lightME.cpp

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#include "lightME.h"
#include "../framework/constants.h"  // for atmospheric_pressure_at_sea_level
 
using physical_constants::atmospheric_pressure_at_sea_level;
 
Light_model lightME(
    double cosine_zenith_angle,        // dimensionless
    double atmospheric_pressure,       // Pa
    double atmospheric_transmittance,  // dimensionless
    double atmospheric_scattering      // dimensionless
)
{
    // Dimensionless quantity used in later calculations.
    double const pressure_ratio =
        atmospheric_pressure / atmospheric_pressure_at_sea_level;
 
    // Equation 11.1 from Campbell & Norman, solving for
    // direct_transmittance = S_p / S_p0 (dimensionless).
    // If the sun is near the horizon, take the limit as cosine_zenith_angle
    // approaches 0 (which is 0).
    double const direct_transmittance =
        cosine_zenith_angle <= 0 ? 0
                                 : pow(atmospheric_transmittance,
                                       (pressure_ratio / cosine_zenith_angle));
 
    // Equation 11.13 from Campbell & Norman, solving for
    // diffuse_transmittance = S_p / S_p0 (dimensionless).
    // If the sun is near the horizon, take the limit as cosine_zenith_angle
    // approaches 0 (which is 0).
    double const diffuse_transmittance =
        cosine_zenith_angle <= 0 ? 0
                                 : atmospheric_scattering *
                                       (1 - direct_transmittance) *
                                       cosine_zenith_angle;
 
    // The fraction of direct irradiance just above the canopy is the ratio of
    // the direct transmittance to the total transmittance (dimensionless).
    // If the sun is near the horizon, take the limit as cosine_zenith_angle
    // approaches 0 (which is 0).
    double const direct_fraction =
        cosine_zenith_angle <= 0 ? 0
                                 : direct_transmittance /
                                       (direct_transmittance + diffuse_transmittance);
 
    // The remaining irradiance is diffuse (dimensionless).
    double const diffuse_fraction = 1.0 - direct_fraction;
 
    return Light_model{
        /* .direct_transmittance = */ direct_transmittance,    // dimensionless
        /* .diffuse_transmittance = */ diffuse_transmittance,  // dimensionless
        /* .direct_fraction = */ direct_fraction,              // dimensionless
        /* .diffuse_fraction = */ diffuse_fraction             // dimensionless
    };
}