html5 canvas use image as mask

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); 

More info on global composite operations