I have been playing with tutorial six and building a basic solar system simulator with texture mapped planets. I am running Windows 2000 with a Matrox G400 graphics card. I am using the GL_MODULATE mode to display my textured spheres with a single light source and I notice that when an object is partially off the screen some (not all) of the facets of the sphere are incorrectly lit - it looks as though they are being lit with diffuse or ambient light but not both. Problem does not occur in GL_DECAL mode. I am using mip-mapped textures but this seems to have no bearing on the issue. I have tried playing with the material properties of the spheres but this does not seem to eliminate the effect. I am using display lists to render the spheres. The problem also occurs on Windows NT. Any ideas? Is this a bug in the Windows implementation of OpenGL or is there some way to set material properties of the textured polygons that resolves this?