<!
//  LightingModel.slh
//  fragment shader
//
//  Created by Sam Gateau on 1/25/14.
//  Copyright 2013 High Fidelity, Inc.
//
//  Distributed under the Apache License, Version 2.0.
//  See the accompanying file LICENSE or http://www.apache.org/licenses/LICENSE-2.0.html
!>
<@if not SHADING_MODEL_SLH@>
<@def SHADING_MODEL_SLH@>

<@include LightingModel.slh@>

<@func declareBeckmannSpecular()@>

LAYOUT(binding=RENDER_UTILS_TEXTURE_SSSC_SPECULAR_BECKMANN) uniform sampler2D scatteringSpecularBeckmann;

float fetchSpecularBeckmann(float ndoth, float roughness) {
    return pow(2.0 * texture(scatteringSpecularBeckmann, vec2(ndoth, roughness)).r, 10.0);
}

vec2 skinSpecular(SurfaceData surface, float intensity) {
    vec2 result = vec2(0.0, 1.0);
    if (surface.ndotl > 0.0) {
        float PH = fetchSpecularBeckmann(surface.ndoth, surface.roughness);
        float F = fresnelSchlickScalar(0.028, surface);
        float frSpec = max(PH * F / dot(surface.halfDir, surface.halfDir), 0.0);
        result.x = surface.ndotl * intensity * frSpec;
        result.y -= F;
    }

    return result;
}
<@endfunc@>

<@func declareEvalPBRShading()@>

float evalSmithInvG1(float roughness4, float ndotd) {
    return ndotd + sqrt(roughness4+ndotd*ndotd*(1.0-roughness4));
}

SurfaceData initSurfaceData(float roughness, vec3 normal, vec3 eyeDir) {
    SurfaceData surface;
    surface.eyeDir = eyeDir;
    surface.normal = normal;
    surface.roughness = mix(0.01, 1.0, roughness);
    surface.roughness2 = surface.roughness * surface.roughness;
    surface.roughness4 = surface.roughness2 * surface.roughness2;
    surface.ndotv = clamp(dot(normal, eyeDir), 0.0, 1.0);
    surface.smithInvG1NdotV = evalSmithInvG1(surface.roughness4, surface.ndotv);

    // These values will be set when we know the light direction, in updateSurfaceDataWithLight
    surface.ndoth = 0.0;
    surface.ndotl = 0.0;
    surface.ldoth = 0.0;
    surface.lightDir = vec3(0,0,1);
    surface.halfDir = vec3(0,0,1);

    return surface;
}

void updateSurfaceDataWithLight(inout SurfaceData surface, vec3 lightDir) {
    surface.lightDir = lightDir;
    surface.halfDir = normalize(surface.eyeDir + lightDir);
    vec3 dots;
    dots.x = dot(surface.normal, surface.halfDir);
    dots.y = dot(surface.normal, surface.lightDir);
    dots.z = dot(surface.halfDir, surface.lightDir);
    dots = clamp(dots, vec3(0), vec3(1));
    surface.ndoth = dots.x;
    surface.ndotl = dots.y;
    surface.ldoth = dots.z;
}

vec3 fresnelSchlickColor(vec3 fresnelColor, SurfaceData surface) {
    float base = 1.0 - surface.ldoth;
    //float exponential = pow(base, 5.0);
    float base2 = base * base;
    float exponential = base * base2 * base2;
    return vec3(exponential) + fresnelColor * (1.0 - exponential);
}

float fresnelSchlickScalar(float fresnelScalar, SurfaceData surface) {
    float base = 1.0 - surface.ldoth;
    //float exponential = pow(base, 5.0);
    float base2 = base * base;
    float exponential = base * base2 * base2;
    return (exponential) + fresnelScalar * (1.0 - exponential);
}

float specularDistribution(SurfaceData surface) {
    // See https://www.khronos.org/assets/uploads/developers/library/2017-web3d/glTF-2.0-Launch_Jun17.pdf
    // for details of equations, especially page 20
    float denom = (surface.ndoth*surface.ndoth * (surface.roughness4 - 1.0) + 1.0);
    denom *= denom;
    // Add geometric factors G1(n,l) and G1(n,v)
    float smithInvG1NdotL = evalSmithInvG1(surface.roughness4, surface.ndotl);
    denom *= surface.smithInvG1NdotV * smithInvG1NdotL;
    // Don't divide by PI as this is part of the light normalization factor
    float power = surface.roughness4 / denom;
    return power;
}

// Frag Shading returns the diffuse amount as W and the specular rgb as xyz
vec4 evalPBRShading(float metallic, vec3 fresnel, SurfaceData surface) {
    // Incident angle attenuation
    float angleAttenuation = surface.ndotl;

    // Specular Lighting
    vec3 fresnelColor = fresnelSchlickColor(fresnel, surface);
    float power = specularDistribution(surface);
    vec3 specular = fresnelColor * power * angleAttenuation;
    float diffuse = (1.0 - metallic) * angleAttenuation * (1.0 - fresnelColor.x);

    // We don't divided by PI, as the "normalized" equations state we should, because we decide, as Naty Hoffman, that
    // we wish to have a similar color as raw albedo on a perfectly diffuse surface perpendicularly lit
    // by a white light of intensity 1. But this is an arbitrary normalization of what light intensity "means".
    // (see http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf
    // page 23 paragraph "Punctual light sources")
    return vec4(specular, diffuse);
}

