/**
 * @file lllegacyatmospherics.cpp
 * @brief LLAtmospherics class implementation
 *
 * $LicenseInfo:firstyear=2001&license=viewerlgpl$
 * Second Life Viewer Source Code
 * Copyright (C) 2010, Linden Research, Inc.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation;
 * version 2.1 of the License only.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 *
 * Linden Research, Inc., 945 Battery Street, San Francisco, CA  94111  USA
 * $/LicenseInfo$
 */

#include "llviewerprecompiledheaders.h"

#include "lllegacyatmospherics.h"

#include "llfeaturemanager.h"
#include "llviewercontrol.h"
#include "llframetimer.h"

#include "llagent.h"
#include "llagentcamera.h"
#include "lldrawable.h"
#include "llface.h"
#include "llglheaders.h"
#include "llsky.h"
#include "llviewercamera.h"
#include "llviewertexturelist.h"
#include "llviewerobjectlist.h"
#include "llviewerregion.h"
#include "llworld.h"
#include "pipeline.h"
#include "v3colorutil.h"

#include "llsettingssky.h"
#include "llenvironment.h"
#include "lldrawpoolwater.h"

class LLFastLn
{
public:
    LLFastLn()
    {
        mTable[0] = 0;
        for( S32 i = 1; i < 257; i++ )
        {
            mTable[i] = log((F32)i);
        }
    }

    F32 ln( F32 x )
    {
        const F32 OO_255 = 0.003921568627450980392156862745098f;
        const F32 LN_255 = 5.5412635451584261462455391880218f;

        if( x < OO_255 )
        {
            return log(x);
        }
        else
        if( x < 1 )
        {
            x *= 255.f;
            S32 index = llfloor(x);
            F32 t = x - index;
            F32 low = mTable[index];
            F32 high = mTable[index + 1];
            return low + t * (high - low) - LN_255;
        }
        else
        if( x <= 255 )
        {
            S32 index = llfloor(x);
            F32 t = x - index;
            F32 low = mTable[index];
            F32 high = mTable[index + 1];
            return low + t * (high - low);
        }
        else
        {
            return log( x );
        }
    }

    F32 pow( F32 x, F32 y )
    {
        return (F32)LL_FAST_EXP(y * ln(x));
    }


private:
    F32 mTable[257]; // index 0 is unused
};

static LLFastLn gFastLn;


// Functions used a lot.

inline F32 LLHaze::calcPhase(const F32 cos_theta) const
{
    const F32 g2 = mG * mG;
    const F32 den = 1 + g2 - 2 * mG * cos_theta;
    return (1 - g2) * gFastLn.pow(den, -1.5);
}

inline void color_pow(LLColor3 &col, const F32 e)
{
    col.mV[0] = gFastLn.pow(col.mV[0], e);
    col.mV[1] = gFastLn.pow(col.mV[1], e);
    col.mV[2] = gFastLn.pow(col.mV[2], e);
}

inline LLColor3 color_norm(const LLColor3 &col)
{
    const F32 m = color_max(col);
    if (m > 1.f)
    {
        return 1.f/m * col;
    }
    else return col;
}

inline void color_gamma_correct(LLColor3 &col)
{
    const F32 gamma_inv = 1.f/1.2f;
    if (col.mV[0] != 0.f)
    {
        col.mV[0] = gFastLn.pow(col.mV[0], gamma_inv);
    }
    if (col.mV[1] != 0.f)
    {
        col.mV[1] = gFastLn.pow(col.mV[1], gamma_inv);
    }
    if (col.mV[2] != 0.f)
    {
        col.mV[2] = gFastLn.pow(col.mV[2], gamma_inv);
    }
}

static LLColor3 calc_air_sca_sea_level()
{
    static LLColor3 WAVE_LEN(675, 520, 445);
    static LLColor3 refr_ind = refr_ind_calc(WAVE_LEN);
    static LLColor3 n21 = refr_ind * refr_ind - LLColor3(1, 1, 1);
    static LLColor3 n4 = n21 * n21;
    static LLColor3 wl2 = WAVE_LEN * WAVE_LEN * 1e-6f;
    static LLColor3 wl4 = wl2 * wl2;
    static LLColor3 mult_const = fsigma * 2.0f/ 3.0f * 1e24f * (F_PI * F_PI) * n4;
    static F32 dens_div_N = F32( ATM_SEA_LEVEL_NDENS / Ndens2);
    return dens_div_N * mult_const.divide(wl4);
}

