[Gtkradiant] [Bug 629] Bumpmapping: proper tangent vector code

[gtkradiant@zerowing.idsoftware.com]

http://zerowing.idsoftware.com/bugzilla/show_bug.cgi?id=629

ydnar@shaderlab.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED



------- Additional Comments From ydnar@shaderlab.com  2002-10-20 18:38 -------
http://shaderlab.com/q3map2/screenshots/bumpmapping_works.jpg

Calculated per-vertex, per-triangle. Haven't seen the formula I used documented 
anywhere, so I'll do so here.

For every triangle, every vertex, you need to compute 2 tangent vectors in 
order for normalmapping to work correctly. They translate from ST shifts to 
world space.

Take vertex ST coordinate. Shift by desired amount (texture space) Calculate 
barycentric coordinates of new ST coordinate. Use the barycentric coordinates 
(triangle space) to calculate XYZ coordinate (world space). Subtract original 
vertex XYZ coordinate and normalize.

Todo:
- Deal with phong shading a bit better, as current tangent vectors lie in each 
triangle's plane, rather than being perpendicular to the interpolated normal at 
the sample point

- Interpolate the modified tangent vectors when subdividing the triangle