You can use a black and white image as a mask using 'source-in' globalCompositeOperation. First you draw your mask image to the canvas, then you change the globalCompositeOperation to 'source-in', finally you draw your final image.
Your final image will only be draw where it overlay the mask.
var ctx = document.getElementById('c').getContext('2d');
ctx.drawImage(YOUR_MASK, 0, 0);
ctx.globalCompositeOperation = 'source-in';
ctx.drawImage(YOUR_IMAGE, 0 , 0);
https://developer.mozilla.org/samples/canvas-tutor...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);
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