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use index buffer to draw a cube, how can each face be one color


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#1 tcige   Members   -  Reputation: 206

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Posted 12 April 2013 - 04:27 AM

or in this situation, must draw 12 triangles



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#2 turch   Members   -  Reputation: 590

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Posted 12 April 2013 - 06:07 AM

You'll have to draw 12 triangles either way. Just instead of using 8 vertices and reusing them for each face, you have 3 unique verts per face with the color.  This also applies to normals, u/v, etc - if two adjacent faces share a vertex, but have different normals, you'll have to split it into two vertices.


Edited by turch, 12 April 2013 - 06:09 AM.


#3 tcige   Members   -  Reputation: 206

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Posted 12 April 2013 - 06:33 AM

You'll have to draw 12 triangles either way. Just instead of using 8 vertices and reusing them for each face, you have 3 unique verts per face with the color.  This also applies to normals, u/v, etc - if two adjacent faces share a vertex, but have different normals, you'll have to split it into two vertices.

 

thanks, so what's the use of index buffer?



#4 kauna   Crossbones+   -  Reputation: 2338

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Posted 12 April 2013 - 07:09 AM

Depending on the needs of your cube you either draw 8 unique or 24 unique vertices. At the first case you must use an index buffer in order to re-use each vertex.

 

The more typical scenario is when you draw 12 triangles with 24 (4 per side) unique vertices (now each side can have different normal and/or texture coordinates). You can draw a non-indexed triangle list by putting 12 * 3 unique vertices in the vertex buffer but you can see that it has 36/24 times more data than it is required. So with the use of index buffer you'll be able to draw the cube with 24 vertices and 36 indices AND the GPU is able to use a post-transform cache for not to execute vertex shader more than required.

 

A cube is so simple an object that in typical scenario you won't notice much of difference with the drawing performance. 

 

Cheers!



#5 tcige   Members   -  Reputation: 206

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Posted 12 April 2013 - 08:28 AM

Depending on the needs of your cube you either draw 8 unique or 24 unique vertices. At the first case you must use an index buffer in order to re-use each vertex.

 

The more typical scenario is when you draw 12 triangles with 24 (4 per side) unique vertices (now each side can have different normal and/or texture coordinates). You can draw a non-indexed triangle list by putting 12 * 3 unique vertices in the vertex buffer but you can see that it has 36/24 times more data than it is required. So with the use of index buffer you'll be able to draw the cube with 24 vertices and 36 indices AND the GPU is able to use a post-transform cache for not to execute vertex shader more than required.

 

A cube is so simple an object that in typical scenario you won't notice much of difference with the drawing performance. 

 

Cheers!

 

thanks a lot

 

i have tried and it works, besides a bit more complicated setting vertices and indices

 

but the performance will increase almost 1/3 and this is the key of index buffer?



#6 kauna   Crossbones+   -  Reputation: 2338

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Posted 12 April 2013 - 09:07 AM

The amount of data is reduced by 1/3 and there is some slight possibilities to use post-transform cache, so it is more optimal solution. Probably the drawing is still more bound by the pixel shader, as the vertex transform cost is quite insignifican in this case. I guess that when drawing some hundred or thousand or so cubes you'll start to notice the difference. 

 

Cheers!



#7 Matias Goldberg   Crossbones+   -  Reputation: 3149

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Posted 12 April 2013 - 09:38 PM

You may be interested in a blog post I made a while back regarding how to draw a cube






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