/**
 * Precomputed Atmospheric Scattering
 * Copyright (c) 2008 INRIA
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the copyright holders nor the names of its
 *    contributors may be used to endorse or promote products derived from
 *    this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
 * THE POSSIBILITY OF SUCH DAMAGE.
 */

/**
 * Author: Eric Bruneton
 */

// ----------------------------------------------------------------------------
// PHYSICAL MODEL PARAMETERS
// ----------------------------------------------------------------------------

const float AVERAGE_GROUND_REFLECTANCE = 0.1;

// Rayleigh
const float HR = 8.0;
const vec3 betaR = vec3(5.8e-3, 1.35e-2, 3.31e-2);

// Mie
// DEFAULT
const float HM = 1.2;
const vec3 betaMSca = vec3(4e-3);
const vec3 betaMEx = betaMSca / 0.9;
const float mieG = 0.8;
// CLEAR SKY
/*const float HM = 1.2;
const vec3 betaMSca = vec3(20e-3);
const vec3 betaMEx = betaMSca / 0.9;
const float mieG = 0.76;*/
// PARTLY CLOUDY
/*const float HM = 3.0;
const vec3 betaMSca = vec3(3e-3);
const vec3 betaMEx = betaMSca / 0.9;
const float mieG = 0.65;*/

// ----------------------------------------------------------------------------
// NUMERICAL INTEGRATION PARAMETERS
// ----------------------------------------------------------------------------

const int TRANSMITTANCE_INTEGRAL_SAMPLES = 500;
const int INSCATTER_INTEGRAL_SAMPLES = 50;
const int IRRADIANCE_INTEGRAL_SAMPLES = 32;
const int INSCATTER_SPHERICAL_INTEGRAL_SAMPLES = 16;

const float M_PI = 3.141592657;

// ----------------------------------------------------------------------------
// PARAMETERIZATION OPTIONS
// ----------------------------------------------------------------------------

#define TRANSMITTANCE_NON_LINEAR
#define INSCATTER_NON_LINEAR

// ----------------------------------------------------------------------------
// PARAMETERIZATION FUNCTIONS
// ----------------------------------------------------------------------------

uniform sampler2D transmittanceSampler;

vec2 getTransmittanceUV(float r, float mu) {
    float uR, uMu;
#ifdef TRANSMITTANCE_NON_LINEAR
	uR = sqrt((r - Rg) / (Rt - Rg));
	uMu = atan((mu + 0.15) / (1.0 + 0.15) * tan(1.5)) / 1.5;
#else
	uR = (r - Rg) / (Rt - Rg);
	uMu = (mu + 0.15) / (1.0 + 0.15);
#endif
    return vec2(uMu, uR);
}

#ifdef _FRAGMENT_
void getTransmittanceRMu(out float r, out float muS) {
    r = gl_FragCoord.y / float(TRANSMITTANCE_H);
    muS = gl_FragCoord.x / float(TRANSMITTANCE_W);
#ifdef TRANSMITTANCE_NON_LINEAR
    r = Rg + (r * r) * (Rt - Rg);
    muS = -0.15 + tan(1.5 * muS) / tan(1.5) * (1.0 + 0.15);
#else
    r = Rg + r * (Rt - Rg);
    muS = -0.15 + muS * (1.0 + 0.15);
#endif
}
#endif

vec2 getIrradianceUV(float r, float muS) {
    float uR = (r - Rg) / (Rt - Rg);
    float uMuS = (muS + 0.2) / (1.0 + 0.2);
    return vec2(uMuS, uR);
}

#ifdef _FRAGMENT_
void getIrradianceRMuS(out float r, out float muS) {
    r = Rg + (gl_FragCoord.y - 0.5) / (float(SKY_H) - 1.0) * (Rt - Rg);
    muS = -0.2 + (gl_FragCoord.x - 0.5) / (float(SKY_W) - 1.0) * (1.0 + 0.2);
}
#endif

