Shader "Hidden/Universal Render Pipeline/CameraMotionVectors"
{
    SubShader
    {
        Pass
        {
            Name "Camera Motion Vectors"

            Cull Off
            ZWrite On

            HLSLPROGRAM
            #pragma target 3.5

            #pragma vertex vert
            #pragma fragment frag

            // -------------------------------------
            // Includes
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/DeclareDepthTexture.hlsl"
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/UnityInput.hlsl"
            #include_with_pragmas "Packages/com.unity.render-pipelines.core/ShaderLibrary/FoveatedRenderingKeywords.hlsl"
            #include "Packages/com.unity.render-pipelines.core/ShaderLibrary/FoveatedRendering.hlsl"

            struct Attributes
            {
                uint vertexID : SV_VertexID;
                UNITY_VERTEX_INPUT_INSTANCE_ID
            };

            struct Varyings
            {
                float4 positionCS : SV_POSITION;
                float2 texcoord   : TEXCOORD0;
                UNITY_VERTEX_OUTPUT_STEREO
            };

            // -------------------------------------
            // Vertex
            Varyings vert(Attributes input)
            {
                Varyings output;
                UNITY_SETUP_INSTANCE_ID(input);
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(output);

                float4 pos = GetFullScreenTriangleVertexPosition(input.vertexID);
                float2 uv  = GetFullScreenTriangleTexCoord(input.vertexID);

                output.positionCS = pos;
                output.texcoord   = uv;

                return output;
            }

            // -------------------------------------
            // Fragment
            half4 frag(Varyings input, out float outDepth : SV_Depth) : SV_Target
            {
                UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input);
                float2 uv = input.texcoord;
                float depth = LoadSceneDepth(uv * _CameraDepthTexture_TexelSize.zw);
                outDepth = depth; // Write depth out unmodified

            #if !UNITY_REVERSED_Z
                depth = lerp(UNITY_NEAR_CLIP_VALUE, 1, SampleSceneDepth(uv).x);
            #endif

            #if defined(SUPPORTS_FOVEATED_RENDERING_NON_UNIFORM_RASTER)
                UNITY_BRANCH if (_FOVEATED_RENDERING_NON_UNIFORM_RASTER)
                {
                    // Get the UVs from non-unifrom space to linear space to determine the right world-space position
                    uv = RemapFoveatedRenderingNonUniformToLinear(uv);
                }
            #endif

                // Reconstruct world position
                float3 posWS = ComputeWorldSpacePosition(uv, depth, UNITY_MATRIX_I_VP);

                // Multiply with current and previous non-jittered view projection
                float4 posCS = mul(_NonJitteredViewProjMatrix, float4(posWS.xyz, 1.0));
                float4 prevPosCS = mul(_PrevViewProjMatrix, float4(posWS.xyz, 1.0));

                // Non-uniform raster needs to keep the posNDC values in float to avoid additional conversions
                // since uv remap functions use floats
                float2 posNDC = posCS.xy * rcp(posCS.w);
                float2 prevPosNDC = prevPosCS.xy * rcp(prevPosCS.w);

                float2 velocity;
                #if defined(SUPPORTS_FOVEATED_RENDERING_NON_UNIFORM_RASTER)
                UNITY_BRANCH if (_FOVEATED_RENDERING_NON_UNIFORM_RASTER)
                {
                    // Convert velocity from NDC space (-1..1) to screen UV 0..1 space since FoveatedRendering remap needs that range.
                    // Also return both position in non-uniform UV space to get the right velocity vector

                    float2 posUV = RemapFoveatedRenderingResolve(posNDC * 0.5f + 0.5f);
                    float2 prevPosUV = RemapFoveatedRenderingPrevFrameLinearToNonUniform(prevPosNDC * 0.5f + 0.5f);

                    // Calculate forward velocity
                    velocity = (posUV - prevPosUV);
                    #if UNITY_UV_STARTS_AT_TOP
                        velocity.y = -velocity.y;
                    #endif
                }
                else
                #endif
                {
                    // Calculate forward velocity
                    velocity = (posNDC - prevPosNDC);

                    // TODO: test that velocity.y is correct
                    #if UNITY_UV_STARTS_AT_TOP
                        velocity.y = -velocity.y;
                    #endif

                    // Convert velocity from NDC space (-1..1) to screen UV 0..1 space
                    // Note: It doesn't mean we don't have negative values, we store negative or positive offset in the UV space.
                    // Note: ((posNDC * 0.5 + 0.5) - (prevPosNDC * 0.5 + 0.5)) = (velocity * 0.5)
                    velocity.xy *= 0.5;
                }

                return float4(velocity, 0, 0);
            }

            ENDHLSL
        }
    }
}