html5 canvas使用图像作为蒙版

Question

是否可以将具有形状的图像用作画布内整个画布或图像的蒙版？

我想将图像置于画布上，并在画布上放置图像，然后将其保存为新图像。

Answer 1

您可以使用'source-in'globalCompositeOperation将黑白图像用作蒙版。首先将遮罩图像绘制到画布上，然后将globalCompositeOperation更改为“源代码”，最后绘制最终图像。

您的最终图像只能在覆盖面具的地方绘制。

var ctx = document.getElementById('c').getContext('2d'); 

ctx.drawImage(YOUR_MASK, 0, 0); 
ctx.globalCompositeOperation = 'source-in'; 
ctx.drawImage(YOUR_IMAGE, 0 , 0); 

More info on global composite operations

Answer 2

除了皮埃尔的回答，你也可以使用一个黑白图像作为图像掩码源通过复制其数据到CanvasPixelArray，如：

var 
dimensions = {width: XXX, height: XXX}, //your dimensions 
imageObj = document.getElementById('#image'), //select image for RGB 
maskObj = document.getElementById('#mask'), //select B/W-mask 
image = imageObj.getImageData(0, 0, dimensions.width, dimensions.height), 
alphaData = maskObj.getImageData(0, 0, dimensions.width, dimensions.height).data; //this is a canvas pixel array 

for (var i = 3, len = image.data.length; i < len; i = i + 4) { 

    image.data[i] = alphaData[i-1]; //copies blue channel of BW mask into A channel of the image 

} 

//displayCtx is the 2d drawing context of your canvas 
displayCtx.putImageData(image, 0, 0, 0, 0, dimensions.width, dimensions.height);