ShaderX Shader Programming Tips & Tricks With DirectX 9

under D3DCAPS9.DeclType, the specific bit is D3DTCAPS_datatype, and the
type to use is D3DDECLTYPE_datatype (where the data type is from the list
above).
Reclaiming Two Bits
When DEC3N or UDEC3 formats are used, we seem to have lost two bits, but
even two bits can be used quite effectively, so we want them back (e.g., if you
have per-vertex branching, you could store the number of bones here). By causing
two different vertex elements to point to the same memory in the vertex
buffer, we can get access to our two bits (this requires the overlapped stream
offset cap bit to be set).
The vertex stream declaration for a single stream if we stored normals (a common
use) as UDEC3 and wanted to reclaim our two bits is below. The vertex
shader can now bind NORMAL0 to access the data as UDEC3 and NORMAL1 as
UBYTE4.
D3DVERTEXELEMENT9 decl[] =
{
Section I — Geometry Manipulation Tricks
Using Vertex Shaders for Geometry Compression
Figure 1: Data from vertex stream element to vertex register
// first element, a ‘normal’ UDEC3 declaration
{ 0, // stream number
0, // stream offset in bytes
D3DDECLTYPE UDEC3, // vertex type for this access
D3DDECLMETHOD DEFAULT, // not used so leave at default
D3DDECLUSAGE NORMAL, // usage (used to bind in the vertex shader)
0 // usage number (you can have n normals)
},
// second element, a UBYTE4 that accesses the same memory as the normal above
{ 0, // stream number, same as first element
0, // stream offset, same as first element
D3DDECLTYPE UBYTE4, // vertex type for this access
D3DDECLMETHOD DEFAULT, // not used so leave at default
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