dcl_usage (Vertex Shader)

Declare the association between a vertex element usage and usage index, to a vertex shader position register.

Syntax

Where:

• _usage identifies how the register data will be used. This is the same value as the members of D3DDECLUSAGE without the "D3DDECLUSAGE" prefix.
• [usage_index] is an optional integer index between 0 and 15. It modifies the usage data. The index matches the usage index in a vertex declaration. See Decoupling Vertex Shaders and Vertex Declarations. The index is appended to the usage value with no space. If it is not supplied, it is assumed to be 0.
• dest is a destination register. A position register (v#) is a valid destination register in all vertex shader versions. See Position Registers.

  Version vs_3_0 supports output registers (o#) as valid destination registers. See Output Registers.

  Version vs_3_0 must have at least one o register with _position0 usage when not used for process-vertices. The _position0 and _psize0 usages are the only usages that have a meaning that extends beyond linkage from vertex shader outputs to pixel shader inputs. For shaders with flow control, it is assumed that the worst case output is declared. There are no defaults if a shader does not actually output what it declares it should (due to flow control).

• [.mask] is an optional write mask that controls which components of the destination register that can be written to. See Destination Register Masking.

  The write mask option is supported in versions 2_x and later.

  For version vs_3_0, the same o register may be declared multiple times (so different usages can be applied to individual components), each time with a unique write mask. However, the same usage may not be used multiple times in a declaration. This means that vectors must be 4 components or less, and cannot go across 4-component register boundaries (individual o registers). When the _psize usage is specified, it must have a full write mask since it is considered a scalar. When the _position usage is specified, it must have a full write mask since all 4 components have to be written.

Remarks

Vertex shader versions	1_1	2_0	2_x	2_sw	3_0	3_sw
dcl_usage	x	x	x	x

All dcl_usage instructions must appear before the first executable instruction.