// static constants.
LLColor3 const LLHaze::sAirScaSeaLevel = calc_air_sca_sea_level();
F32 const LLHaze::sAirScaIntense = color_intens(LLHaze::sAirScaSeaLevel);
F32 const LLHaze::sAirScaAvg = LLHaze::sAirScaIntense / 3.f;

/***************************************
        Atmospherics
***************************************/

LLAtmospherics::LLAtmospherics()
:   mCloudDensity(0.2f),
    mWind(0.f),
    mWorldScale(1.f)
{
    /// WL PARAMS
    mInitialized = false;
    mAmbientScale = gSavedSettings.getF32("SkyAmbientScale");
    mNightColorShift = gSavedSettings.getColor3("SkyNightColorShift");
    mFogColor.mV[VRED] = mFogColor.mV[VGREEN] = mFogColor.mV[VBLUE] = 0.5f;
    mFogColor.mV[VALPHA] = 0.0f;
    mFogRatio = 1.2f;
    mHazeConcentration = 0.f;
    mInterpVal = 0.f;
}


LLAtmospherics::~LLAtmospherics()
{
}

void LLAtmospherics::init()
{
    const F32 haze_int = color_intens(mHaze.calcSigSca(0));
    mHazeConcentration = haze_int / (color_intens(mHaze.calcAirSca(0)) + haze_int);
    mInitialized = true;
}

// This cubemap is used as "environmentMap" in indra/newview/app_settings/shaders/class2/deferred/softenLightF.glsl
LLColor4 LLAtmospherics::calcSkyColorInDir(const LLSettingsSky::ptr_t &psky, AtmosphericsVars& vars, const LLVector3 &dir, bool isShiny, bool low_end)
{
    const F32 sky_saturation = 0.25f;
    const F32 land_saturation = 0.1f;

    if (isShiny && dir.mV[VZ] < -0.02f)
    {
        LLColor4 col;
        LLColor3 desat_fog = LLColor3(mFogColor);
        F32 brightness = desat_fog.brightness();// NOTE: Linear brightness!
        // So that shiny somewhat shows up at night.
        if (brightness < 0.15f)
        {
            brightness = 0.15f;
            desat_fog = smear(0.15f);
        }
        F32 greyscale_sat = brightness * (1.0f - land_saturation);
        desat_fog = desat_fog * land_saturation + smear(greyscale_sat);
        if (low_end)
        {
            col = LLColor4(desat_fog, 0.f);
        }
        else
        {
            col = LLColor4(desat_fog * 0.5f, 0.f);
        }
        float x = 1.0f-fabsf(-0.1f-dir.mV[VZ]);
        x *= x;
        col.mV[0] *= x*x;
        col.mV[1] *= powf(x, 2.5f);
        col.mV[2] *= x*x*x;
        return col;
    }

    // undo OGL_TO_CFR_ROTATION and negate vertical direction.
    LLVector3 Pn = LLVector3(-dir[1] , -dir[2], -dir[0]);

    //calculates hazeColor
    calcSkyColorWLVert(psky, Pn, vars);

    if (isShiny)
    {
        F32 brightness = vars.hazeColor.brightness();
        F32 greyscale_sat = brightness * (1.0f - sky_saturation);
        LLColor3 sky_color = vars.hazeColor * sky_saturation + smear(greyscale_sat);
        //sky_color *= (0.5f + 0.5f * brightness); // SL-12574 EEP sky is being attenuated too much
        return LLColor4(sky_color, 0.0f);
    }

