了解画布如何将图像转换为黑白

Can anyone explain in a little more detail what is happening here:

function grayscale(src){ //Creates a canvas element with a grayscale version of the color image
    var canvas = document.createElement('canvas');
    var ctx = canvas.getContext('2d');
    var imgObj = new Image();
    imgObj.src = src;
    canvas.width = imgObj.width;
    canvas.height = imgObj.height; 
    ctx.drawImage(imgObj, 0, 0); //Are these CTX functions documented somewhere where I can see what parameters they require / what those parameters mean?
    var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
    for(var y = 0; y < imgPixels.height; y++){
        for(var x = 0; x < imgPixels.width; x++){
            var i = (y * 4) * imgPixels.width + x * 4; //Why is this multiplied by 4?
            var avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3; //Is this getting the average of the values of each channel R G and B, and converting them to BW(?)
            imgPixels.data[i] = avg; 
            imgPixels.data[i + 1] = avg; 
            imgPixels.data[i + 2] = avg;
        }
    }
    ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height); 
    return canvas.toDataURL();
}

推荐答案

1. canvas 函数与大多数函数一样，在 中进行了描述官方规范.此外，MDC 对更多非正式"文章很有帮助.例如.MDC 上的 drawImage 函数在 这里.