Stencil Buffer Read/Write mask.

Question

In Direct3D10 the Stencil Read/Write mask is a byte (from 0x00 to 0xFF)

In Direct3D9 the Stencil Read/Write mask is a int (from 0x00000000 to 0xFFFFFFFF)

The question is :

How the stencil read/write mask in Direct3D10 relate to the Direct3D9 one?

Direct3D10 | 0x00FFFFFF or Direct3D10 | 0xFFFFFF00 ?

And another question :

Why the Direct3D9 one is a 32 bit integer when the stencil buffer can be max 8 bit? o.O

Thanks.

Answer 1

Direct3D10 | 0xFFFFFF00

The least significant bits are the relevant ones in D3D9, the docs describe the stencil operations in terms of DWORDs but ultimately the stencil buffer only stores a single byte so it is only the least significant byte of the mask that is important.

The reason D3D9 uses a DWORD is that the value is set through SetRenderState which takes two parameters, a D3DRENDERSTATETYPE enum specifying the state to change and a DWORD value. All render states must therefore use a DWORD value regardless of how they are ultimately used. In some cases this means doing a reinterpret_cast on a floating point number. For the stencil mask it means passing a 32 bit value where only the least significant 8 bits are really needed. D3D10 sets states through typed structures and so avoids this issue.