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Same vertex, but different UV?


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#1 Tispe   Members   -  Reputation: 879

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Posted 20 January 2014 - 01:35 AM

Hello

 

I found this old thread http://www.gamedev.net/topic/171751-texture-coordinates-and-index-buffers-can-it-be-done/ which describes a problem I wanted to know if newer DirectX versions has addressed.

 

Suppose you have a Cube, which has 8 corners. But depending on which side of the cube that is being rendered, different UVs and Normals would have to be used. The thread concludes that the vertices would have to be duplicated in the seams that use different components.

 

I immediately start thinking of Streams and how they could possibly be used for solving this issue, but I can't figure out exactly how it can be done, or if it can be done.

 

Is it possible to move UV and Normal components over to the Index buffer in some way, such that a vertex may be drawn multiple times with different UVs?



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#2 tonemgub   Members   -  Reputation: 563

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Posted 20 January 2014 - 06:16 AM

http://msdn.microsoft.com/en-us/library/windows/desktop/ff476335%28v=vs.85%29.aspx

 

You could put the vertex positions, normals and UVs each in a separate structured buffer, and the vertex buffer would simply contain indices into these three structured buffers.

Then, in the vertex or geometry shader, you simply use three structured buffers at the indices from the input vertex buffer.



#3 eppo   Crossbones+   -  Reputation: 2065

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Posted 20 January 2014 - 06:17 AM

You can indeed bind multiple vertex buffers, but you can only bind a single index buffer. You'll always read from the vertex streams with a shared index.

 

Mesh resolutions have become higher and thus smoother, resulting in less duplication needed, so in a way it has been addressed/minimized.

Compared to a few duplicated vertices, having multiple index buffers would only have higher demands on memory, because then every vertex is guaranteed to carry extra information (a fixed set of indices).



#4 BarrySkellern   Members   -  Reputation: 548

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Posted 20 January 2014 - 06:21 AM

The case of the cube is an extreme example because every vertex is on a seam. In a realistic sensibly-UV-mapped model the proportion of vertices that need to be duplicated for such a reason is small as a proportion of the total number of vertices: the vertex count is far more likely to be dominated by those internal to smoothly-varying portions of the model, which have shared positions, normals and textures.

 

Basically, under normal circumstances the overhead of duplicated vertex data at seams is unlikely to be a cause for concern in terms of performance or memory.


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#5 mhagain   Crossbones+   -  Reputation: 6332

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Posted 20 January 2014 - 01:16 PM

For the specific case of a cube, you can use instancing to achieve a similar result.  With this setup, you create a single fully-specified unit cube in one vertex buffer, then use a per-instance buffer to store the center position and size of each cube you wish to draw.  Each cube therefore only needs 4 floats (assuming it doesn't need to be rotated) and this can be done even with D3D9.

 

For D3D10+ you can specify a cube using only positions and output normals and texcoords from a geometry shader, but that's likely to be slower than the instancing method.

 

Either way a second index buffer isn't even necessary, even if it was possible.


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#6 Tispe   Members   -  Reputation: 879

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Posted 20 January 2014 - 03:12 PM


Mesh resolutions have become higher and thus smoother, resulting in less duplication needed, so in a way it has been addressed/minimized.

 

So smooth surfaces do not duplicate vertices at the same position? I figure the Normal must then be a blend of the 3 adjacent face normals?



#7 eppo   Crossbones+   -  Reputation: 2065

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Posted 20 January 2014 - 03:35 PM


So smooth surfaces do not duplicate vertices at the same position? I figure the Normal must then be a blend of the 3 adjacent face normals?

 

Yes, for a cube specifically, that would be problematic. If you tie the SV_VertexID semantic to a vertex shader input (D3D10+), you can forego vertex buffers altogether and construct a cube in its entirety in the vertex shader based of the index value that's passed in.



#8 tonemgub   Members   -  Reputation: 563

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Posted 21 January 2014 - 12:10 PM


but you can only bind a single index buffer

That's why I said he can put the indices into a vertex buffer:


and the vertex buffer would simply contain indices into these three structured buffers





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