GPU Instancing

GPU instancing is an optimization where multiple instances of the same model with the same material can be drawn in a single draw call. This is a big optimization when rendering things that appear many times in a scene such as foliage.

Standard Instancing

Everything is automatically instanced by default, if the renderer can batch your models into 1 draw call, it will.

Instance ID

Standard vertex input comes with a property for ID of your instance already, it is called i.nInstanceTransformID. Bone index (1D blend index for skinned meshes) can be fetched from i.nBoneIndex.

To add batched instance ID and bone index to your custom vertex struct, add this:

uint nInstanceTransformID : TEXCOORD13 < Semantic( InstanceTransformUv ); >;
uint nBoneIndex 		  : BLENDINDICES < Semantic( BlendIndices ); >;

Helpers

• To get the Object→World matrix in your vertex shader you use the following function:

float3x4 GetTransformMatrix( uint nTransformIndexID, uint nBlendIndex = 0 )

• Extra per instance data can be grabbed by using:

ExtraShaderData_t GetExtraPerInstanceShaderData( uint nTransformIndexID );

ExtraShaderData allows you to still have instances even though some data is different between them, currently only tint color is changeable within instances:

struct ExtraShaderData_t
{
	float4 vTint;
	uint nVertexCacheOffset;
	uint nBlendWeightCount;	// if D_SKINNING, blend weight count
};

• There's also a helper function which allows quickly getting transform scale from object->world matrix of your instance:

float3 ComputeMatrixScale( float3x4 m )

Procedural Instancing

Procedural instancing is useful for programmatic shaders that are to be invoked from C# or indirectly.
No transforms are passed, only the count of instance ids, you would derive some form of transform yourself from them within the shader.

You can get the instance id from SV_InstanceID in your vertex function. This is also useful for indirect draw calls.

PixelInput MainVs( VertexInput i, uint instanceID : SV_InstanceID )
{ 
    float3 offsetPosition = float3( 0, 0, 64 * instanceID );
}

See:

• Graphics.DrawModelInstanced( Model, count )
• Graphics.DrawModelInstancedIndirect( Model, GpuBuffer )