vec4 texture4D(sampler3D table, float r, float mu, float muS, float nu)
{
    float H = sqrt(Rt * Rt - Rg * Rg);
    float rho = sqrt(r * r - Rg * Rg);
#ifdef INSCATTER_NON_LINEAR
    float rmu = r * mu;
    float delta = rmu * rmu - r * r + Rg * Rg;
    vec4 cst = rmu < 0.0 && delta > 0.0 ? vec4(1.0, 0.0, 0.0, 0.5 - 0.5 / float(RES_MU)) : vec4(-1.0, H * H, H, 0.5 + 0.5 / float(RES_MU));
	float uR = 0.5 / float(RES_R) + rho / H * (1.0 - 1.0 / float(RES_R));
    float uMu = cst.w + (rmu * cst.x + sqrt(delta + cst.y)) / (rho + cst.z) * (0.5 - 1.0 / float(RES_MU));
    // paper formula
    //float uMuS = 0.5 / float(RES_MU_S) + max((1.0 - exp(-3.0 * muS - 0.6)) / (1.0 - exp(-3.6)), 0.0) * (1.0 - 1.0 / float(RES_MU_S));
    // better formula
    float uMuS = 0.5 / float(RES_MU_S) + (atan(max(muS, -0.1975) * tan(1.26 * 1.1)) / 1.1 + (1.0 - 0.26)) * 0.5 * (1.0 - 1.0 / float(RES_MU_S));
#else
	float uR = 0.5 / float(RES_R) + rho / H * (1.0 - 1.0 / float(RES_R));
    float uMu = 0.5 / float(RES_MU) + (mu + 1.0) / 2.0 * (1.0 - 1.0 / float(RES_MU));
    float uMuS = 0.5 / float(RES_MU_S) + max(muS + 0.2, 0.0) / 1.2 * (1.0 - 1.0 / float(RES_MU_S));
#endif
    float lerp = (nu + 1.0) / 2.0 * (float(RES_NU) - 1.0);
    float uNu = floor(lerp);
    lerp = lerp - uNu;
    return texture3D(table, vec3((uNu + uMuS) / float(RES_NU), uMu, uR)) * (1.0 - lerp) +
           texture3D(table, vec3((uNu + uMuS + 1.0) / float(RES_NU), uMu, uR)) * lerp;
}

#ifdef _FRAGMENT_
void getMuMuSNu(float r, vec4 dhdH, out float mu, out float muS, out float nu) {
    float x = gl_FragCoord.x - 0.5;
    float y = gl_FragCoord.y - 0.5;
#ifdef INSCATTER_NON_LINEAR
    if (y < float(RES_MU) / 2.0) {
        float d = 1.0 - y / (float(RES_MU) / 2.0 - 1.0);
        d = min(max(dhdH.z, d * dhdH.w), dhdH.w * 0.999);
        mu = (Rg * Rg - r * r - d * d) / (2.0 * r * d);
        mu = min(mu, -sqrt(1.0 - (Rg / r) * (Rg / r)) - 0.001);
    } else {
        float d = (y - float(RES_MU) / 2.0) / (float(RES_MU) / 2.0 - 1.0);
        d = min(max(dhdH.x, d * dhdH.y), dhdH.y * 0.999);
        mu = (Rt * Rt - r * r - d * d) / (2.0 * r * d);
    }
    muS = mod(x, float(RES_MU_S)) / (float(RES_MU_S) - 1.0);
    // paper formula
    //muS = -(0.6 + log(1.0 - muS * (1.0 -  exp(-3.6)))) / 3.0;
    // better formula
    muS = tan((2.0 * muS - 1.0 + 0.26) * 1.1) / tan(1.26 * 1.1);
    nu = -1.0 + floor(x / float(RES_MU_S)) / (float(RES_NU) - 1.0) * 2.0;
#else
    mu = -1.0 + 2.0 * y / (float(RES_MU) - 1.0);
    muS = mod(x, float(RES_MU_S)) / (float(RES_MU_S) - 1.0);
    muS = -0.2 + muS * 1.2;
    nu = -1.0 + floor(x / float(RES_MU_S)) / (float(RES_NU) - 1.0) * 2.0;
#endif
}
#endif

// ----------------------------------------------------------------------------
// UTILITY FUNCTIONS
// ----------------------------------------------------------------------------

// nearest intersection of ray r,mu with ground or top atmosphere boundary
// mu=cos(ray zenith angle at ray origin)
float limit(float r, float mu) {
    float dout = -r * mu + sqrt(r * r * (mu * mu - 1.0) + RL * RL);
    float delta2 = r * r * (mu * mu - 1.0) + Rg * Rg;
    if (delta2 >= 0.0) {
        float din = -r * mu - sqrt(delta2);
        if (din >= 0.0) {
            dout = min(dout, din);
        }
    }
    return dout;
}

