Rendering both faces with different materials

[James W. <ja...@fr...>]

Some 3D formats allow applying different materials to the front and back 
faces of materials, and I was thinking about how to do that with Quesa. 
   Preferably, I'd want to do it without duplicating the geometry, to 
save memory.  At first I thought that backface culling plus orientation 
styles would do the trick:

Counterclockwise orientation + backfacing-remove means that we define 
the counterclockwise faces to the the front, and cull out the clockwise 
ones.  Clockwise orientation + backfacing-remove means that we define 
the clockwise faces to the the front, and cull out the counterclockwise 
ones.

However, we need to take the vertex normals into account.  If the 
normals were created to face toward the counterclockwise face, then when 
rendering the clockwise face, we need to flip the normals.  Currently, 
Quesa will only flip normals (OpenGL two-sided lighting) when it's not 
doing backface culling.

What I'm considering is adding a new backfacing style, call it 
kQ3BackfacingStyleRemoveFront, which will cull out the front faces 
(where "front" respects the orientation style), but also use two-sided 
lighting.  With this addition, one could render two faces with a 
structure like this:

Group
   front attribute set
   backfacing style remove
   geometry
Group
   back attribute set
   backfacing style remove-front
   another reference to the same geometry

Does that make sense?
-- 
   James W. Walker, Innoventive Software LLC
   <http://www.frameforge3d.com/>