Shader data flow questions

1. Is there any decent way to get a small chunk of data (4-8

bytes) from a vertex shader back to the CPU? I would only

do this in direct response to a mouse click, so it need not be fast,

just possible.

2. The MSDN reference materialhere would suggest that a variable declaredstatic

could carry information from one shader invocation to the next, so that

one could, for example, calculate a running min, max, or average of

some property over all vertices. But in practice it seems that my

static variable is zeroed afresh for every vertex.

Below are the bones of my test case. The CPU program is providing

each vertex with the "extra" variable, of which extra.x is a unique id

for each vertex, starting with 0.0 for the first, counting by 1.0 each

time. This information is demonstrably getting through.

However, reading the documentation would suggest that the line labelled

"HERE'S THE ACTION" should give me all vertices red. In fact, I

get only one red vertex, namely the first, suggesting that pickd is NOT

retaining a value.

Am I missing something really obvious? Should I be reading a

different reference? Is there one document somewhere that is The

Definitive Reference for HLSL?

[using DirectX 9 (June SDK), 6800

Ultra graphics card, Windows XP Pro, using D3DXCreateEffectFromFile to

set up the shader.]
uniform extern float4x4 gWVP;

OutputVS MyDemoVS(float3 posL : POSITION0, float2 extra : TEXCOORD0) {

static float pickd = 0;

if (extra.x == 0) pickd = 100; // reset picking algorithm, once per frame

OutputVS outVS = (OutputVS)0;

outVS.color.y = extra.x / 4000.0; // visually demonstrates that the ID is getting through

// Transform to homogeneous clip space:

outVS.posH = mul(float4(posL, 1.0f), gWVP);

if (pickd == 100) outVS.color = red; // HERE'S THE ACTION

return outVS;

}

[2128 byte] By [Sparklight] at [2007-12-25]