Canvas Pixel Manipulation

JavaScript performance comparison

Revision 6 of this test case created by Andrew J. Baker

Info

Tests two different methods of manipulating pixels using the canvas.

NOTE: Now takes endianness into account.

Preparation code

<canvas id="canvas" height="256" width="256"></canvas>
<script>
Benchmark.prototype.setup = function() {
    var canvas = document.getElementById('canvas');
    var canvasWidth  = canvas.width;
    var canvasHeight = canvas.height;
    var ctx = canvas.getContext('2d');
    var imageData = ctx.getImageData(0, 0, canvasWidth, canvasHeight);
   
    var data = imageData.data;
   
    var buf = new ArrayBuffer(imageData.data.length);
    var buf8 = new Uint8ClampedArray(buf);
    var data32 = new Uint32Array(buf);
   
    // Determine whether Uint32 is little- or big-endian.
    data32[1] = 0x0a0b0c0d;
   
    var isLittleEndian = true;
    if (buf[4] === 0x0a && buf[5] === 0x0b && buf[6] === 0x0c &&
        buf[7] === 0x0d) {
        isLittleEndian = false;
    }
};

Benchmark.prototype.teardown = function() {
    ctx.putImageData(imageData, 0, 0);
};
</script>

Preparation code output

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Testing in unknown unknown
Test Ops/sec
8-bit Pixel Manipulation
for (var y = 0; y < canvasHeight; ++y) {
    for (var x = 0; x < canvasWidth; ++x) {
        var index = (y * canvasWidth + x) * 4;

        var value = x * y & 0xff;

        data[index]   = value;    // red
        data[++index] = value;    // green
        data[++index] = value;    // blue
        data[++index] = 255;      // alpha
    }
}
pending…
32-bit Pixel Manipulation
if (isLittleEndian) {
    for (var y = 0; y < canvasHeight; ++y) {
        for (var x = 0; x < canvasWidth; ++x) {
            var value = x * y & 0xff;

            data32[y * canvasWidth + x] =
                (255   << 24) |    // alpha
                (value << 16) |    // blue
                (value <<  8) |    // green
                 value;            // red
        }
    }
} else {
    for (y = 0; y < canvasHeight; ++y) {
        for (x = 0; x < canvasWidth; ++x) {
            value = x * y & 0xff;

            data32[y * canvasWidth + x] =
                (value << 24) |    // red
                (value << 16) |    // green
                (value <<  8) |    // blue
                 255;              // alpha
        }
    }
}

imageData.data.set(buf8);
pending…

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2 comments

CCalvez commented :

According to my tests, 8-bit Pixel Manipulation is ~50% slower than 32-bit Pixel Manipulation, both on FF8 and 11. And FF8 is is ~50% slower than FF11. But my figures are way lower than those shown in the graphs (972 & 2351 in FF1)

mantas commented :

Hi Andrew. Can you write me to email magimantas@yahoo.com? I want to talk more about browser based online games on canvas.

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