Binding an Instance Matrix with an InputLayout

Question

If I want to bind an instance matrix to a vertex shader, how do I go about it?

For example, here is a super-simple VS input:

struct VS_IN {
    float4 Position : POSITION;
    row_major float4x4 WorldMat : INSTANCE_TRANSFORM;
};

How would the corresponding bindings look when creating the ID3D11InputLayout?

I figured it would be something akin to:

D3D11_INPUT_ELEMENT_DESC[] bindings = ...;
bindings[0] = { 
    "POSITION", // SemanticName
    0U, // SemanticIndex
    DXGI_FORMAT::DXGI_FORMAT_R32G32B32A32_FLOAT, // Format
    0U, // InputSlot
    0U, // AlignedByteOffset
    D3D11_INPUT_CLASSIFICATION::D3D11_INPUT_PER_VERTEX_DATA, // InputSlotClass
    0U // InstanceDataStepRate
};
bindings[1] = { 
    "INSTANCE_TRANSFORM", // SemanticName
    0U, // SemanticIndex
    DXGI_FORMAT::DXGI_FORMAT_R32G32B32A32_FLOAT, // Format
    1U, // InputSlot
    0U, // AlignedByteOffset
    D3D11_INPUT_CLASSIFICATION::D3D11_INPUT_PER_INSTANCE_DATA, // InputSlotClass
    1U // InstanceDataStepRate
};
bindings[2] = { 
    "INSTANCE_TRANSFORM", // SemanticName
    0U, // SemanticIndex
    DXGI_FORMAT::DXGI_FORMAT_R32G32B32A32_FLOAT, // Format
    2U, // InputSlot
    0U, // AlignedByteOffset
    D3D11_INPUT_CLASSIFICATION::D3D11_INPUT_PER_INSTANCE_DATA, // InputSlotClass
    1U // InstanceDataStepRate
};
bindings[3] = { 
    "INSTANCE_TRANSFORM", // SemanticName
    0U, // SemanticIndex
    DXGI_FORMAT::DXGI_FORMAT_R32G32B32A32_FLOAT, // Format
    3U, // InputSlot
    0U, // AlignedByteOffset
    D3D11_INPUT_CLASSIFICATION::D3D11_INPUT_PER_INSTANCE_DATA, // InputSlotClass
    1U // InstanceDataStepRate
};
bindings[4] = { 
    "INSTANCE_TRANSFORM", // SemanticName
    0U, // SemanticIndex
    DXGI_FORMAT::DXGI_FORMAT_R32G32B32A32_FLOAT, // Format
    4U, // InputSlot
    0U, // AlignedByteOffset
    D3D11_INPUT_CLASSIFICATION::D3D11_INPUT_PER_INSTANCE_DATA, // InputSlotClass
    1U // InstanceDataStepRate
};

In the example above, I'm increasing the InputSlot value with each 'row' of the instance matrix, because I would expect that each row of the matrix would take up the next 128-bit input register.

However, in other examples I've seen on the internet, people seem to increase the SemanticIndex with each row instead. I thought that might because they declare the input as four float4 fields instead of a matrix, but then I looked at this question, and that doesn't seem to be the case.

Anyway, to cut to the point, if I want to use VS_IN in the format I've provided, what is the correct way to lay out the input elements?

Answer 1

If you use same semantic for SemanticName, then you need to increase number for SemanticIndex.

I also see that you are increasing InputSlot. This may be correct or not depending on how your data is layed out. If you really have 4 vertex streams (vertex buffers) then it's fine, but I assume that your instance transform is stored in one vertex buffer. Then you need to set same stream number to InputSlot in every instance binding.

assume that:

stream_number = 4;

You'll also need to set this stream:

//  most probably offset_in_vertices will be 0 in your case
// vert_buf - contains your instrancing data
IASetVertexBuffers(stream_number, 1, &vert_buf, &stride, &offset_in_vertices);

Also while AlignedByteOffset is optional i suggest at least use D3D11_APPEND_ALIGNED_ELEMENT to specify that data go sequentially one after another.

so, setup code will look like this:

bindings[0] = { 
    // nothing changed
};
bindings[1] = { 
    "INSTANCE_TRANSFORM", // SemanticName
    0U, // SemanticIndex
    stream_number, // InputSlot
    0, // AlignedByteOffset: zero because start of a matrix
    // ...
};
bindings[2] = { 
   "INSTANCE_TRANSFORM", 
    1U, // SemanticIndex
    stream_number, // InputSlot
    D3D11_APPEND_ALIGNED_ELEMENT, // or 16 (4 float32)    AlignedByteOffset
    // ...
};
bindings[3] = { 
    "INSTANCE_TRANSFORM", 
    2U, // SemanticIndex
    stream_number, // InputSlot
    D3D11_APPEND_ALIGNED_ELEMENT, //or 32, AlignedByteOffset +4 float32
    // ...
};
bindings[4] = { 
    "INSTANCE_TRANSFORM", // SemanticName
    3U, // SemanticIndex
    stream_number, // InputSlot
    D3D11_APPEND_ALIGNED_ELEMENT, // or 48 AlignedByteOffset +4 float32
    // ...
};

Then your vertex shader input structure will look like this:

struct VS_IN
{
    float4 Pos          : POSITION;
    float4 r0           : INSTANCE_TRANSFORM0;
    float4 r1           : INSTANCE_TRANSFORM1;
    float4 r2           : INSTANCE_TRANSFORM2;
    float4 r3           : INSTANCE_TRANSFORM3;
};

Or like this:

struct VS_IN
{
    float4 Pos                  : POSITION;
    row_major float4x4 inst_mat : INSTANCE_TRANSFORM;
};