Partial Re-rendering

JavaScript performance comparison

Revision 2 of this test case created by Boris

Info

How much faster is it to render just the parts of the canvas that changed, compared to the whole thing?

Rendering a white dot that moves across a black background.

Preparation code

<canvas id="c" width="640" height="480"></canvas>
<script>
  var canvas = document.getElementById('c');
  var context = canvas.getContext('2d');
 
  var HEIGHT = 100;
</script>

Preparation code output

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Testing in unknown unknown
Test Ops/sec
re-render whole scene
for (var i = 0; i < canvas.width; i++) {
  context.fillStyle = '#000';
  context.fillRect(0, 0, canvas.width, canvas.height);
  context.fillStyle = '#fff';
  context.fillRect(i, HEIGHT, 1, 1);
}
pending…
only render differences
for (var i = 0; i < canvas.width; i++) {
  context.fillStyle = 'black';
  context.fillRect(i-1, HEIGHT, 1, 1);
  context.fillStyle = 'white';
  context.fillRect(i, HEIGHT, 1, 1);
}
pending…

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2 comments

Brennan commented :

Is there a way to get a real life example with this in use?

Bonsaiheld commented :

Hm... Very nice. The differences are HUGE. I need this badly for my game.

Question: So this works with pixels... How to do it with sprite images? I think i did it wrong. When i make each player in my game itself clearing its surrounding rectangle, then - when two players overlap each other - the one player cuts off the other inside its rectangle. :/

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