#version 400

// original PBR shader by @seece (Public Domain). link: https://github.com/Gargaj/Foxotron/pull/12

//#define textureQueryLod(t,c) vec2(0.0,0.0)    // #version 400 required

uniform vec2 resolution = vec2(640.0,480.0);    // debug options below use this (USE_MAP_DEBUGGING, USE_AMBIENT_DEBUGGING)

#define USE_BRUTEFORCE_IRRADIANCE false         // Samples irradiance from tex_skysphere when enabled.
#define USE_WRAPAROUND_SPECULAR true            // Makes silhouettes more reflective to avoid black pixels.
#define USE_SPECULAR_AO_ATTENUATION true        // Dampens IBL specular ambient with AO if enabled.
#define USE_NORMAL_VARIATION_TO_ROUGHNESS true  // Increases roughness if normal map has variation and was minified.
#define USE_MAP_DEBUGGING false                 // Shows all ColorMaps as horizontal bars
#define USE_AMBIENT_DEBUGGING false             // Splits the screen in two and shows image-based specular (left), full shading (middle), diffuse shading (right).
#define BOOST_LIGHTING  2.00f                   // Multiplies analytic light's color with this constant because otherwise they look really pathetic.
#define BOOST_SPECULAR  1.50f
#define BOOST_NOISE     2.50f

struct Light
{
    vec3 direction;
    vec3 color;
};

struct ColorMap
{
    bool has_tex;
    vec4 color;
};

uniform ColorMap map_albedo;    uniform sampler2D map_albedo_tex;
uniform ColorMap map_diffuse;   uniform sampler2D map_diffuse_tex;
uniform ColorMap map_specular;  uniform sampler2D map_specular_tex; // not used
uniform ColorMap map_normals;   uniform sampler2D map_normals_tex;
uniform ColorMap map_roughness; uniform sampler2D map_roughness_tex;
uniform ColorMap map_metallic;  uniform sampler2D map_metallic_tex;
uniform ColorMap map_ao;        uniform sampler2D map_ao_tex;
uniform ColorMap map_ambient;   uniform sampler2D map_ambient_tex;
uniform ColorMap map_emissive;  uniform sampler2D map_emissive_tex;

#define sample_colormap(ColorMap_, uv_) \
    (ColorMap_.has_tex ? texture( ColorMap_##_tex, uv_ ) : ColorMap_.color)

in vec3 out_normal;
in vec3 out_tangent;
in vec3 out_binormal;
in vec2 out_texcoord;
in vec3 out_worldpos;
in vec3 out_to_camera;

uniform float skysphere_rotation;
uniform float skysphere_mip_count;
uniform float exposure;
uniform uint frame_count;
uniform float specular_shininess;

uniform vec3 camera_position;
uniform Light lights[3];

uniform sampler2D tex_skysphere;
uniform sampler2D tex_skyenv;
uniform sampler2D tex_brdf_lut;

uniform bool has_tex_skysphere;
uniform bool has_tex_skyenv;

out vec4 frag_color;

const float PI = 3.1415926536;

// MurMurHash 3 finalizer. Implementation is in public domain.
uint hash( uint h )
{
    h ^= h >> 16;
    h *= 0x85ebca6bU;
    h ^= h >> 13;
    h *= 0xc2b2ae35U;
    h ^= h >> 16;
    return h;
}

// Random function using the idea of StackOverflow user "Spatial" https://stackoverflow.com/a/17479300
// Creates random 23 bits and puts them into the fraction bits of an 32-bit float.
float random( uvec3 h )
{
    uint m = hash(h.x ^ hash( h.y ) ^ hash( h.z ));
    return uintBitsToFloat( ( m & 0x007FFFFFu ) | 0x3f800000u ) - 1.;
}

float random( vec3 v )
{
    return random(floatBitsToUint( v ));
}

vec3 fresnel_schlick( vec3 H, vec3 V, vec3 F0 )
{
    float cosTheta = clamp( dot( H, V ), 0., 1. );
    return F0 + ( vec3( 1.0 ) - F0 ) * pow( 1. - cosTheta, 5.0 );
}

