💿🐜 Antkeeper source code
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346 lines
12 KiB
346 lines
12 KiB
/*
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* Copyright (C) 2021 Christopher J. Howard
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*
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* This file is part of Antkeeper source code.
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*
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* Antkeeper source code is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* Antkeeper source code is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with Antkeeper source code. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include "ecs/systems/astronomy-system.hpp"
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#include "astro/apparent-size.hpp"
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#include "ecs/components/orbit-component.hpp"
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#include "ecs/components/blackbody-component.hpp"
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#include "ecs/components/atmosphere-component.hpp"
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#include "ecs/components/transform-component.hpp"
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#include "geom/intersection.hpp"
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#include "color/color.hpp"
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#include "physics/orbit/orbit.hpp"
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#include "physics/time/ut1.hpp"
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#include "physics/light/blackbody.hpp"
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#include "physics/light/photometry.hpp"
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#include "physics/light/luminosity.hpp"
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#include "geom/cartesian.hpp"
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#include <iostream>
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namespace ecs {
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/**
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* Calculates the optical depth between two points.
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*
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* @param start Start point.
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* @param end End point.
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* @param sample_count Number of samples to take between the start and end points.
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* @param scale_height Scale height of the atmospheric particles to measure.
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*/
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template <class T>
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T transmittance(math::vector3<T> start, math::vector3<T> end, std::size_t sample_count, T scale_height)
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{
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const T inverse_scale_height = T(1) / -scale_height;
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math::vector3<T> direction = end - start;
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T distance = length(direction);
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direction /= distance;
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// Calculate the distance between each sample point
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T step_distance = distance / T(sample_count);
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// Sum the atmospheric particle densities at each sample point
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T total_density = 0.0;
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math::vector3<T> sample_point = start;
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for (std::size_t i = 0; i < sample_count; ++i)
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{
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// Determine altitude of sample point
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T altitude = length(sample_point);
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// Calculate atmospheric particle density at sample altitude
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T density = exp(altitude * inverse_scale_height);
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// Add density to the total density
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total_density += density;
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// Advance to next sample point
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sample_point += direction * step_distance;
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}
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// Scale the total density by the step distance
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return total_density * step_distance;
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}
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astronomy_system::astronomy_system(ecs::registry& registry):
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entity_system(registry),
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universal_time(0.0),
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time_scale(1.0),
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reference_body(entt::null),
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reference_body_axial_tilt(0.0),
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reference_body_axial_rotation(0.0),
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sun_light(nullptr),
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sky_pass(nullptr)
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{}
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void astronomy_system::update(double t, double dt)
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{
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// Add scaled timestep to current time
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set_universal_time(universal_time + dt * time_scale);
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// Abort if reference body has not been set
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if (reference_body == entt::null)
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return;
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// Abort if reference body has no orbit component
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if (!registry.has<ecs::orbit_component>(reference_body))
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return;
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// Update axial rotation of reference body
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reference_body_axial_rotation = physics::time::ut1::era(universal_time);
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// Get orbit component of reference body
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const auto& reference_orbit = registry.get<ecs::orbit_component>(reference_body);
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/// Construct reference frame which transforms coordinates from inertial space to reference body BCBF space
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inertial_to_bcbf = physics::orbit::inertial::to_bcbf
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(
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reference_orbit.state.r,
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reference_orbit.elements.i,
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reference_body_axial_tilt,
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reference_body_axial_rotation