// transmittance(=transparency) of atmosphere for infinite ray (r,mu)
// (mu=cos(view zenith angle)), intersections with ground ignored
vec3 transmittance(float r, float mu) {
	vec2 uv = getTransmittanceUV(r, mu);
    return texture2D(transmittanceSampler, uv).rgb;
}

// transmittance(=transparency) of atmosphere for infinite ray (r,mu)
// (mu=cos(view zenith angle)), or zero if ray intersects ground
vec3 transmittanceWithShadow(float r, float mu) {
    return mu < -sqrt(1.0 - (Rg / r) * (Rg / r)) ? vec3(0.0) : transmittance(r, mu);
}

// transmittance(=transparency) of atmosphere between x and x0
// assume segment x,x0 not intersecting ground
// r=||x||, mu=cos(zenith angle of [x,x0) ray at x), v=unit direction vector of [x,x0) ray
vec3 transmittance(float r, float mu, vec3 v, vec3 x0) {
    vec3 result;
    float r1 = length(x0);
    float mu1 = dot(x0, v) / r;
    if (mu > 0.0) {
        result = min(transmittance(r, mu) / transmittance(r1, mu1), 1.0);
    } else {
        result = min(transmittance(r1, -mu1) / transmittance(r, -mu), 1.0);
    }
    return result;
}

// optical depth for ray (r,mu) of length d, using analytic formula
// (mu=cos(view zenith angle)), intersections with ground ignored
// H=height scale of exponential density function
float opticalDepth(float H, float r, float mu, float d) {
    float a = sqrt((0.5/H)*r);
    vec2 a01 = a*vec2(mu, mu + d / r);
    vec2 a01s = sign(a01);
    vec2 a01sq = a01*a01;
    float x = a01s.y > a01s.x ? exp(a01sq.x) : 0.0;
    vec2 y = a01s / (2.3193*abs(a01) + sqrt(1.52*a01sq + 4.0)) * vec2(1.0, exp(-d/H*(d/(2.0*r)+mu)));
    return sqrt((6.2831*H)*r) * exp((Rg-r)/H) * (x + dot(y, vec2(1.0, -1.0)));
}

// transmittance(=transparency) of atmosphere for ray (r,mu) of length d
// (mu=cos(view zenith angle)), intersections with ground ignored
// uses analytic formula instead of transmittance texture
vec3 analyticTransmittance(float r, float mu, float d) {
    return exp(- betaR * opticalDepth(HR, r, mu, d) - betaMEx * opticalDepth(HM, r, mu, d));
}

// transmittance(=transparency) of atmosphere between x and x0
// assume segment x,x0 not intersecting ground
// d = distance between x and x0, mu=cos(zenith angle of [x,x0) ray at x)
vec3 transmittance(float r, float mu, float d) {
    vec3 result;
    float r1 = sqrt(r * r + d * d + 2.0 * r * mu * d);
    float mu1 = (r * mu + d) / r1;
    if (mu > 0.0) {
        result = min(transmittance(r, mu) / transmittance(r1, mu1), 1.0);
    } else {
        result = min(transmittance(r1, -mu1) / transmittance(r, -mu), 1.0);
    }
    return result;
}

vec3 irradiance(sampler2D sampler, float r, float muS) {
    vec2 uv = getIrradianceUV(r, muS);
    return texture2D(sampler, uv).rgb;
}

// Rayleigh phase function
float phaseFunctionR(float mu) {
    return (3.0 / (16.0 * M_PI)) * (1.0 + mu * mu);
}

// Mie phase function
float phaseFunctionM(float mu) {
	return 1.5 * 1.0 / (4.0 * M_PI) * (1.0 - mieG*mieG) * pow(1.0 + (mieG*mieG) - 2.0*mieG*mu, -3.0/2.0) * (1.0 + mu * mu) / (2.0 + mieG*mieG);
}

// approximated single Mie scattering (cf. approximate Cm in paragraph "Angular precision")
vec3 getMie(vec4 rayMie) { // rayMie.rgb=C*, rayMie.w=Cm,r
	return rayMie.rgb * rayMie.w / max(rayMie.r, 1e-4) * (betaR.r / betaR);
}