Patrice,
I understand and acknowledge your pain, blood, sweat, and tears. But please don't fall into that trap of cheap double-sided lighting. Otherwise we are going to end up with yet more of those good-for-nothing half-baked models that are so abundant on the net.
Most of the models you rework were designed with only one goal in mind: be rendered just once or twice for a few bucks and not in an OpenGL viewer but rather in a turtle-slow physically based ray tracer that has no notion of polygon surface normals and other fast mathematical approximations. It is based on real world laws of light emission, radiance, scattering, etc. where z-buffer fighting, unlit backfacing geometry and other OpenGL/Direct3D simplification artifacts just don't exist.
1. Automatic two-sided lighting is extremely strenuous for OpenGL. Look at the pictures below; they were taken on a box with just
one of my older nVidia GTX
550Ti w/
1GB of VRAM installed. While such a video card is still quite competitive, rendering the Event Horizon model lit from both sides is practically unbearable for it even in the FFP mode.
2. GL_LIGHT_MODEL_TWO_SIDE == GL_TRUE
isn't supported in PPL. The shader must explicitly implement additional light sources that would light the model from the opposite side(s).
In other words, I'm afraid your "pangs of creativity" would be a lesser sacrifice to the project than the programmatic PITA and model quality losses we are going to face if we follow your suggestion, however understandable your motivation might be.
