Function mask

pattern/canvas.js:3206–3218

(img1, img2, dx, dy, alpha)

}

function mask(img1, img2, dx, dy, alpha) {
    /* Applies the second image as an alpha mask to the first image.
     * The second image must be a grayscale image, where the black areas
     * make the first image transparent (e.g. punch holes in it).
     * - dx: horizontal offset (in pixels) of the blend layer.
     * - dy: vertical offset (in pixels) of the blend layer.
     */
    alpha = (alpha || alpha == 0)? alpha : 1;
    return composite(img1, img2, dx, dy, function(p1, p2) {
        p1[3] = p1[3] * p2[0]/255 * p2[3]/255 * alpha;
        return p1;
    });
}

var ADD       = "add";       // Pixels are added.
var SUBTRACT  = "subtract";  // Pixels are subtracted.