Add builtin GaussianSplat forward pass baseline

engine/assets/builtin/shaders/gaussian-splat.shader

@@ -0,0 +1,113 @@
+Shader "Builtin Gaussian Splat"
+{
+    Properties
+    {
+        _PointScale ("Point Scale", Float) = 1.0
+        _OpacityScale ("Opacity Scale", Float) = 1.0
+    }
+    HLSLINCLUDE
+    cbuffer PerObjectConstants
+    {
+        float4x4 gProjectionMatrix;
+        float4x4 gViewMatrix;
+        float4x4 gModelMatrix;
+        float4 gCameraRight;
+        float4 gCameraUp;
+    };
+
+    cbuffer MaterialConstants
+    {
+        float4 gSplatParams;
+    };
+
+    struct GaussianSplatOtherData
+    {
+        float4 rotation;
+        float4 scaleReserved;
+    };
+
+    StructuredBuffer<float3> GaussianSplatPositions;
+    StructuredBuffer<GaussianSplatOtherData> GaussianSplatOther;
+    StructuredBuffer<float4> GaussianSplatColor;
+
+    struct VSOutput
+    {
+        float4 position : SV_POSITION;
+        float2 localUv : TEXCOORD0;
+        float4 colorOpacity : TEXCOORD1;
+    };
+
+    float2 ResolveQuadCorner(uint vertexId)
+    {
+        switch (vertexId)
+        {
+        case 0u: return float2(-1.0, -1.0);
+        case 1u: return float2( 1.0, -1.0);
+        case 2u: return float2( 1.0,  1.0);
+        case 3u: return float2(-1.0, -1.0);
+        case 4u: return float2( 1.0,  1.0);
+        default: return float2(-1.0,  1.0);
+        }
+    }
+
+    VSOutput MainVS(uint vertexId : SV_VertexID, uint instanceId : SV_InstanceID)
+    {
+        VSOutput output;
+        const float2 corner = ResolveQuadCorner(vertexId);
+        const float3 localCenter = GaussianSplatPositions[instanceId];
+        const GaussianSplatOtherData otherData = GaussianSplatOther[instanceId];
+        const float4 colorOpacity = GaussianSplatColor[instanceId];
+
+        const float3 worldCenter = mul(gModelMatrix, float4(localCenter, 1.0)).xyz;
+        const float maxAxisScale =
+            max(max(otherData.scaleReserved.x, otherData.scaleReserved.y), otherData.scaleReserved.z);
+        const float radius = max(maxAxisScale * gSplatParams.x, 0.0001);
+        const float3 worldPosition =
+            worldCenter +
+            gCameraRight.xyz * (corner.x * radius) +
+            gCameraUp.xyz * (corner.y * radius);
+
+        output.position = mul(gProjectionMatrix, mul(gViewMatrix, float4(worldPosition, 1.0)));
+        output.localUv = corner;
+        output.colorOpacity = colorOpacity;
+        return output;
+    }
+
+    float4 MainPS(VSOutput input) : SV_TARGET
+    {
+        const float radiusSq = dot(input.localUv, input.localUv);
+        if (radiusSq > 1.0)
+        {
+            discard;
+        }
+
+        const float gaussianFalloff = exp(-radiusSq * 2.5);
+        const float alpha = saturate(input.colorOpacity.a * gSplatParams.y * gaussianFalloff);
+        if (alpha <= 0.001)
+        {
+            discard;
+        }
+
+        return float4(input.colorOpacity.rgb, alpha);
+    }
+    ENDHLSL
+
+    SubShader
+    {
+        Tags { "Queue" = "Transparent" }
+        Pass
+        {
+            Name "GaussianSplat"
+            Tags { "LightMode" = "GaussianSplat" }
+            Cull Off
+            ZWrite Off
+            ZTest LEqual
+            Blend SrcAlpha OneMinusSrcAlpha
+            HLSLPROGRAM
+            #pragma target 4.5
+            #pragma vertex MainVS
+            #pragma fragment MainPS
+            ENDHLSL
+        }
+    }
+}