    LLColor3 sky_color = low_end ? vars.hazeColor * 2.0f : psky->gammaCorrect(vars.hazeColor * 2.0f, vars.gamma);

    return LLColor4(sky_color, 0.0f);
}

// NOTE: Keep these in sync!
//       indra\newview\app_settings\shaders\class1\deferred\skyV.glsl
//       indra\newview\app_settings\shaders\class1\deferred\cloudsV.glsl
//       indra\newview\lllegacyatmospherics.cpp
void LLAtmospherics::calcSkyColorWLVert(const LLSettingsSky::ptr_t &psky, LLVector3 & Pn, AtmosphericsVars& vars)
{
    const LLColor3    blue_density = vars.blue_density;
    const LLColor3    blue_horizon = vars.blue_horizon;
    const F32         haze_horizon = vars.haze_horizon;
    const F32         haze_density = vars.haze_density;
    const F32         density_multiplier = vars.density_multiplier;

    F32         max_y = vars.max_y;
    LLVector4   sun_norm = vars.sun_norm;

    // project the direction ray onto the sky dome.
    F32 phi = acos(Pn[1]);
    F32 sinA = sin(F_PI - phi);
    if (fabsf(sinA) < 0.01f)
    { //avoid division by zero
        sinA = 0.01f;
    }

    F32 Plen = vars.dome_radius * sin(F_PI + phi + asin(vars.dome_offset * sinA)) / sinA;

    Pn *= Plen;

    // Set altitude
    if (Pn[1] > 0.f)
    {
        Pn *= (max_y / Pn[1]);
    }
    else
    {
        Pn *= (-32000.f / Pn[1]);
    }

    Plen = Pn.length();
    Pn /= Plen;

    // Initialize temp variables
    LLColor3 sunlight = vars.sunlight;
    LLColor3 ambient = vars.ambient;

    LLColor3 glow = vars.glow;
    F32 cloud_shadow = vars.cloud_shadow;

    // Sunlight attenuation effect (hue and brightness) due to atmosphere
    // this is used later for sunlight modulation at various altitudes
    LLColor3 light_atten = vars.light_atten;
    LLColor3 light_transmittance = psky->getLightTransmittanceFast(vars.total_density, vars.density_multiplier, Plen);
    (void)light_transmittance; // silence Clang warn-error

    // Calculate relative weights
    LLColor3 temp2(0.f, 0.f, 0.f);
    LLColor3 temp1 = vars.total_density;

    LLColor3 blue_weight = componentDiv(blue_density, temp1);
    LLColor3 blue_factor = blue_horizon * blue_weight;
    LLColor3 haze_weight = componentDiv(smear(haze_density), temp1);
    LLColor3 haze_factor = haze_horizon * haze_weight;


    // Compute sunlight from P & lightnorm (for long rays like sky)
    temp2.mV[1] = llmax(F_APPROXIMATELY_ZERO, llmax(0.f, Pn[1]) * 1.0f + sun_norm.mV[1] );

    temp2.mV[1] = 1.f / temp2.mV[1];
    componentMultBy(sunlight, componentExp((light_atten * -1.f) * temp2.mV[1]));
    componentMultBy(sunlight, light_transmittance);

    // Distance
    temp2.mV[2] = Plen * density_multiplier;

    // Transparency (-> temp1)
    temp1 = componentExp((temp1 * -1.f) * temp2.mV[2]);

    // Compute haze glow
    temp2.mV[0] = Pn * LLVector3(sun_norm);

    temp2.mV[0] = 1.f - temp2.mV[0];
        // temp2.x is 0 at the sun and increases away from sun
    temp2.mV[0] = llmax(temp2.mV[0], .001f);
        // Set a minimum "angle" (smaller glow.y allows tighter, brighter hotspot)

    // Higher glow.x gives dimmer glow (because next step is 1 / "angle")
    temp2.mV[0] *= glow.mV[0];

    temp2.mV[0] = pow(temp2.mV[0], glow.mV[2]);
        // glow.z should be negative, so we're doing a sort of (1 / "angle") function

