Is it possible to use a image with a shape as a mask for a whole canvas or images within the canvas?

I want to place images in a canvas with a mask over the images, and then save it as a new image.

Is it possible to use a image with a shape as a mask for a whole canvas or images within the canvas?

I want to place images in a canvas with a mask over the images, and then save it as a new image.

• javascript
• html
• canvas

• yes, if there is really some transparent area on the mask image you could easily use drawImage() to draw the image on top of the canvas. The transparent part will let the underlying image/canvas shine through. – devnull69 Commented Oct 1, 2012 at 13:02
• 1 That's not what i mean, i want the shape to clip the image, so that the rest of the image becomes transparent. – Jaap Commented Oct 1, 2012 at 13:11

2 Answers 2

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

In addition to Pierre's answer you can also use a black and white image as a mask source for your image by copying its data into a CanvasPixelArray like:

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);