Function subtract

pattern/canvas.js:3298–3300

(img1, img2, dx, dy, alpha)

    return blend(ADD, img1, img2, dx, dy, alpha);
}
function subtract(img1, img2, dx, dy, alpha) {
    return blend(SUBTRACT, img1, img2, dx, dy, alpha);
}
function lighten(img1, img2, dx, dy, alpha) {
    return blend(LIGHTEN, img1, img2, dx, dy, alpha);
}