    // Add "minimum anti-solar illumination"
    temp2.mV[0] += .25f;


    // Haze color above cloud
    vars.hazeColor = (blue_factor * (sunlight + ambient) + componentMult(haze_factor, sunlight * temp2.mV[0] + ambient));

    // Increase ambient when there are more clouds
    LLColor3 tmpAmbient = ambient + (LLColor3::white - ambient) * cloud_shadow * 0.5f;

    // Dim sunlight by cloud shadow percentage
    sunlight *= (1.f - cloud_shadow);

    // Haze color below cloud
    vars.hazeColorBelowCloud = (blue_factor * (sunlight + tmpAmbient) + componentMult(haze_factor, sunlight * temp2.mV[0] + tmpAmbient));

    // Final atmosphere additive
    componentMultBy(vars.hazeColor, LLColor3::white - temp1);

/*
    // SL-12574

    // Attenuate cloud color by atmosphere
    temp1 = componentSqrt(temp1);   //less atmos opacity (more transparency) below clouds

    // At horizon, blend high altitude sky color towards the darker color below the clouds
    vars.hazeColor += componentMult(vars.hazeColorBelowCloud - vars.hazeColor, LLColor3::white - componentSqrt(temp1));
*/
}

void LLAtmospherics::updateFog(const F32 distance, const LLVector3& tosun_in)
{
    LLVector3 tosun = tosun_in;

    if (!gPipeline.hasRenderDebugFeatureMask(LLPipeline::RENDER_DEBUG_FEATURE_FOG))
    {
        return;
    }

    LLColor4 target_fog(0.f, 0.2f, 0.5f, 0.f);

    const F32 water_height = gAgent.getRegion() ? gAgent.getRegion()->getWaterHeight() : 0.f;
    // LLWorld::getInstance()->getWaterHeight();
    F32 camera_height = gAgentCamera.getCameraPositionAgent().mV[2];

    F32 near_clip_height = LLViewerCamera::getInstance()->getAtAxis().mV[VZ] * LLViewerCamera::getInstance()->getNear();
    camera_height += near_clip_height;

    F32 fog_distance = 0.f;
    LLColor3 res_color[3];

    LLColor3 sky_fog_color = LLColor3::white;
    LLColor3 render_fog_color = LLColor3::white;

    const F32 tosun_z = tosun.mV[VZ];
    tosun.mV[VZ] = 0.f;
    tosun.normalize();
    LLVector3 perp_tosun;
    perp_tosun.mV[VX] = -tosun.mV[VY];
    perp_tosun.mV[VY] = tosun.mV[VX];
    LLVector3 tosun_45 = tosun + perp_tosun;
    tosun_45.normalize();

    F32 delta = 0.06f;
    tosun.mV[VZ] = delta;
    perp_tosun.mV[VZ] = delta;
    tosun_45.mV[VZ] = delta;
    tosun.normalize();
    perp_tosun.normalize();
    tosun_45.normalize();

    // Sky colors, just slightly above the horizon in the direction of the sun, perpendicular to the sun, and at a 45 degree angle to the sun.
    AtmosphericsVars vars;

    LLSettingsSky::ptr_t psky = LLEnvironment::instance().getCurrentSky();

    // NOTE: This is very similar to LLVOSky::cacheEnvironment()
    // Differences:
    //     vars.sun_norm
    //     vars.sunlight
    // invariants across whole sky tex process...
    vars.blue_density = psky->getBlueDensity();
    vars.blue_horizon = psky->getBlueHorizon();
    vars.haze_density = psky->getHazeDensity();
    vars.haze_horizon = psky->getHazeHorizon();
    vars.density_multiplier = psky->getDensityMultiplier();
    vars.distance_multiplier = psky->getDistanceMultiplier();
    vars.max_y = psky->getMaxY();
    vars.sun_norm = LLEnvironment::instance().getSunDirectionCFR();
    vars.sunlight = psky->getSunlightColor();
    vars.ambient = psky->getAmbientColor();
    vars.glow = psky->getGlow();
    vars.cloud_shadow = psky->getCloudShadow();
    vars.dome_radius = psky->getDomeRadius();
    vars.dome_offset = psky->getDomeOffset();
    vars.light_atten = psky->getLightAttenuation(vars.max_y);
    vars.light_transmittance = psky->getLightTransmittance(vars.max_y);
    vars.total_density = psky->getTotalDensity();
    vars.gamma = psky->getGamma();

