Mapping between a Direct3D 9 Declaration and a Direct3D 8 Declaration

This table maps members of a D3DVERTEXELEMENT9 declaration to a Direct3D 8 declaration.

Direct3D 9 Usage	Direct3D 9 Usage index	Direct3D 8
D3DDECLUSAGE_POSITION	0	D3DVSDE_POSITION
D3DDECLUSAGE_POSITION	1	D3DVSDE_POSITION2
D3DDECLUSAGE_NORMAL	0	D3DVSDE_NORMAL
D3DDECLUSAGE_NORMAL	1	D3DVSDE_NORMAL2
D3DDECLUSAGE_BLENDWEIGHT	0	D3DVSDE_BLENDWEIGHT
D3DDECLUSAGE_BLENDINDICES	0	D3DVSDE_BLENDINDICES
D3DDECLUSAGE_PSIZE	0	D3DVSDE_PSIZE
D3DDECLUSAGE_COLOR	0	D3DVSDE_DIFFUSE
D3DDECLUSAGE_COLOR	1	D3DVSDE_SPECULAR
D3DDECLUSAGE_TEXCOORD	n	D3DVSDE_TEXCOORDn

When a declaration is used with hardware vertex processing on a Direct3D 7 driver, the Direct3D runtime converts it to a FVF with the following rules:

Only stream 0 should be used (evident from the MaxStreams cap).
The order of vertex elements should be the same as order of FVF bits.
Gaps in texture coordinates are not allowed.
Any vertex element not described the table cannot be converted into a valid FVF for all pre-DirectX 8 drivers and, hence, cannot be used on those drivers.
Only D3DDECLTYPE_FLOAT2 is allowed for vertex elements with D3DDECLUSAGE_TEXCOORD if device does not set either of the D3DPTEXTURECAPS_PROJECTED or D3DPTEXTURECAPS_CUBEMAP caps.