//  Generated on <$_SCRIBE_DATE$>
//
//  Created by Sam Gateau on 6/8/16.
//  Copyright 2016 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 SUBSURFACE_SCATTERING_SLH@>
<@def SUBSURFACE_SCATTERING_SLH@>

<@include render-utils/ShaderConstants.h@>

<@func declareSubsurfaceScatteringProfileSource()@>

float gaussian(float v, float r) {
    const float _PI = 3.14159265358979523846;
    return (1.0 / sqrt(2.0 * _PI * v)) * exp(-(r*r) / (2.0 * v));
}

vec3 scatter(float r) {
    // r is the distance expressed in millimeter
    // returns the scatter reflectance
    // Values from GPU Gems 3 "Advanced Skin Rendering".
    // Originally taken from real life samples.
    const vec4 profile[6] = vec4[6](
        vec4(0.0064, 0.233, 0.455, 0.649),
        vec4(0.0484, 0.100, 0.336, 0.344),
        vec4(0.1870, 0.118, 0.198, 0.000),
        vec4(0.5670, 0.113, 0.007, 0.007),
        vec4(1.9900, 0.358, 0.004, 0.000),
        vec4(7.4100, 0.078, 0.000, 0.000)
        );
    const int profileNum = 6;

    vec3 ret = vec3(0.0);
    for (int i = 0; i < profileNum; i++) {
        float v = profile[i].x * 1.414;
        float g = gaussian(v, r);
        ret += g * profile[i].yzw;
    }

    return ret;
}

<@endfunc@>

<@func declareSubsurfaceScatteringGenerateProfileMap()@>
<$declareSubsurfaceScatteringProfileSource()$>

vec3 generateProfile(vec2 uv) {
    return scatter(uv.x * 2.0);
}

<@endfunc@>

<@func declareSubsurfaceScatteringProfileMap()@>

LAYOUT(binding=RENDER_UTILS_TEXTURE_SSSC_PROFILE) uniform sampler2D scatteringProfile;

vec3 scatter(float r) {
    return texture(scatteringProfile, vec2(r * 0.5, 0.5)).rgb;
}

<@endfunc@>


<@func declareSubsurfaceScatteringIntegrate(NumIntegrationSteps)@>


vec3 integrate(float cosTheta, float skinRadius) {
    // Angle from lighting direction.
    float theta = acos(cosTheta);
    vec3 totalWeights = vec3(0.0);
    vec3 totalLight = vec3(0.0);

    const float _PI = 3.14159265358979523846;
    const float step = 2.0 * _PI / float(<$NumIntegrationSteps$>);
    float a = -(_PI);


    while (a <= (_PI)) {
        float sampleAngle = theta + a;
        float diffuse = clamp(cos(sampleAngle), 0.0, 1.0);

        // Distance.
        float sampleDist = abs(2.0 * skinRadius * sin(a * 0.5));

        // Profile Weight.
        vec3 weights = scatter(sampleDist);

        totalWeights += weights;
        totalLight += diffuse * weights;
        a += step;
    }

    vec3 result = (totalLight / totalWeights);
    return clamp(result, vec3(0.0), vec3(1.0));

}
<@endfunc@>

<@func declareSubsurfaceScatteringResource()@>

LAYOUT(binding=RENDER_UTILS_TEXTURE_SSSC_LUT) uniform sampler2D scatteringLUT;

vec3 fetchBRDF(float LdotN, float curvature) {
    return texture(scatteringLUT, vec2( clamp(LdotN * 0.5 + 0.5, 0.0, 1.0), clamp(2.0 * curvature, 0.0, 1.0))).xyz;
}

vec3 fetchBRDFSpectrum(vec3 LdotNSpectrum, float curvature) {
    return vec3(
        fetchBRDF(LdotNSpectrum.r, curvature).r,
        fetchBRDF(LdotNSpectrum.g, curvature).g,
        fetchBRDF(LdotNSpectrum.b, curvature).b);
}

// Subsurface Scattering parameters
struct ScatteringParameters {
    vec4 normalBendInfo; // R, G, B, factor
    vec4 curvatureInfo;// Offset, Scale, level
    vec4 debugFlags;
};

LAYOUT(binding=RENDER_UTILS_BUFFER_SSSC_PARAMS) uniform subsurfaceScatteringParametersBuffer {
    ScatteringParameters parameters;
};

vec3 getBendFactor() {
    return parameters.normalBendInfo.xyz * parameters.normalBendInfo.w;
}

float getScatteringLevel() {
    return parameters.curvatureInfo.z;
}

bool showBRDF() {
    return parameters.curvatureInfo.w > 0.0;
}

bool showCurvature() {
    return parameters.debugFlags.x > 0.0;
}
bool showDiffusedNormal() {
    return parameters.debugFlags.y > 0.0;
}


float tuneCurvatureUnsigned(float curvature) {
    return abs(curvature) * parameters.curvatureInfo.y + parameters.curvatureInfo.x;
}

float unpackCurvature(float packedCurvature) {
    return (packedCurvature * 2.0 - 1.0);
}

vec3 evalScatteringBentNdotL(vec3 normal, vec3 midNormal, vec3 lowNormal, vec3 lightDir) {
    vec3 bendFactorSpectrum = getBendFactor();
    // vec3 rN = normalize(mix(normal, lowNormal, bendFactorSpectrum.x));
    vec3 rN = normalize(mix(midNormal, lowNormal, bendFactorSpectrum.x));
    vec3 gN = normalize(mix(midNormal, lowNormal, bendFactorSpectrum.y));
    vec3 bN = normalize(mix(midNormal, lowNormal, bendFactorSpectrum.z));

    vec3 NdotLSpectrum = vec3(dot(rN, lightDir), dot(gN, lightDir), dot(bN, lightDir));

    return NdotLSpectrum;
}



<@endfunc@>

<@func declareSubsurfaceScatteringBRDF()@>
<$declareSubsurfaceScatteringResource()$>

vec3 evalSkinBRDF(vec3 lightDir, vec3 normal, vec3 midNormal, vec3 lowNormal, float curvature) {
    if (showDiffusedNormal()) {
        return lowNormal * 0.5 + vec3(0.5);
    }
    if (showCurvature()) {
        float check = float(curvature > 0.0);
        return vec3(check * curvature, 0.0, (1.0 - check) * -curvature);
    }

    vec3 bentNdotL = evalScatteringBentNdotL(normal, midNormal, lowNormal, lightDir);

    float tunedCurvature = tuneCurvatureUnsigned(curvature);

    vec3 brdf = fetchBRDFSpectrum(bentNdotL, tunedCurvature);
    return brdf;
}

<@endfunc@>

<@endif@>