// A Fresnel term that dampens rough specular reflections.
// https://seblagarde.wordpress.com/2011/08/17/hello-world/
vec3 fresnel_schlick_roughness( vec3 H, vec3 V, vec3 F0, float roughness )
{
    float cosTheta = clamp( dot( H, V ), 0., 1. );
    return F0 + ( max( vec3( 1.0 - roughness ), F0 ) - F0 ) * pow( 1. - cosTheta, 5.0 );
}

float distribution_ggx( vec3 N, vec3 H, float roughness )
{
    float a = roughness * roughness;
    float a2 = a * a;
    float NdotH = max( 0., dot( N, H ) );
    float factor = NdotH * NdotH * ( a2 - 1. ) + 1.;

    return a2 / ( PI * factor * factor );
}

float geometry_schlick_ggx( vec3 N, vec3 V, float k )
{
    float NdotV = max( 0., dot( N, V ) );
    return NdotV / (NdotV * ( 1. - k ) + k );
}

float geometry_smith( vec3 N, vec3 V, vec3 L, float roughness )
{
#if 1 // original
    float r = roughness + 1.;
    float k = (r * r) / 8.;
#elif 0 // vries
    float a = roughness;
    float k = (a * a) / 2.0;
#elif 0 // vries improved?
    float a = roughness * roughness;
    float k = a / 2.0;
#endif
    return geometry_schlick_ggx( N, V, k ) * geometry_schlick_ggx( N, L, k );
}

vec2 sphere_to_polar( vec3 normal )
{
    normal = normalize( normal );
    return vec2( ( atan( normal.z, normal.x ) + skysphere_rotation ) / PI / 2.0 + 0.5, acos( normal.y ) / PI );
}

// Our vertically GL_CLAMPed textures seem to blend towards black when sampling the half-pixel edge.
// Not sure if it has a border, or this if is a driver bug, but can repro on multiple nvidia cards.
// Knowing the texture height we can limit sampling to the centers of the top and bottom pixel rows.
vec2 sphere_to_polar_clamp_y( vec3 normal, float texture_height )
{
    normal = normalize( normal );
    return vec2( ( atan( normal.z, normal.x ) + skysphere_rotation ) / PI / 2.0 + 0.5, clamp(acos( normal.y ) / PI, 0.5 / texture_height, 1.0 - 0.5 / texture_height) );
}

vec3 sample_sky( vec3 normal )
{
    vec2 polar = sphere_to_polar( normal );
    return texture( tex_skysphere, polar ).rgb * exposure;
}

// Takes samples around the hemisphere, converts them to radiances via weighting and
// returns a normalized sum.
vec3 sample_irradiance_slow( vec3 normal, vec3 vertex_tangent )
{
    float delta = 0.10;

    vec3 up = abs( normal.y ) < 0.999 ? vec3( 0., 1., 0. ) : vec3( 0., 0., 1. );
    vec3 tangent_x = normalize( cross( up, normal ) );
    vec3 tangent_y = cross( normal, tangent_x );

    int numIrradianceSamples = 0;

    vec3 irradiance = vec3(0.);

    for ( float phi = 0.; phi < 2. * PI ; phi += delta )
    {
        for ( float theta = 0.; theta < 0.5 * PI; theta += delta )
        {
            vec3 tangent_space = vec3(
                sin( theta ) * cos( phi ),
                sin( theta ) * sin( phi ),
                cos( theta ) );

            vec3 world_space = tangent_space.x * tangent_x + tangent_space.y + tangent_y + tangent_space.z * normal;

            vec3 color = sample_sky( world_space );
            irradiance += color * cos( theta ) * sin( theta );
            numIrradianceSamples++;
        }
    }

    irradiance = PI * irradiance / float( numIrradianceSamples );
    return irradiance;
}

