javascript – 在html5中将图像的颜色更改为灰度

我有一个png图像,我想转换成灰度级.我设法将其转换为灰度,但它也会改变图像透明度的颜色.如何在不改变图像的透明区域的情况下更改其颜色？

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

var imageData = ctx.getimageData(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);

解决方法

阿尔法是“不是一种颜色”.您应该将其原样复制,使透明部分透明.只需删除行：

px[i+3] = greyScale;