[vtkusers] Re: API

[Eckhoff, Michael A]

I believe this x component refers to the alpha channel, which determines
the opaqueness (or transparency) of an object. Rather than RGB space
(the unit cube I^3), one now has RGBA space I^4.

This is a standard part of computer graphics. 
See section 4.5 ("Transparency, Alpha, and Compositing") of the book
Real-Time Rendering, by Moller and Haines (1999); in particular, look
at the concept of "blending", which sounds a lot like what Jens Fisseler
told you.

> c = (x - x')/(x'' - x') * c'' + (x'' - x)/(x'' - x') * c'

Here, c' and c'' are two interpolating alphas, so that c is a
weighted average.

Mike Eckhoff

PS The 2nd edition of Real-Time Rendering (2002) is supposedly very good.