// Combine two images: a*x + b*y. If you set a within [0,1] and b=1-a,
// you will get a fade; if not, you may get surprising results (consider alpha).
// Combine two images: a*x + b*y. If you set a within [0,1] and b=1-a,
// you will get a fade; if not, you may get surprising results (consider alpha).