[Algorithms] Re: Light Scissors Rect Algorithm

[Bryan C. <br...@si...>]

Hello,

I was going through the archives of the gdalgorithms list and noticed 
your post regarding scissor rectangles. I, too, am implementing Eric 
Lengyel's algorithm, but am running into some problems that I think are 
related to focal length calculations. I did have one question on your 
implementation regarding the following line:

float xa = 2 * Nza / (Nxa * viewport->cam->width);

This seems to differ a bit from equation 10.32 in the book, which is

x = Nz * e / Nx,

where e is the focal length. How does (2 / viewport->cam->width) 
correspond to the focal length, and just what does that width variable 
represent? Can you clarify how you arrived at your version? That would 
be a great help.

Thanks,
Bryan