vec3 sample_irradiance_fast( vec3 normal, vec3 vertex_tangent )
{
    // Sample the irradiance map if it exists, otherwise fall back to blurred reflection map.
    if ( has_tex_skyenv )
    {
        vec2 polar = sphere_to_polar_clamp_y( normal, 180.0 );
        return textureLod( tex_skyenv, polar, 0.0 ).rgb * exposure;
    }
    else
    {
        vec2 polar = sphere_to_polar( normal );
        return textureLod( tex_skysphere, polar, 0.80 * skysphere_mip_count ).rgb * exposure;
    }
}


vec3 specular_ibl( vec3 V, vec3 N, float roughness, vec3 fresnel )
{
    // What we'd like to do here is take a LOT of skybox samples around the reflection
    // vector R according to the BRDF lobe.
    //
    // Unfortunately it's not possible in real time so we use the following UE4 style approximations:
    // 1. Integrate incoming light and BRDF separately ("split sum approximation")
    // 2. Assume V = R = N so that we can just blur the skybox and sample that.
    // 3. Bake the BRDF integral into a lookup texture so that it can be computed in constant time.
    //
    // Here we also simplify approximation #2 by using bilinear mipmaps with a magic formula instead
    // of properly convolving it with a GGX lobe.
    //
    // For details, see Brian Karis, "Real Shading in Unreal Engine 4", 2013.

    vec3 R = 2. * dot( V, N ) * N - V;

    vec2 polar = sphere_to_polar( R );

    // Map roughness from range [0, 1] into a mip LOD [0, skysphere_mip_count].
    // The magic numbers were chosen empirically.

    float mip = 0.9 * skysphere_mip_count * pow(roughness, 0.25 * BOOST_SPECULAR);

    vec3 prefiltered = textureLod( tex_skysphere, polar, mip ).rgb * exposure;

    float NdotV = dot( N, V );

    // dot( N, V ) seems to produce negative values so we can try to stretch it a bit behind the silhouette
    // to avoid black pixels.
    if (USE_WRAPAROUND_SPECULAR)
    {
        NdotV = NdotV * 0.9 + 0.1;
    }

    NdotV = min(0.99, max(0.01, NdotV));

    // A precomputed lookup table contains a scale and a bias term for specular intensity (called "fresnel" here).
    // See equation (8) in Karis' course notes mentioned above.
    vec2 envBRDF = texture( tex_brdf_lut, vec2(NdotV, 1.0-roughness) ).xy; // (NdotV,1-roughtness) for green top-left (NdotV,roughness) for green bottom-left
    vec3 specular = prefiltered * (fresnel * envBRDF.x + vec3(envBRDF.y));

    return specular;
}


void main(void)
{
    vec3 baseColor = vec3( 0.5, 0.5, 0.5 );
    float roughness = 1.0;
    float metallic = 0.0;
    float ao = 1.0;
    float alpha = 1.0;

    vec4 baseColor_alpha;
    if ( map_albedo.has_tex )
        baseColor_alpha = sample_colormap( map_albedo, out_texcoord );
    else
        baseColor_alpha = sample_colormap( map_diffuse, out_texcoord );
    baseColor = baseColor_alpha.xyz;
    alpha = baseColor_alpha.w;

    if( map_metallic.has_tex && map_roughness.has_tex ) {
        metallic = sample_colormap( map_metallic, out_texcoord ).x;
        roughness = sample_colormap( map_roughness, out_texcoord ).x;
    }
    else if( map_roughness.has_tex ) {
        //< @r-lyeh, metalness B, roughness G, (@todo: self-shadowing occlusion R; for now, any of R/B are metallic)
        metallic = sample_colormap( map_roughness, out_texcoord ).b + sample_colormap( map_roughness, out_texcoord ).r;
        roughness = sample_colormap( map_roughness, out_texcoord ).g;
    }

    if ( map_ao.has_tex )
        ao = sample_colormap( map_ao, out_texcoord ).x;
    else if ( map_ambient.has_tex )
        ao = sample_colormap( map_ambient, out_texcoord ).x;

