Shader "Custom/HeatHaze"
{
    Properties
    {
        [Header(Global)]
        _Weight         ("Global Weight",        Range(0, 1))     = 0.0

        [Header(Distortion)]
        _Intensity      ("Distortion Intensity", Range(0, 0.05))  = 0.02
        _Speed          ("Distortion Speed",     Range(0, 5))     = 1.0
        _Scale          ("Noise Scale",          Range(0, 20))    = 6.0

        [Header(Heat Blur)]
        _BlurAmount     ("Blur Amount",          Range(0, 1))     = 0.6
        _BlurSamples    ("Blur Sample Spread",   Range(0, 0.02))  = 0.005

        [Header(Heat Color)]
        _HeatTint           ("Heat Tint Color",  Color)           = (1.0, 0.45, 0.1, 1)
        _HeatTintStrength   ("Tint Strength",    Range(0, 1))     = 0.4

        [Header(Heat Vignette)]
        _VignetteStrength   ("Vignette Strength", Range(0, 1))    = 0.5
        _VignettePower      ("Vignette Power",    Range(0.5, 5))  = 2.0
    }

    SubShader
    {
        Tags { "RenderType"="Opaque" "RenderPipeline"="UniversalPipeline" }
        ZWrite Off ZTest Always Cull Off

        Pass
        {
            Name "HeatHazePass"

            HLSLPROGRAM
            #pragma vertex Vert
            #pragma fragment Frag
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
            #include "Packages/com.unity.render-pipelines.core/Runtime/Utilities/Blit.hlsl"

            float  _Weight;
            float  _Intensity;
            float  _Speed;
            float  _Scale;
            float  _BlurAmount;
            float  _BlurSamples;
            float4 _HeatTint;
            float  _HeatTintStrength;
            float  _VignetteStrength;
            float  _VignettePower;

            // ─── NOISE ───────────────────────────────────────────────

            float SmoothNoise(float2 p)
            {
                float2 i = floor(p);
                float2 f = frac(p);
                f = f * f * (3.0 - 2.0 * f);

                float a = frac(sin(dot(i,               float2(127.1, 311.7))) * 43758.5453);
                float b = frac(sin(dot(i + float2(1,0), float2(127.1, 311.7))) * 43758.5453);
                float c = frac(sin(dot(i + float2(0,1), float2(127.1, 311.7))) * 43758.5453);
                float d = frac(sin(dot(i + float2(1,1), float2(127.1, 311.7))) * 43758.5453);

                return lerp(lerp(a, b, f.x), lerp(c, d, f.x), f.y);
            }

            // ─── BLUR ────────────────────────────────────────────────

            half3 RadialBlur(float2 uv, float strength)
            {
                half3  col    = 0;
                float  spread = _BlurSamples * strength;

                col += SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, uv + float2( spread,  0)).rgb;
                col += SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, uv + float2(-spread,  0)).rgb;
                col += SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, uv + float2( 0,  spread)).rgb;
                col += SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, uv + float2( 0, -spread)).rgb;
                col += SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, uv + float2( spread,  spread) * 0.7071).rgb;
                col += SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, uv + float2(-spread,  spread) * 0.7071).rgb;
                col += SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, uv + float2( spread, -spread) * 0.7071).rgb;
                col += SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, uv + float2(-spread, -spread) * 0.7071).rgb;

                return col / 8.0;
            }

            // ─── FRAGMENT ────────────────────────────────────────────

            half4 Frag(Varyings input) : SV_Target
            {
                float2 uv = input.texcoord;

                // Early exit — no effect at weight zero
                if (_Weight <= 0.001)
                    return SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, uv);

                float t = _Time.y * _Speed;

                // ── 1. Distortion ─────────────────────────────────────
                float n1 = SmoothNoise(uv * _Scale       + float2( t * 0.7,  t * 0.3));
                float n2 = SmoothNoise(uv * _Scale * 1.7 + float2(-t * 0.4,  t * 0.6));

                // Heat rises — distortion stronger at the bottom
                float  heightMask  = 1.0 - uv.y;
                float2 distort     = float2(n1 - 0.5, n2 - 0.5) * _Intensity * heightMask * _Weight;
                float2 distortedUV = uv + distort;

                // ── 2. Base sample + blur ─────────────────────────────
                half3 sharp   = SAMPLE_TEXTURE2D(_BlitTexture, sampler_LinearClamp, distortedUV).rgb;
                half3 blurred = RadialBlur(distortedUV, 1.0);
                half3 col     = lerp(sharp, blurred, _BlurAmount * heightMask * _Weight);

                // ── 3. Heat color tint ────────────────────────────────
                half3 tint = _HeatTint.rgb;
                col = lerp(col, col * tint * 1.5, _HeatTintStrength * heightMask * _Weight);

                // ── 4. Heat vignette ──────────────────────────────────
                float2 vigUV    = abs(uv - 0.5) * 2.0;
                float  vignette = pow(max(vigUV.x, vigUV.y), _VignettePower);
                col = lerp(col, col * tint * 2.0, vignette * _VignetteStrength * _Weight);

                return half4(col, 1.0);
            }
            ENDHLSL
        }
    }
}