    res_color[0] = calcSkyColorInDir(psky, vars, tosun);
    res_color[1] = calcSkyColorInDir(psky, vars, perp_tosun);
    res_color[2] = calcSkyColorInDir(psky, vars, tosun_45);

    sky_fog_color = color_norm(res_color[0] + res_color[1] + res_color[2]);

    F32 full_off = -0.25f;
    F32 full_on = 0.00f;
    F32 on = (tosun_z - full_off) / (full_on - full_off);
    on = llclamp(on, 0.01f, 1.f);
    sky_fog_color *= 0.5f * on;


    // We need to clamp these to non-zero, in order for the gamma correction to work. 0^y = ???
    S32 i;
    for (i = 0; i < 3; i++)
    {
        sky_fog_color.mV[i] = llmax(0.0001f, sky_fog_color.mV[i]);
    }

    color_gamma_correct(sky_fog_color);

    render_fog_color = sky_fog_color;

    fog_distance = mFogRatio * distance;

    if (camera_height > water_height)
    {
        LLColor4 fog(render_fog_color);
        mGLFogCol = fog;
    }
    else
    {
        LLSettingsWater::ptr_t pwater = LLEnvironment::instance().getCurrentWater();
        F32 depth = water_height - camera_height;
        LLColor4 water_fog_color(pwater->getWaterFogColor());

        // adjust the color based on depth.  We're doing linear approximations
        float depth_scale = gSavedSettings.getF32("WaterGLFogDepthScale");
        float depth_modifier = 1.0f - llmin(llmax(depth / depth_scale, 0.01f),
            gSavedSettings.getF32("WaterGLFogDepthFloor"));

        LLColor4 fogCol = water_fog_color * depth_modifier;
        fogCol.setAlpha(1);

        // set the gl fog color
        mGLFogCol = fogCol;
    }

    mFogColor = sky_fog_color;
    mFogColor.setAlpha(1);

    LLDrawPoolWater::sWaterFogEnd = fog_distance*2.2f;

    stop_glerror();
}

// Functions used a lot.
F32 color_norm_pow(LLColor3& col, F32 e, bool postmultiply)
{
    F32 mv = color_max(col);
    if (0 == mv)
    {
        return 0;
    }

    col *= 1.f / mv;
    color_pow(col, e);
    if (postmultiply)
    {
        col *= mv;
    }
    return mv;
}

// Returns angle (RADIANs) between the horizontal projection of "v" and the x_axis.
// Range of output is 0.0f to 2pi //359.99999...f
// Returns 0.0f when "v" = +/- z_axis.
F32 azimuth(const LLVector3 &v)
{
    F32 azimuth = 0.0f;
    if (v.mV[VX] == 0.0f)
    {
        if (v.mV[VY] > 0.0f)
        {
            azimuth = F_PI * 0.5f;
        }
        else if (v.mV[VY] < 0.0f)
        {
            azimuth = F_PI * 1.5f;// 270.f;
        }
    }
    else
    {
        azimuth = (F32) atan(v.mV[VY] / v.mV[VX]);
        if (v.mV[VX] < 0.0f)
        {
            azimuth += F_PI;
        }
        else if (v.mV[VY] < 0.0f)
        {
            azimuth += F_PI * 2;
        }
    }
    return azimuth;
}

bool operator==(const AtmosphericsVars& a, const AtmosphericsVars& b)
{
    if (a.hazeColor != b.hazeColor)
    {
        return false;
    }

    if (a.hazeColorBelowCloud != b.hazeColorBelowCloud)
    {
        return false;
    }