    vec3 emissive = sample_colormap( map_emissive, out_texcoord ).rgb;

    vec3 normalmap = texture( map_normals_tex, out_texcoord ).xyz * vec3(2.0) - vec3(1.0);
    float normalmap_mip = textureQueryLod( map_normals_tex, out_texcoord ).x;
    float normalmap_length = length(normalmap);
    normalmap /= normalmap_length;

    vec3 normal = out_normal;

    if ( map_normals.has_tex )
    {
        // Mikkelsen's tangent space normal map decoding. See http://mikktspace.com/ for rationale.
        vec3 bi = cross( out_normal, out_tangent );
        vec3 nmap = normalmap.xyz;
        normal = nmap.x * out_tangent + nmap.y * bi + nmap.z * out_normal;
    }

    normal = normalize( normal );

    if( USE_MAP_DEBUGGING && !USE_AMBIENT_DEBUGGING )
    {
        vec3 c = vec3(1., 0., 0.);
        float x = gl_FragCoord.x / resolution.x;
        float y = gl_FragCoord.y / resolution.y;
        if ( y < (7.0/7.0) ) c = vec3(.5) + .5*out_normal;
        if ( y < (6.0/7.0) ) c = vec3(.5) + .5*normalmap;
        if ( y < (5.0/7.0) ) c = vec3(ao);
        if ( y < (4.0/7.0) ) c = vec3(emissive);
        if ( y < (3.0/7.0) ) c = vec3(metallic);
        if ( y < (2.0/7.0) ) c = vec3(roughness);
        if ( y < (1.0/7.0) ) c = baseColor;
        frag_color = vec4(c, 1.);
        return;
    }

    if (USE_NORMAL_VARIATION_TO_ROUGHNESS)
    {
        // Try to reduce specular aliasing by increasing roughness when minified normal maps have high variation.
        float variation = 1. - pow( normalmap_length, 8. );
        float minification = clamp( normalmap_mip - 2., 0., 1. );
        roughness = mix( roughness, 1.0, variation * minification );
    }


    vec3 N = normal;
    vec3 V = normalize( out_to_camera );

    vec3 Lo = vec3(0.);
    vec3 F0 = vec3(0.04);
    F0 = mix( F0, baseColor, metallic );

    bool use_ibl = has_tex_skysphere;

    // Add contributions from analytic lights only if we don't have a skybox.

    {
        int num_lights = use_ibl ? 1 : lights.length();
        for ( int i = 0; i < num_lights; i++ )
        {
            vec3 radiance = lights[ i ].color * BOOST_LIGHTING;

            vec3 L = -normalize( lights[ i ].direction );
            vec3 H = normalize( V + L );

            vec3 F = fresnel_schlick( H, V, F0 );
            vec3 kS = F;
            vec3 kD = vec3(1.0) - kS;
            kD *= 1.0 - metallic;

            // Premultiplied alpha applied to the diffuse component only
            kD *= alpha;

            float D = distribution_ggx( N, H, roughness );
            float G = geometry_smith( N, V, L, roughness );

            vec3 num = D * F * G;
            float denom = 4. * max( 0., dot( N, V ) ) * max( 0., dot( N, L ) );

            vec3 specular = kS * (num / max( 0.001, denom ));

            float NdotL = max( 0., dot( N, L ) );

            Lo += ( kD * ( baseColor / PI ) + specular ) * radiance * NdotL;
        }
    }

    vec3 ambient = sample_colormap( map_ambient, out_texcoord ).xyz;
    vec3 diffuse_ambient;
    vec3 specular_ambient;

    if ( use_ibl )
    {
        // Image based lighting.
        // Based on https://learnopengl.com/PBR/IBL/Diffuse-irradiance

        vec3 irradiance = vec3(0.);

        if ( USE_BRUTEFORCE_IRRADIANCE )
        {
            irradiance = sample_irradiance_slow( normal, out_tangent );
        }
        else
        {
            irradiance = sample_irradiance_fast( normal, out_tangent );
        }

