[Cin] User Defined Blend Algebra

[Andrea paz]

Great, everything works perfectly even with this third patch. For
example I had already done a test with my Subtracts function, where I
put alpha=1, and it comes back the same as your new arith_subtract.ba.
Reading your explanations makes it clear how many cases your plugins
can be applied to. You really did a great job, thank you.


I was thinking of adding the following note to the manual, in the
chapter on overlays. Do you think it is correct or should it be made
more precise?

"OVERLAY
Remember that when making the various blends of 2 tracks you should
always consider:
1- The colors of the 2 tracks that will be blended. We often consider
only these channels to guess the final result.
2- The Alpha channel of the 2 tracks that can interact with each other
in ways that are intuitively (but not mathematically) unexpected.
3- The choice of Source track and destination track (top or bottom).
4- The presence of a third black background track (or even the color
of the canvas, which in CinGG is black by default but whose color can
be varied manually), which can interfere with viewing the blend in the
Compositor and show unexpected results.

Ultimately, if we get a result that looks wrong to us, we need to look
at the formula for that blend and calculate our mathematical result.
Then compare our calculation with the result we see on the screen."