Ok, this is a follow-up to http://www.experts-exchange.com/Web/Web_Languages/PHP/Q_21165330.html
which was about using masks.
I have a white(ish) BG and a grayscale sketch. Currently, I'm able to put the sketch onto the page, using a mask to define an outline. This works well enough on a white BG but if I wanted to use, say, a light blue BG, the BG would be blue but the sketch would still appear to be on "white" so the edge of the mask would be visible.
What I really need is: To take the "whiteness" of the sketch to mean the % alpha blend, so the white bits are transparent and the black bits are opaque and the bits in the middle are halway in-between.
Is it possible and if so, how can it be done?