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Modifying Vertex Stream Frequency

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A DirectX 9.0 version driver for a device that supports vertex shader version 3.0 and later must implement vertex stream frequency division. For version 2.0 and earlier models of vertex shader (including fixed function), the vertex shader is called once per vertex; for each call, the input vertex registers are initialized with unique vertex elements from the vertex streams. However, using vertex stream frequency division, the vertex shader (3.0 and later) can be called to initialize applicable input registers at a less frequent rate.

When an application calls the IDirect3DDevice9::SetStreamSourceFreq method to set the frequency for a given stream, the DirectX 9.0 runtime in turn calls the driver's D3dDrawPrimitives2 function using the D3DDP2OP_SETSTREAMSOURCEFREQ operation code.

After the stream's frequency divisor is set--for example, to 2, then the driver must fetch data from the stream and pass this data into applicable input vertex registers every 2 vertices. This divisor affects each element in the stream.

The driver uses this divisor to compute the vertex offset into the vertex buffer according to the following formula:

VertexOffset = VertexIndex / Divider * StreamStride + StreamOffset

For each vertex stream used, if the driver receives a start-vertex value during a call to the driver's D3dDrawPrimitives2 function using the D3DDP2OP_DRAWPRIMITIVE operation code, the driver also divides this start-vertex value by the frequency divisor and factors the result in the formula. This start-vertex value is provided in the VStart member of the D3DHAL_DP2DRAWPRIMITIVE structure. The following formula factors in the start-vertex value:

VertexOffset = StartVertex / Divider + 
               VertexIndex / Divider * StreamStride + StreamOffset

Note that the preceding formulas use integer division.

The application passes the D3DSBT_VERTEXSTATE state type in a call to the IDirect3DDevice9::CreateStateBlock method to capture the current vertex state.

The driver ignores the setting of a stream's frequency divisor either for indexed primitives or if the driver only supports a vertex shader model that is earlier than version 3.0 (including fixed function).

For more information about IDirect3DDeviceXxx::SetStreamSourceFreq and IDirect3DDeviceXxx::CreateStateBlock, see the latest DirectX SDK documentation.