Question

Would someone provide/point me to an explanation of or tutorial on using multiple vertex streams in HLSL and XNA? I’m interested in how they’re stored/accessed by the GPU, advantages of or uses for streams in custom shaders, etc.

I’ve seen a few examples on using multiple vertex streams for instanced geometry, but I’m having a hard time wrapping my head around the underlying mechanism.

Update

If I have a vertex shader which accepts two parameters (borrowed from this tutorial)

InstancingVSoutput InstancingVS(InstancingVSinput input, float4x4 instanceTransform : TEXCOORD0, float4 color : TEXCOORD4)
{
 InstancingVSoutput output;
 float4 pos = input.Position;
 pos = mul(pos, transpose(instanceTransform));
 pos = mul(pos, WVP);
 output.Position = pos;
 output.Color = color;
 return output;
}

It seems from the example that I pulled this from that instanceTransform and input are pulled from separate streams. However, in this case, the input stream is a list of six vertices, and the instanceTransform comes from a stream of a much larger number of elements, consisting of translation matrices. This is supposed to be used for instanced geometry.

I’m confused about how many times this shader gets executed – is it VertexBuffer0.VertexCount*VertexBuffer1.VertexCount? The problem with this kind of thing is that, once someone’s figured it out, they don’t bother contributing a well-written document back to the community detailing their discovery.

Argh.

Answer 1

Since no one else has chimed in yet, I’ll give it a go 🙂 this is a great thread on the apphub forums about Vertex Streams:
http://forums.create.msdn.com/forums/p/46229/276901.aspx

from one of the answers:

The gist is this: different streams
can have different data layouts, and
your VertexDeclaration determines what
data gets pulled from what stream.
So, for instance, you could have one
buffer that stores all your positions
and one buffer which stores all your
colors, and you could set those to
different streams; alternatively you
could munge them into a single stream,
but this isn’t always convenient.

Hope it helps 😉