[vtkusers] Re: API

Amy Henderson amy.henderson at kitware.com
Thu Jan 13 16:13:29 EST 2005

At 03:59 PM 1/13/2005, Eckhoff, Michael A wrote:
>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.

No, in vtkColorTransferFunction::AddRGBPoint, the first parameter, x, is 
the scalar value for which the specified RGB color should be used.

>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.
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