Introduction to Vertex Shaders - Introduction to 3D Game Programming with DirectX 9.0 - page 298

where _decl is a pointer to an IDirect3DVertexDeclaration9
interface.
17.2 Vertex Data Usages
Consider the vertex declaration:
D3DVERTEXELEMENT9 decl[] =
{
{0, 0, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_
POSITION, 0},
{0, 12, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_
NORMAL, 0},
{0, 24, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_
NORMAL, 1},
{0, 36, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_
NORMAL, 2},
D3DDECL_END()
};
We need a way to define a map from the elements of the vertex declara-
tion to the data members of the vertex shader's input structure. We
define this map in the input structure by specifying a semantic
( : usage-type[usage-index] ) for each data member. The seman-
tic identifies an element in the vertex declaration by its usage type and
usage index. The vertex element identified by a data member's seman-
tic is the element that gets mapped to that data member. For example,
an input structure for the previous vertex declaration is:
struct VS_INPUT
{
vector position : POSITION;
vector normal : NORMAL0;
vector faceNormal1 : NORMAL1;
vector faceNormal2 : NORMAL2;
};
Note: If we leave off the usage index, it implies usage index zero.
For example, POSITION is the same thing as POSITION0 .
Here element 0 in decl , identified by usage POSITION and usage
index 0, is mapped to position . Element 1 in decl , identified by
usage NORMAL and usage index 0, is mapped to normal . Element 2 in
decl , identified by usage NORMAL and usage index 1, is mapped to
faceNormal1 . Element 3 in decl , identified by usage NORMAL and
usage index 2, is mapped to faceNormal2 .