Rendering to a full 3D Render Target in one pass - directx-11

Question

Using DirectX 11, I created a 3D volume texture that can be bound as a render target:
D3D11_TEXTURE3D_DESC texDesc3d;
// ...
texDesc3d.Usage = D3D11_USAGE_DEFAULT;
texDesc3d.BindFlags = D3D11_BIND_RENDER_TARGET;
// Create volume texture and views
m_dxDevice->CreateTexture3D(&texDesc3d, nullptr, &m_tex3d);
m_dxDevice->CreateRenderTargetView(m_tex3d, nullptr, &m_tex3dRTView);
I would now like to update the whole render target and fill it with procedural data generated in a pixel shader, similar to updating a 2D render target with a 'fullscreen pass'. Everything I need to generate the data is the UVW coordinates of the pixel in question.
For 2D, a simple vertex shader that renders a full screen triangle can be built:
struct VS_OUTPUT
{
float4 position : SV_Position;
float2 uv: TexCoord;
};
// input: three empty vertices
VS_OUTPUT main( uint vertexID : SV_VertexID )
{
VS_OUTPUT result;
result.uv = float2((vertexID << 1) & 2, vertexID & 2);
result.position = float4(result.uv * float2(2.0f, -2.0f) + float2(-1.0f, 1.0f), 0.0f, 1.0f);
return result;
}
I have a hard time wrapping my head around how to adopt this principle for 3D. Is this even possible in DirectX 11, or do I have to render to individual slices of the volume texture as described here?

Answer 1

Here is some sample code doing it with pipeline version. You basically batch N triangles and route each instance to a volume slice using Geometry Shader.
struct VS_OUTPUT
{
float4 position : SV_Position;
float2 uv: TexCoord;
uint index: SLICEINDEX;
};
VS_OUTPUT main( uint vertexID : SV_VertexID, uint ii : SV_InstanceID )
{
VS_OUTPUT result;
result.uv = float2((vertexID << 1) & 2, vertexID & 2);
result.position = float4(result.uv * float2(2.0f, -2.0f) + float2(-1.0f, 1.0f), 0.0f, 1.0f);
result.index= ii;
return result;
}
Now you need to call DrawInstanced with 3 vertices and N instances where N is your volume slices count
Then you assign triangles to GS like this:
struct psInput
{
float4 pos : SV_POSITION;
float2 uv: TEXCOORD0;
uint index : SV_RenderTargetArrayIndex; //This will write your vertex to a specific slice, which you can read in pixel shader too
};
[maxvertexcount(3)]
void GS( triangle VS_OUTPUT input[3], inout TriangleStream<psInput> gsout )
{
psInput output;
for (uint i = 0; i < 3; i++)
{
output.pos = input[i].pos;
output.uv = input[i].uv;
output.index= input[0].index; //Use 0 as we need to push a full triangle to the slice
gsout.Append(output);
}
gsout.RestartStrip();
}
Now you have access to slice index in your pixel shader:
float4 PS(psInput input) : SV_Target
{
//Do something with uvs, and use slice input as Z
}
Compute shader version (don't forget to create a UAV for your volume), and numthreads here is totally arbirtary
[numthreads(8,8,8)]
void CS(uint3 tid : SV_DispatchThreadID)
{
//Standard overflow safeguards
//Generate data using tid coordinates
}
Now instead you need to call dispatch with
width/8, height/8, depth/8