);
  rafID = window.requestAnimationFrame(drawLoop);
}

drawLoop();

Now I would need to go into more performance exploration to find out why, or if it actually does anything at all.

However you could employ a different drawing technique, such as just moving a sprite or a mask back and forth over some bitmap data, that will make this much less hard for the renderer to handle. I will not post that here as it goes beyond the scope of this question.

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