Shader "Custom/TiltShift"
{
    SubShader
    {
        Tags { "RenderType" = "Opaque" "RenderPipeline" = "UniversalPipeline" }
        ZTest Always ZWrite Off Cull Off Blend Off

        HLSLINCLUDE
        #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
        #include "Packages/com.unity.render-pipelines.core/Runtime/Utilities/Blit.hlsl"

        float _TiltShift_CenterOffset;
        float _TiltShift_FocusWidth;
        float _TiltShift_FalloffRange;
        float _TiltShift_MaxBlurRadius;
        int   _TiltShift_SampleCount;

        // _BlitTexture and _BlitTexture_TexelSize are provided by Blit.hlsl
        // _BlitTexture_TexelSize: (1/w, 1/h, w, h)
        // sampler_LinearClamp is provided by GlobalSamplers.hlsl (included via Blit.hlsl)

        // Returns blur radius in pixels for a given vertical UV coordinate.
        float ComputeBlurRadius(float uvY)
        {
            float dist = abs(uvY - 0.5 + _TiltShift_CenterOffset);
            float halfFocus = _TiltShift_FocusWidth * 0.5;
            float range = max(_TiltShift_FalloffRange, 0.001);
            return smoothstep(halfFocus, halfFocus + range, dist) * _TiltShift_MaxBlurRadius;
        }

        // Gaussian blur in an arbitrary direction.
        // direction: offset per sample step in UV space (already multiplied by texelSize)
        float4 GaussianBlur(float2 uv, float2 stepUV)
        {
            int n = max(1, _TiltShift_SampleCount);
            float4 color = 0;
            float totalWeight = 0;

            [loop]
            for (int i = -n; i <= n; i++)
            {
                // t in [-1, 1]; weight peaks at centre and falls near edges.
                float t = float(i) / float(n);
                float weight = exp(-4.5 * t * t);
                float2 sampleUV = uv + stepUV * float(i);
                color += SAMPLE_TEXTURE2D_X_LOD(_BlitTexture, sampler_LinearClamp, sampleUV, 0) * weight;
                totalWeight += weight;
            }

            return color / totalWeight;
        }

        float4 HorizontalBlur(Varyings input) : SV_Target
        {
            UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input);
            float2 uv = input.texcoord;
            float radius = ComputeBlurRadius(uv.y);
            if (radius < 0.5)
                return SAMPLE_TEXTURE2D_X_LOD(_BlitTexture, sampler_LinearClamp, uv, 0);

            // stepUV: one step in the horizontal direction, spread over [−radius, +radius] pixels
            int n = max(1, _TiltShift_SampleCount);
            float2 stepUV = float2((radius / float(n)) * _BlitTexture_TexelSize.x, 0.0);
            return GaussianBlur(uv, stepUV);
        }

        float4 VerticalBlur(Varyings input) : SV_Target
        {
            UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input);
            float2 uv = input.texcoord;
            float radius = ComputeBlurRadius(uv.y);
            if (radius < 0.5)
                return SAMPLE_TEXTURE2D_X_LOD(_BlitTexture, sampler_LinearClamp, uv, 0);

            int n = max(1, _TiltShift_SampleCount);
            float2 stepUV = float2(0.0, (radius / float(n)) * _BlitTexture_TexelSize.y);
            return GaussianBlur(uv, stepUV);
        }
        ENDHLSL

        // Pass 0 — Horizontal blur
        Pass
        {
            Name "TiltShift_H"
            HLSLPROGRAM
            #pragma vertex Vert
            #pragma fragment HorizontalBlur
            ENDHLSL
        }

        // Pass 1 — Vertical blur
        Pass
        {
            Name "TiltShift_V"
            HLSLPROGRAM
            #pragma vertex Vert
            #pragma fragment VerticalBlur
            ENDHLSL
        }
    }
}