// NOTE: Based on URP Lighting.hlsl which replaced some half3 with floats to avoid lighting artifacts on mobile


#ifndef LIGHTWEIGHT_SKINLIGHTING_INCLUDED
#define LIGHTWEIGHT_SKINLIGHTING_INCLUDED


TEXTURE2D(_SkinLUT); SAMPLER(sampler_SkinLUT); float4 _SkinLUT_TexelSize;


// Based on Minimalist CookTorrance BRDF
// Implementation is slightly different from original derivation: http://www.thetenthplanet.de/archives/255
//
// * NDF [Modified] GGX
// * Modified Kelemen and Szirmay-​Kalos for Visibility term
// * Fresnel approximated with 1/LdotH
half3 DirectBDRF_Lux(BRDFData brdfData, half3 normalWS, half3 lightDirectionWS, half3 viewDirectionWS)
{
#ifndef _SPECULARHIGHLIGHTS_OFF
    float3 halfDir = SafeNormalize(lightDirectionWS + viewDirectionWS);

    float NoH = saturate(dot(normalWS, halfDir));
    half LoH = saturate(dot(lightDirectionWS, halfDir));

    // GGX Distribution multiplied by combined approximation of Visibility and Fresnel
    // BRDFspec = (D * V * F) / 4.0
    // D = roughness² / ( NoH² * (roughness² - 1) + 1 )²
    // V * F = 1.0 / ( LoH² * (roughness + 0.5) )
    // See "Optimizing PBR for Mobile" from Siggraph 2015 moving mobile graphics course
    // https://community.arm.com/events/1155

    // Final BRDFspec = roughness² / ( NoH² * (roughness² - 1) + 1 )² * (LoH² * (roughness + 0.5) * 4.0)
    // We further optimize a few light invariant terms
    // brdfData.normalizationTerm = (roughness + 0.5) * 4.0 rewritten as roughness * 4.0 + 2.0 to a fit a MAD.
    float d = NoH * NoH * brdfData.roughness2MinusOne + 1.00001f;

    half LoH2 = LoH * LoH;
    half specularTerm = brdfData.roughness2 / ((d * d) * max(0.1h, LoH2) * brdfData.normalizationTerm);

    // On platforms where half actually means something, the denominator has a risk of overflow
    // clamp below was added specifically to "fix" that, but dx compiler (we convert bytecode to metal/gles)
    // sees that specularTerm have only non-negative terms, so it skips max(0,..) in clamp (leaving only min(100,...))
#if defined (SHADER_API_MOBILE) || defined (SHADER_API_SWITCH)
    specularTerm = specularTerm - HALF_MIN;
    specularTerm = clamp(specularTerm, 0.0, 100.0); // Prevent FP16 overflow on mobiles
#endif

    half3 color = specularTerm * brdfData.specular; // + brdfData.diffuse;
    return color;
#else
    return brdfData.diffuse;
#endif
}


half3 GlobalIllumination_Lux(BRDFData brdfData, half3 bakedGI, half occlusion, half3 normalWS, half3 viewDirectionWS, 
    half specOccluison)
{
    half3 reflectVector = reflect(-viewDirectionWS, normalWS);
    half fresnelTerm = Pow4(1.0 - saturate(dot(normalWS, viewDirectionWS)));

    half3 indirectDiffuse = bakedGI * occlusion;
    half3 indirectSpecular = GlossyEnvironmentReflection(reflectVector, brdfData.perceptualRoughness, occlusion)        * specOccluison;

    return EnvironmentBRDF(brdfData, indirectDiffuse, indirectSpecular, fresnelTerm);
}


