Culling...

[Sylus101]

Okay, I know I'm saturating the forum with 3D questions, but hopefully at least one other person out there is picking up some valuable information at the same time.

I understand what Front and Back Culling do, but is there any advantage to using it? I understand the correct order (counter clockwise) of vertex creation, so if I turn on Back Culling and have an enclosed object, it looks the same whether I'd used Back Culling or not... so is there a good reason to use it i.e. using less memory somehow?

[elhobbs]

culled triangles do not count towards the ~2000 trangle limit.

[Sylus101]

I'm sure I'm missing something... my understanding was that there was a 2048 triangle limit... (~1500 quads).

So... if I cull the back side and start making a ton of triangles I can just keep on going without limit? I'm sure I'm missing something...

Is it that the front and back both count as separate polygons perhaps and culling could reduce the used amount?

[Discostew]

I'm sure I'm missing something... my understanding was that there was a 2048 triangle limit... (~1500 quads).

So... if I cull the back side and start making a ton of triangles I can just keep on going without limit? I'm sure I'm missing something...

Is it that the front and back both count as separate polygons perhaps and culling could reduce the used amount?

That's why he said ~2000.
With culling of the back, the only time you'll see the polygon is if it's normal is pointed towards the camera, even if it barely is. Should it be pointing away from the camera with this culling on (for which you'd be seeing the back side of the polygon), the hardware will prevent the polygon from being loaded into the buffer.

If you have back culling on, and you try to load 1000 polygons that are pointing away from the camera, none of those 1000 will get loaded, and therefore, won't affect the remaining number you can use for the frame.

[Sylus101]

Okay, the lightbulb just went off. If it weren't for the quote I'd edit out what seems like such a stupid question now...

Excellent, so in practice, for any enclosed 3D object you should pretty much always have Back Culling turned on.(?)

[elhobbs]

backface culling is nice but keep in mind you are still sending the vertex data (maybe color and normals too) to the 3d hardware - which is not free, moving data takes time and the 3 pipeline still needs to process it to determine if it should be culled. so you will need to still do some software visibility checking.

[Sylus101]

Well, it just goes to show that attempting this without knowing any OpenGL to start with is definitely a learning experience... the forefront drawing and matrix manipulation is getting very simple, but understanding fully the stages the hardware is taking is still something I'm trying to learn. I understand the idea of the double buffer but the rest is taking a bit more time to sink in.