   Normal problem
  I am really stubled here and I was wondering if anyone had a good solution.Alright here is my problem:I have a geometry that takes a while to compute, but once computed I use display lists to rotate and zoom (its a CAD software). The problem that I am having is that I was the normals to always face user, so when I original compute I negate the normal depending on which direction it is facing. Now, here is the issue that I am having. Since all normals face the user when the geometry is computed if you pan 180 degrees everything appears screwed up. My question is, is there a way to change the normals in a display list without having to recompute the scene ?   
  You could use a vertex shader. You get to transform the normal however you want in a shader.What effect is it you are trying to achieve by flipping the normals? Two sided lighting?   ;  Maybe you mean to turn backface culling off?   ;  I am trying to achieve two sided lighting.Imagine you have a rectangle and you are staring straight down at it, now I want to have the same lighting if you stare at the back face (the light source is the "camera")   ;  I am trying to achieve two sided lighting.Imagine you have a rectangle and you are staring straight down at it, now I want to have the same lighting if you stare at the back face (the light source is the "camera")   ;  Well, if you rotate 180 degrees, you would not be looking at a backface.  When you look at a cube, the backfaces are the ones that you cannot see unless you are INSIDE of the cube.  The back of the cube's backface is the face you would see if the front was removed.  So you should only flip normals when you are able to see the faces insid the cube.Levi   ;Quote:Original post by levjsWell, if you rotate 180 degrees, you would not be looking at a backface.  When you look at a cube, the backfaces are the ones that you cannot see unless you are INSIDE of the cube.  The back of the cube's backface is the face you would see if the front was removed.  So you should only flip normals when you are able to see the faces insid the cube.LeviThat is the problem I am using a display list to display the results and make them interactive.... Reseting the normals would require me to recompute the geometry, which takes a decent amount of time.   ;  Backfaces' normals are the exact same except they are flipped.  Couldn't you use this to your advantage.  For example, instead of recomputing the whole thing, couldn't you just multiply the vectors by -1 to flip it?  That would take much less time.  Levi   ;Quote:Original post by levjsBackfaces' normals are the exact same except they are flipped.  Couldn't you use this to your advantage.  For example, instead of recomputing the whole thing, couldn't you just multiply the vectors by -1 to flip it?  That would take much less time.  LeviYes I could, but the problem is How to I get to the normal without recomputing my scene. I set the normal when I compute the geometry and then as soon as I am done it all gets dumped into a display list. My question is how do I change the normals in the display list ?;  Well if you really don't want to use a vertex shader, then you can still recreate your display list without recalculating the geometry (assuming you don't delete it as soon as the list is created). I'm sure there must be a cleverer way of doing it though.   