    if (a.cloudColorSun != b.cloudColorSun)
    {
        return false;
    }

    if (a.cloudColorAmbient != b.cloudColorAmbient)
    {
        return false;
    }

    if (a.cloudDensity != b.cloudDensity)
    {
        return false;
    }

    if (a.density_multiplier != b.density_multiplier)
    {
        return false;
    }

    if (a.haze_horizon != b.haze_horizon)
    {
        return false;
    }

    if (a.haze_density != b.haze_density)
    {
        return false;
    }

    if (a.blue_horizon != b.blue_horizon)
    {
        return false;
    }

    if (a.blue_density != b.blue_density)
    {
        return false;
    }

    if (a.dome_offset != b.dome_offset)
    {
        return false;
    }

    if (a.dome_radius != b.dome_radius)
    {
        return false;
    }

    if (a.cloud_shadow != b.cloud_shadow)
    {
        return false;
    }

    if (a.glow != b.glow)
    {
        return false;
    }

    if (a.ambient != b.ambient)
    {
        return false;
    }

    if (a.sunlight != b.sunlight)
    {
        return false;
    }

    if (a.sun_norm != b.sun_norm)
    {
        return false;
    }

    if (a.gamma != b.gamma)
    {
        return false;
    }

    if (a.max_y != b.max_y)
    {
        return false;
    }

    if (a.distance_multiplier != b.distance_multiplier)
    {
        return false;
    }

    // light_atten, light_transmittance, total_density
    // are ignored as they always change when the values above do
    // they're just shared calc across the sky map generation to save cycles

    return true;
}

bool approximatelyEqual(const F32 &a, const  F32 &b, const F32 &fraction_treshold)
{
    F32 diff = fabs(a - b);
    if (diff < F_APPROXIMATELY_ZERO || diff < llmax(fabs(a), fabs(b)) * fraction_treshold)
    {
        return true;
    }
    return false;
}

bool approximatelyEqual(const LLColor3 &a, const  LLColor3 &b, const F32 &fraction_treshold)
{
    return approximatelyEqual(a.mV[0], b.mV[0], fraction_treshold)
           && approximatelyEqual(a.mV[1], b.mV[1], fraction_treshold)
           && approximatelyEqual(a.mV[2], b.mV[2], fraction_treshold);
}

bool approximatelyEqual(const LLVector4 &a, const  LLVector4 &b, const F32 &fraction_treshold)
{
    return approximatelyEqual(a.mV[0], b.mV[0], fraction_treshold)
        && approximatelyEqual(a.mV[1], b.mV[1], fraction_treshold)
        && approximatelyEqual(a.mV[2], b.mV[2], fraction_treshold)
        && approximatelyEqual(a.mV[3], b.mV[3], fraction_treshold);
}

bool approximatelyEqual(const AtmosphericsVars& a, const AtmosphericsVars& b, const F32 fraction_treshold)
{
    if (!approximatelyEqual(a.hazeColor, b.hazeColor, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.hazeColorBelowCloud, b.hazeColorBelowCloud, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.cloudColorSun, b.cloudColorSun, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.cloudColorAmbient, b.cloudColorAmbient, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.cloudDensity, b.cloudDensity, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.density_multiplier, b.density_multiplier, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.haze_horizon, b.haze_horizon, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.haze_density, b.haze_density, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.blue_horizon, b.blue_horizon, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.blue_density, b.blue_density, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.dome_offset, b.dome_offset, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.dome_radius, b.dome_radius, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.cloud_shadow, b.cloud_shadow, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.glow, b.glow, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.ambient, b.ambient, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.sunlight, b.sunlight, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.sun_norm, b.sun_norm, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.gamma, b.gamma, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.max_y, b.max_y, fraction_treshold))
    {
        return false;
    }

    if (!approximatelyEqual(a.distance_multiplier, b.distance_multiplier, fraction_treshold))
    {
        return false;
    }

    // light_atten, light_transmittance, total_density
    // are ignored as they always change when the values above do
    // they're just shared calc across the sky map generation to save cycles

    return true;
}