        // Compute the Fresnel term for a perfect mirror reflection with L = R.
        // In this case the halfway vector H = N.
        //
        // We use a modified Fresnel function that dampens specular reflections of very
        // rough surfaces to avoid too bright pixels at grazing angles.
        vec3 F = fresnel_schlick_roughness( N, V, F0, roughness );
        vec3 kS = F;

        // Subtract the amount of reflected light (specular) to get the energy left for
        // absorbed (diffuse) light.
        vec3 kD = vec3(1.) - kS;

        // Metallic surfaces have only a specular reflection.
        kD *= 1.0 - metallic;

        // Premultiplied alpha applied to the diffuse component only
        kD *= alpha;

        // Modulate the incoming lighting with the diffuse color: some wavelengths get absorbed.
        diffuse_ambient = irradiance * baseColor;

        // Ambient light also has a specular part.
        specular_ambient = specular_ibl( V, normal, roughness, F );

        // Ambient occlusion tells us the fraction of sky light that reaches this point.
        if (USE_SPECULAR_AO_ATTENUATION)
        {
            ambient = ao * (kD * diffuse_ambient + specular_ambient);
        }
        else
        {
            // We don't attenuate specular_ambient ambient here with AO which might cause flickering in dark cavities.
            ambient = ao * (kD * diffuse_ambient) + specular_ambient;
        }
    }

    vec3 color = (ambient + Lo) + emissive;

    if ( USE_AMBIENT_DEBUGGING )
    {
        float y = gl_FragCoord.y / resolution.y;
        if( USE_MAP_DEBUGGING && y > 0.5 )
        {
            if ( (y-0.5) < (7.0/7.0/2.0) ) color = vec3(.5) + .5*out_normal;
            if ( (y-0.5) < (6.0/7.0/2.0) ) color = vec3(.5) + .5*normalmap;
            if ( (y-0.5) < (5.0/7.0/2.0) ) color = vec3(ao);
            if ( (y-0.5) < (4.0/7.0/2.0) ) color = vec3(emissive);
            if ( (y-0.5) < (3.0/7.0/2.0) ) color = vec3(metallic);
            if ( (y-0.5) < (2.0/7.0/2.0) ) color = vec3(roughness);
            if ( (y-0.5) < (1.0/7.0/2.0) ) color = baseColor;
        } else {
            float x = gl_FragCoord.x / resolution.x;
            if ( x < 0.33 )
                    color = specular_ambient;
            else if( x > 0.66 )
                    color = diffuse_ambient;
        }
    }

#if 0 // original
    // basic tonemap and gamma correction
    color = color / ( vec3(1.) + color );
    color = pow( color, vec3(1. / 2.2) );
#elif 0
    // filmic tonemapper
    vec3 linearColor = color;
    vec3 x = max(vec3(0.0), linearColor - 0.004);
    color = (x * (6.2 * x + 0.5)) / (x * (6.2 * x + 1.7) + 0.06);
    // gamma correction
    // color = pow( color, vec3(1. / 2.2) );
#elif 1
    // aces film (CC0, src: https://knarkowicz.wordpress.com/2016/01/06/aces-filmic-tone-mapping-curve/)
    vec3 x = color;
    float a = 2.51f;
    float b = 0.03f;
    float c = 2.43f;
    float d = 0.59f;
    float e = 0.14f;
    color = clamp((x*(a*x+b))/(x*(c*x+d)+e), 0.0, 1.0);
    // gamma correction
    // color = pow( color, vec3(2.2) );
#endif

    // dither with noise.
    // float dither = random( uvec3( floatBitsToUint( gl_FragCoord.xy ), frame_count ) );
    // color += BOOST_NOISE * vec3( (-1.0/256.) + (2./256.) * dither );

    // Technically this alpha may be too transparent, if there is a lot of reflected light we wouldn't
    // see the background, maybe we can approximate it well enough by adding a fresnel term
    frag_color = vec4( color, alpha );
}