Partial Re-rendering

JavaScript performance comparison

Test case created by Boris and last updated

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, 0, canvas.width, canvas.height);
  context.fillStyle = 'white';
  context.fillRect(i, HEIGHT, 1, 1);
}
pending…

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