Feature request: Add more composition modes.

[Evi1M4chine]

Especially ones that allow one, to see differences.
I found the following algorithm to be quite useful:

(In Imagemagick)

composite 1.bmp 2.bmp -compose Difference temp1.bmp;
convert temp1.bmp -fill red -tint 200 temp2.bmp;
composite 1.bmp temp2.bmp -compose Screen out.bmp;

Nicely tints all differences red.

Also, a simple subtraction can be useful. (Just the first operation.)

I suggest, to keep things simple, to allow above two operations, and a custom one, where you can enter a JS expression to calculate the target pixel value from the source ones. With the canvas element and its 2D context, this should be not that hard.

Plus: With Firefox 4, WebGL enters the stage. And that one already has lots of compositing functionality, which can be used. :) (Although I don’t t know about the compositing power of the 2D context.)