// Frag Shading returns the diffuse amount as W and the specular rgb as xyz
vec4 evalPBRShadingDielectric(SurfaceData surface, float fresnel) {
    // Incident angle attenuation
    float angleAttenuation = surface.ndotl;

    // Specular Lighting
    float fresnelScalar = fresnelSchlickScalar(fresnel, surface);
    float power = specularDistribution(surface);
    vec3 specular = vec3(fresnelScalar) * power * angleAttenuation;
    float diffuse = angleAttenuation * (1.0 - fresnelScalar);

    // We don't divided by PI, as the "normalized" equations state we should, because we decide, as Naty Hoffman, that
    // we wish to have a similar color as raw albedo on a perfectly diffuse surface perpendicularly lit
    // by a white light of intensity 1. But this is an arbitrary normalization of what light intensity "means".
    // (see http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf
    // page 23 paragraph "Punctual light sources")
    return vec4(specular, diffuse);
}

vec4 evalPBRShadingMetallic(SurfaceData surface, vec3 fresnel) {
    // Incident angle attenuation
    float angleAttenuation = surface.ndotl;

    // Specular Lighting
    vec3 fresnelColor = fresnelSchlickColor(fresnel, surface);
    float power = specularDistribution(surface);
    vec3 specular = fresnelColor * power * angleAttenuation;

    // We don't divided by PI, as the "normalized" equations state we should, because we decide, as Naty Hoffman, that
    // we wish to have a similar color as raw albedo on a perfectly diffuse surface perpendicularly lit
    // by a white light of intensity 1. But this is an arbitrary normalization of what light intensity "means".
    // (see http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf
    // page 23 paragraph "Punctual light sources")
    return vec4(specular, 0.f);
}

<@endfunc@>


<$declareEvalPBRShading()$>

void evalFragShading(out vec3 diffuse, out vec3 specular,
    float metallic, vec3 fresnel, SurfaceData surface, vec3 albedo) {
    vec4 shading = evalPBRShading(metallic, fresnel, surface);
    diffuse = vec3(shading.w);
    diffuse *= mix(vec3(1.0), albedo, isAlbedoEnabled());
    specular = shading.xyz;
}

<$declareBeckmannSpecular()$>
<@include SubsurfaceScattering.slh@>
<$declareSubsurfaceScatteringBRDF()$>


void evalFragShading(out vec3 diffuse, out vec3 specular,
    float metallic, vec3 fresnel, SurfaceData surface, vec3 albedo,
    float scattering, vec4 midNormalCurvature, vec4 lowNormalCurvature) {
    if (scattering * isScatteringEnabled() > 0.0) {
        vec3 brdf = evalSkinBRDF(surface.lightDir, surface.normal, midNormalCurvature.xyz, lowNormalCurvature.xyz, lowNormalCurvature.w);
        diffuse = mix(vec3(surface.ndotl), brdf, scattering);

        // Specular Lighting
        vec2 specularBrdf = skinSpecular(surface, 1.0);
        
        diffuse *= specularBrdf.y;
        specular = vec3(specularBrdf.x);
    } else {
        vec4 shading = evalPBRShading(metallic, fresnel, surface);
        diffuse = vec3(shading.w);
        specular = shading.xyz;
    }
    diffuse *= mix(vec3(1.0), albedo, isAlbedoEnabled());
}


void evalFragShadingScattering(out vec3 diffuse, out vec3 specular,
    float metallic, vec3 fresnel, SurfaceData surface, vec3 albedo,
    float scattering, vec4 midNormalCurvature, vec4 lowNormalCurvature) {
    vec3 brdf = evalSkinBRDF(surface.lightDir, surface.normal, midNormalCurvature.xyz, lowNormalCurvature.xyz, lowNormalCurvature.w);
    float NdotL = surface.ndotl;
    diffuse = mix(vec3(NdotL), brdf, scattering);

    // Specular Lighting
    vec2 specularBrdf = skinSpecular(surface, 1.0);
        
    diffuse *= specularBrdf.y;
    specular = vec3(specularBrdf.x);
    diffuse *= mix(vec3(1.0), albedo, isAlbedoEnabled());
}

void evalFragShadingGloss(out vec3 diffuse, out vec3 specular,
    float metallic, vec3 fresnel, SurfaceData surface, vec3 albedo) {
    vec4 shading = evalPBRShading(metallic, fresnel, surface);
    diffuse = vec3(shading.w);
    diffuse *= mix(vec3(1.0), albedo, isAlbedoEnabled());
    specular = shading.xyz;
}

vec3 evalSpecularWithOpacity(vec3 specular, float opacity) {
    return specular / mix(1.0, opacity, float(opacity > 0.0));
}

<@if not GETFRESNEL0@>
<@def GETFRESNEL0@>
vec3 getFresnelF0(float metallic, vec3 metalF0) {
    // Enable continuous metallness value by lerping between dielectric
    // and metal fresnel F0 value based on the "metallic" parameter
    return mix(vec3(0.03), metalF0, metallic);
}
<@endif@>

<@endif@>