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    <p>I think you're confused between whether the <em>logic</em> is running and whether the <em>rendering</em> is happening. Many browsers now hardware-accelerate their canvases so all rendering happens on the GPU, so actual pixel pushing takes no CPU time anyway. However your tick function has non-trivial code to generate random noise on the CPU. So you're only really concerned over whether the tick function is running. If the canvas is offscreen, it certainly won't be rendered to the display (it's not visible). As for the canvas draw calls, it probably depends on the browser. It could render all draw calls to an off-screen canvas in case you suddenly scroll it back in to view, or it could just queue up all the draw calls and not actually do anything with them until you scroll the canvas in to view. I'm not sure what each browser does there.</p> <p>However, <strong>you shouldn't use <code>setInterval</code> or <code>setTimeout</code> for animating Canvas</strong>. Use the new <a href="https://developer.mozilla.org/en/DOM/window.requestAnimationFrame"><code>requestAnimationFrame</code></a> API. Browsers don't know what you do in a timer call so will always call the timer. <code>requestAnimationFrame</code> on the other hand is designed specifically for visual things, so the browser has the opportunity to not call the tick function, or to reduce the rate it's called at, if the canvas or page is not visible.</p> <p>As for how browsers <em>actually</em> handle it, I'm not sure. However, you should definitely prefer it since future browsers may be able to better optimise <code>requestAnimationFrame</code> in ways they cannot optimise <code>setInterval</code> or <code>setTimeout</code>. I think modern browsers also reduce the ordinary timers to 1 Hz if the page is not visible, but it's definitely much easier for the browser to optimise <code>requestAnimationFrame</code>, plus some browsers get you V-syncing and other niceness with it.</p> <p>So I'm not certain <code>requestAnimationFrame</code> will mean your tick function is not called if the canvas is scrolled out of view. So I'd recommend using <em>both</em> <code>requestAnimationFrame</code> and the existing visibility check. That should guarantee you the most efficient rendering.</p>
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    1. COWow, thanks so much for that super thorough answer! I've heard about `requestAnimationFrame` before, but what always bugged me about it is that it seems to aim for animation running at 60fps no matter what (which I do think looks too smooth for many kinds of animation). The only way to limit this I could think of once again included a `setInterval`.... Do you know if there is any way to restrict / alter that?
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    2. COToo smooth? I've never heard of an animation that's too smooth, what's wrong with that? :S
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    3. COMight sound weird, yet I do have a background in classical animation so I really do think at 12fps, thinking like this is part of my brain by now. My objection might be silly in 95% of the cases, yet for certain cases it would be awesome to have more control.
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