js 装换灰度图

grayscale.js

var canvasData = context.getImageData(0, 0, canvas.width, canvas.height);
// 转换灰度图
for (var x = 0; x < canvasData.width; x++) {
    for (var y = 0; y < canvasData.height; y++) {

        // Index of the pixel in the array  
        var idx = (x + y * canvasData.width) * 4;
        var r = canvasData.data[idx + 0];
        var g = canvasData.data[idx + 1];
        var b = canvasData.data[idx + 2];
        var gray = Math.round(.299 * r + .587 * g + .114 * b);

        // assign gray scale value  
        canvasData.data[idx + 0] = gray; // Red channel  
        canvasData.data[idx + 1] = gray; // Green channel  
        canvasData.data[idx + 2] = gray; // Blue channel  
        canvasData.data[idx + 3] = 255; // Alpha channel  
    }
}
context.putImageData(canvasData, 0, 0);