half3 LightingPhysicallyBasedSkin(BRDFData brdfData, half3 lightColor, half3 lightDirectionWS, half lightAttenuation, half3 normalWS, half3 viewDirectionWS, half NdotL, half NdotLUnclamped, half curvature, half skinMask)
{
    //half3 radiance = lightColor * NdotL;
    half3 diffuseLighting = brdfData.diffuse * SAMPLE_TEXTURE2D_LOD(_SkinLUT, sampler_SkinLUT, float2( (NdotLUnclamped * 0.5 + 0.5), curvature), 0).rgb;
    diffuseLighting = lerp(brdfData.diffuse * NdotL, diffuseLighting, skinMask);
    return ( DirectBDRF_Lux(brdfData, normalWS, lightDirectionWS, viewDirectionWS) * NdotL + diffuseLighting ) * lightColor * lightAttenuation;
}

half3 LightingPhysicallyBasedSkin(BRDFData brdfData, Light light, half3 normalWS, half3 viewDirectionWS, half NdotL, half NdotLUnclamped, half curvature, half skinMask)
{
    return LightingPhysicallyBasedSkin(brdfData, light.color, light.direction, light.distanceAttenuation * light.shadowAttenuation, normalWS, viewDirectionWS, NdotL, NdotLUnclamped, curvature, skinMask);
}


half4 LuxLWRPSkinFragmentPBR(InputData inputData, half3 albedo, half metallic, half3 specular,
    half smoothness, half occlusion, half3 emission, half alpha, half4 translucency, half AmbientReflection, half3 diffuseNormalWS, half3 subsurfaceColor, half curvature, half skinMask)
{
    BRDFData brdfData;
    InitializeBRDFData(albedo, metallic, specular, smoothness, alpha, brdfData);

    Light mainLight = GetMainLight(inputData.shadowCoord);
    MixRealtimeAndBakedGI(mainLight, inputData.normalWS, inputData.bakedGI, half4(0, 0, 0, 0));

half3 color = GlobalIllumination_Lux(brdfData, inputData.bakedGI, occlusion, inputData.normalWS, inputData.viewDirectionWS,     AmbientReflection);

    half NdotLUnclamped = dot(diffuseNormalWS, mainLight.direction);
    half NdotL = saturate( dot(inputData.normalWS, mainLight.direction) );
    color += LightingPhysicallyBasedSkin(brdfData, mainLight, inputData.normalWS, inputData.viewDirectionWS, NdotL, NdotLUnclamped, curvature, skinMask);

//  Subsurface Scattering
    half transPower = translucency.y;
    half3 transLightDir = mainLight.direction + inputData.normalWS * translucency.w;
    half transDot = dot( transLightDir, -inputData.viewDirectionWS );
    transDot = exp2(saturate(transDot) * transPower - transPower);
    color += skinMask * subsurfaceColor * transDot * (1.0 - saturate(NdotLUnclamped)) * mainLight.color * lerp(1.0h, mainLight.shadowAttenuation, translucency.z) * translucency.x;


    #ifdef _ADDITIONAL_LIGHTS
        int pixelLightCount = GetAdditionalLightsCount();
        for (int i = 0; i < pixelLightCount; ++i)
        {
            Light light = GetAdditionalLight(i, inputData.positionWS);

            half NdotLUnclamped = dot(diffuseNormalWS, light.direction);
            NdotL = saturate( dot(inputData.normalWS, light.direction) );
            color += LightingPhysicallyBasedSkin(brdfData, light, inputData.normalWS, inputData.viewDirectionWS, NdotL, NdotLUnclamped, curvature, skinMask);

        //  Subsurface Scattering
            transLightDir = light.direction + inputData.normalWS * translucency.w;
            transDot = dot( transLightDir, -inputData.viewDirectionWS );
            transDot = exp2(saturate(transDot) * transPower - transPower);
            color += skinMask * subsurfaceColor * transDot * (1.0 - NdotL) * light.color * lerp(1.0h, light.shadowAttenuation, translucency.z) * light.distanceAttenuation * translucency.x;
        }
    #endif
    #ifdef _ADDITIONAL_LIGHTS_VERTEX
        color += inputData.vertexLighting * brdfData.diffuse;
    #endif
    color += emission;
    return half4(color, alpha);
}


#endif
