Using tranformation matrixes with GPU instancing

Unity Engine
, ,
1

I’m trying to apply a billboard shader to meshes drew on top of a terrain with DrawMeshInstancedIndirect, but I can’t find a way to make it work. The compute buffer contains a 4x4Matrix,with position, rotation and scale, and the normal. It looks like the built in matrixes are relative to a mesh drew at the terrain origin, even though if I replace o.vertex with UnityObjectToClipPos(pos) it works fine (without the effect). I’ve tried different techniques but all give the same result. I’m using urp.
Any help is appreciated!

Current result

1234×613 219 KB

Shader

Shader "Custom/InstancedIndirectGrass" {
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader {
        Tags { "RenderType" = "Opaque" }
        LOD 100

        Pass {
            ZTest On
            Cull Off

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata_t {
                float4 vertex   : POSITION;
                float4 color    : COLOR;
                float3 normal   : NORMAL;
                float2 uv       : TEXCOORD0;
            };

            struct v2f {
                float4 vertex   : SV_POSITION;
                fixed4 color    : COLOR;
                float3 normal   : NORMAL;
                float2 uv       : TEXCOORD0;
            };

            struct MeshProperties {
                float4x4 mat;
                float3 normal;
            };

            StructuredBuffer<MeshProperties> _Properties;

            Texture2D _MainTex;
            SamplerState sampler_MainTex;
            float4 _MainTex_ST;

            v2f vert(appdata_t i, uint instanceID: SV_InstanceID) {
                v2f o;
                // data
                float4 pos = mul(_Properties[instanceID].mat, i.vertex);
               
                float4 worldOrigin = mul(unity_ObjectToWorld, float4(0, 0, 0, 1));
                float4 viewOrigin = float4(UnityObjectToViewPos(float3(0, 0, 0)), 1);
               
                // Billboard effect
                float4 worldPosition = mul(unity_ObjectToWorld, pos);
                // Removed rotation
                float4 viewPosition = worldPosition - worldOrigin + viewOrigin;
                float4 clipPosition = mul(UNITY_MATRIX_P, viewPosition);

                o.vertex = clipPosition;

                o.normal = _Properties[instanceID].normal;
                o.uv = TRANSFORM_TEX(i.uv, _MainTex);
               
                return o;
            }

            fixed4 frag(v2f i) : SV_Target {
                fixed4 col = _MainTex.Sample(sampler_MainTex, i.uv);
                clip(col.a - 0.5);
                return col;
            }

            ENDCG
        }
    }
}
2

I fixed it, it was my fault. I had to change worldOrigin and viewOrigin using the mesh origin position, which I added to the buffer.