gfx_procrect's mysterious 768-entry table

[papagirafe]

Hi dear video processor gurus!

Nowadays I am trying one by one the non-obvious gfx functions and today's winner is gfx_procrect()! Apart from the fact that 768 is 3x256 and that the table seems to be composed of 3 groups 256 0/1 values, I do not know how to populate this table with useful values. It modifies colors but does it also allow to alter the alpha values?

Also the gfx_evalrect() seems to use a similar "table" logic but again the "eval code" is a pure mystery for me.

Thanks in advance!