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Reverse Lighting

Started by questors_endgame Oct 12, 2006 at 11:58 AM 2 replies 400+ views

Original Post

Oct 12, 2006 at 11:58 AM

What causes lighting to light the wrong side of a mesh, so the back of a cube is lit when light shines on the front. I can't flip the normals because the faces will not be visible from the correct side then. This is with opengl, I don't know many more details, it's just drawing triangles from indices. This seems partly related to flipping the mesh horizontally, if that is happening how would one flip it but not get messed up normals or lighting?

Zakwayda

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Zakwayda

Oct 12, 2006 at 12:17 PM

If the normals are pointing the 'wrong way', then flip them. If that results in the wrong side of the faces being visible, then reverse the winding of the faces, or change OpenGL's cull mode or winding order.

[Edit: My wording was poor (see Kalidor's post below). What I meant was that if the initial flipping of the mesh results in incorrect face culling, you can correct this by changing the winding order or cull mode.]

[Edited by - jyk on October 12, 2006 12:17:23 PM]

Kalidor

Oct 12, 2006 at 12:33 PM

Normals have no effect on face culling. They are just like any other per-vertex data. Since they are per-vertex, that means a face doesn't have a single corresponding normal.

Imagine you have a screen-aligned quad and the top two vertices have normals which point out of the screen and the bottom two vertices' normals point into the screen. If normals affected face culling, would the quad be visible or invisible? Would half of it be visible?

questors_endgame

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questors_endgame

Oct 12, 2006 at 12:41 PM

I don't understand about normals having no effect. Winding of the faces sounds like it may be the issue, can someone explain how to do this and why winding order matters?

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