OpenGL Lighting Quads

Question

just a quick question. I have a quad in 3D OpenGL scene. I define a normal to the plane counter clockwise. So that the normal points out one side of the plane. In the direction of my light source. The quad is light but on both sides.

Should it not only be light on one side of the quad? Or is it the fact that a primitive like a quad is finitely thin and thus looks light from both sides. So if i wanted to make a wall I would use two quads. One for each side of the wall.

Thanks

Answer 1

The default OpenGL lighting behavior for two sided polygons is to calculate lighting for the front face and apply it to both sides.

You can get around this by using a front and back polygon with seperate normals for each of your double sided polygons.

Alternatively, you can enable GL_LIGHT_MODEL_TWO_SIDE for lighting calculations using glLightModeli(GL_LIGHT_MODEL_TWO_SIDE, GL_TRUE) . See the glLightModel reference for more information.