Resolved How do I use Matrix4x4.TRS inside a vertex shader?

I'm trying to instantiate a lot of objects with

Graphics.DrawMeshInstancedIndirect()

I have a shader with a compute buffer that contains an array of Matrix4x4 TRS.
But how do I use these matrices to position, scale and rotate the object inside a vertex shader?

Or is there a better approach to do this?

 

Those transform matrices are how (almost) all shaders transform any mesh from local object space to world, or view, or clip space, or any other kind of transform. They fast and efficient. For example most Unity shaders have something like this line in the vertex shader:

Code (csharp):

1. o.pos = UnityObjectToClipPos(v.vertex);

That

UnityObjectToClipPos()

funciton is defined like this:

Code (csharp):

1. float4 UnityObjectToClipPos(float3 vertex)
2. {
3.   return mul(UNITY_MATRIX_VP, mul(unity_ObjectToWorld, float4(vertex, 1.0));
4. }

Those

UNITY_MATRIX_VP

and

unity_ObjectToWorld

variables are

float4x4

transform matrices, equivalent to Unity's C# Matrix4x4. One transforms the vertex position form object to world space, as you can probably guess from the name, and the other from world space to clip space. Clip space is a sort of special screen space position used by the GPU to calculate the actual screen space position, as well as handle perspective correct UV distortions. You don't really need to understand that last part too much, but if your transform matrices are to transform your instanced mesh from local object space to world space, then you can just replace that

unity_ObjectToWorld

transform before that

UnityObjectToClipPos()

function gets called. If the shader is using normal mapping, you'll also need to override the

unity_WorldToObject

transform matrix, which if you're only using uniform scaling, can be approximated with:

Code (csharp):

1. unity_ObjectToWorld = MyTRSBuffer[Index];
2. unity_WorldToObject = transpose(unity_ObjectToWorld);