在html5中将图像颜色更改为灰度

Question

我有一个png图像,我想将其转换为灰度.我设法将其转换为灰度,但它也改变了图像透明区域的颜色.如何在不更改图像透明区域的情况下更改颜色？

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
var img = new Image();
img.src = 'image.png';
ctx.drawImage(img, 0, 0);

var imageData = ctx.getImageData(0, 0, 420, 420);
var px = imageData.data;

var len = px.length;

for (var i = 0; i < len; i+=4) {
    var redPx = px[i];
    var greenPx = px[i+1];
    var bluePx = px[i+2];
    var alphaPx = px[i+3];

    var greyScale = redPx * .3 + greenPx * .59 + bluePx * .11;

    px[i] = greyScale;
    px[i+1] = greyScale;
    px[i+2] = greyScale;
    px[i+3] = greyScale;
}

ctx.putImageData(imageData, 0, 0);

Answer 1

阿尔法是"不是一种颜色".您应该按原样复制它,使透明部分保持透明.只需删除该行:

px[i+3] = greyScale;