Canvas 颜色反转

ImageData中的元素反转颜色 255-data[i,i+1,i+2]
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>converse color</title>
</head>

<body>
    <canvas id='myCanvas' width='800' height='600'>
        your browser does not support canvas
    </canvas>
    <script type="text/javascript">
    var c = document.getElementById('myCanvas');
    var ctx = c.getContext('2d');
    var img = new Image();
    img.src = 'face.jpg';
    img.onload = function() {
        ctx.drawImage(img, 0, 0);
        var imgData = ctx.getImageData(0, 0, 250, 250);
        var pixels = imgData.data;
        for (var i = 0, n = pixels.length; i < n; i += 4) {
            pixels[i] = 255 - pixels[i];
            pixels[i + 1] = 255 - pixels[i + 1];
            pixels[i + 2] = 255 - pixels[i + 2];
        }
        ctx.putImageData(imgData, 250, 0);
    };
    </script>
</body>

</html>