Shader Constants (HLSL)
In Shader Model 4, shader constants are stored in one or more buffer resources in memory. They can be organized into two types of buffers: constant buffers (cbuffers) and texture buffers (tbuffers). Constant buffers are optimized for constant-variable usage, which is characterized by lower-latency access and more frequent update from the CPU. For this reason, additional size, layout, and access restrictions apply to these resources. Texture buffers are accessed like textures and perform better for arbitrarily indexed data. Regardless of which type of resource you use, there is no limit to the number of constant buffers or texture buffers an application can create.
Declaring a constant buffer or a texture buffer looks very much like a structure declaration in C, with the addition of the register and packoffset keywords for manually assigning registers or packing data.
BufferType Name [: register(b#)] { VariableDeclaration [: packoffset(c#.xyzw)]; ... };
Parameters
-
BufferType
-
[in] The buffer type.
BufferType Description cbuffer constant buffer tbuffer texture buffer -
Name
-
[in] ASCII string containing a unique buffer name.
-
register(b#)
-
[in] Optional keyword, used to manually pack constant data. Constants can be packed in a register only in a constant buffer, where the starting register is given by the register number (#).
-
VariableDeclaration
-
[in] Variable declaration, similar to a structure member declaration. This can be any HLSL type or effect object (except a texture or a sampler object).
-
packoffset(c#.xyzw)
-
[in] Optional keyword, used to manually pack constant data. Constants can be packed in any constant buffer, where the register number is given by (#). Sub-component packing (using xyzw swizzling) is available for constants whose size fit within a single register (do not cross a register boundary). For instance, a float4 could not be packed in a single register starting with the y-component because it would not fit in a four-component register.
Remarks
Constant buffers reduce the bandwidth required to update shader constants by allowing shader constants to be grouped together and committed at the same time rather than making individual calls to commit each constant separately.
A constant buffer is a specialized buffer resource that is accessed like a buffer. Each constant buffer can hold up to 4096 vectors; each vector contains up to four 32-bit values. You can bind up to 14 constant buffers per pipeline stage (2 additional slots are reserved for internal use).
A texture buffer is a specialized buffer resource that is accessed like a texture. Texture access (as compared with buffer access) can have better performance for arbitrarily indexed data. You can bind up to 128 texture buffers per pipeline stage.
A buffer resource is designed to minimize the overhead of setting shader constants. The effect framework (see ID3D10Effect Interface) will manage updating constant and texture buffers, or you can use the Direct3D API to update buffers (see Copying and Accessing Resource Data (Direct3D 10) for information). An application can also copy data from another buffer (such as a render target or a stream-output target) into a constant buffer.
For more info on using constant buffers in a D3D10 application, see Resource Types (Direct3D 10) and Creating Buffer Resources (Direct3D 10).
For morel info on using constant buffers in a D3D11 application, see Introduction to Buffers in Direct3D 11 and How to: Create a Constant Buffer.
A constant buffer does not require a view to be bound to the pipeline. A texture buffer, however, requires a view and must be bound to a texture slot (or must be bound with SetTextureBuffer when using an effect).
There are two ways to pack constants data: using the register (DirectX HLSL) and packoffset (DirectX HLSL) keywords.
Differences between Direct3D 9 and Direct3D 10 and 11:
- Unlike the auto-allocation of constants in Direct3D 9, which did not perform packing and instead assigned each variable to a set of float4 registers, HLSL constant variables follow packing rules in Direct3D 10 and 11.
Organizing constant buffers
Constant buffers reduce the bandwidth required to update shader constants by allowing shader constants to be grouped together and committed at the same time rather than making individual calls to commit each constant separately.
The best way to efficiently use constant buffers is to organize shader variables into constant buffers based on their frequency of update. This allows an application to minimize the bandwidth required for updating shader constants. For example, a shader might declare two constant buffers and organize the data in each based on their frequency of update: data that needs to be updated on a per-object basis (like a world matrix) is grouped into a constant buffer which could be updated for each object. This is separate from data that characterizes a scene and is therefore likely to be updated much less often (when the scene changes).
cbuffer myObject
{
float4x4 matWorld;
float3 vObjectPosition;
int arrayIndex;
}
cbuffer myScene
{
float3 vSunPosition;
float4x4 matView;
}
Default constant buffers
There are two default constant buffers available, $Global and $Param. Variables that are placed in the global scope are added implicitly to the $Global cbuffer, using the same packing method that is used for cbuffers. Uniform parameters in the parameter list of a function appear in the $Param constant buffer when a shader is compiled outside of the effects framework. When compiled inside the effects framework, all uniforms must resolve to variables defined in the global scope.
Examples
Here is an example from Skinning10 Sample that is a texture buffer made up of an array of matrices.
tbuffer tbAnimMatrices
{
matrix g_mTexBoneWorld[MAX_BONE_MATRICES];
};
This example declaration manually assigns a constant buffer to start at a particular register, and also packs particular elements by subcomponents.
cbuffer MyBuffer : register(b3)
{
float4 Element1 : packoffset(c0);
float1 Element2 : packoffset(c1);
float1 Element3 : packoffset(c1.y);
}