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);
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/// Construct reference frame which transforms coordinates from inertial space to reference body topocentric space
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inertial_to_topocentric = inertial_to_bcbf * bcbf_to_topocentric;
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// Set the transform component translations of orbiting bodies to their topocentric positions
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registry.view<orbit_component, transform_component>().each(
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[&](ecs::entity entity, auto& orbit, auto& transform)
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{
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// Transform Cartesian position vector (r) from inertial space to topocentric space
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const math::vector3<double> r_topocentric = inertial_to_topocentric * orbit.state.r;
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// Update local transform
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transform.local.translation = math::type_cast<float>(r_topocentric);
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});
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// Get atmosphere component of reference body, if any
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if (registry.has<ecs::atmosphere_component>(reference_body))
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{
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const ecs::atmosphere_component& atmosphere = registry.get<ecs::atmosphere_component>(reference_body);
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}
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if (sun_light != nullptr)
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{
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const math::vector3<double> sun_position_inertial = {0, 0, 0};
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const math::vector3<double> sun_forward_inertial = math::normalize(reference_orbit.state.r - sun_position_inertial);
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const math::vector3<double> sun_up_inertial = {0, 0, 1};
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// Transform sun position, forward, and up vectors into topocentric space
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const math::vector3<double> sun_position_topocentric = inertial_to_topocentric * sun_position_inertial;
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const math::vector3<double> sun_forward_topocentric = inertial_to_topocentric.rotation * sun_forward_inertial;
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const math::vector3<double> sun_up_topocentric = inertial_to_topocentric.rotation * sun_up_inertial;
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// Update sun light transform
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sun_light->set_translation(math::type_cast<float>(sun_position_topocentric));
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sun_light->set_rotation
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(
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math::look_rotation
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(
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math::type_cast<float>(sun_forward_topocentric),
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math::type_cast<float>(sun_up_topocentric)
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)
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);
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// Convert sun topocentric Cartesian coordinates to spherical coordinates
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math::vector3<double> sun_az_el = geom::cartesian::to_spherical(ezs_to_sez * sun_position_topocentric);
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sun_az_el.z = math::pi<double> - sun_az_el.z;
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//std::cout << "el: " << math::degrees(sun_az_el.y) << "; az: " << math::degrees(sun_az_el.z) << std::endl;
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// If the reference body has an atmosphere
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if (registry.has<ecs::atmosphere_component>(reference_body))
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{
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// Get the atmosphere component of the reference body
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const auto& atmosphere = registry.get<ecs::atmosphere_component>(reference_body);
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const double meters_per_au = 1.496e+11;
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const double earth_radius_au = 4.26352e-5;
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const double earth_radius_m = earth_radius_au * meters_per_au;
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const double observer_altitude_m = (observer_location[0] - earth_radius_au) * meters_per_au;
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// Altitude of observer in meters
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geom::ray<double> sample_ray;
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sample_ray.origin = {0, observer_altitude_m, 0};
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sample_ray.direction = math::normalize(sun_position_topocentric);
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geom::sphere<double> exosphere;
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exosphere.center = {0, -earth_radius_m, 0};
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exosphere.radius = atmosphere.exosphere_radius;
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auto intersection_result = geom::ray_sphere_intersection(sample_ray, exosphere);
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if (std::get<0>(intersection_result))
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{
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double3 sample_start = sample_ray.origin;
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double3 sample_end = sample_ray.extrapolate(std::get<2>(intersection_result));
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double transmittance_rayleigh = transmittance(sample_start, sample_end, 16, atmosphere.rayleigh_scale_height);
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double transmittance_mie = transmittance(sample_start, sample_end, 16, atmosphere.mie_scale_height);
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// Calculate attenuation due to atmospheric scattering
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double3 scattering_attenuation =
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atmosphere.rayleigh_scattering_coefficients * transmittance_rayleigh +
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atmosphere.mie_scattering_coefficients * transmittance_mie;
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scattering_attenuation.x = std::exp(-scattering_attenuation.x);
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scattering_attenuation.y = std::exp(-scattering_attenuation.y);
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scattering_attenuation.z = std::exp(-scattering_attenuation.z);
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double scattering_mean = (scattering_attenuation.x + scattering_attenuation.y + scattering_attenuation.z) / 3.0;
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const double sun_temperature = 5777.0;
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const double sun_radius = 6.957e+8;
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const double sun_surface_area = 4.0 * math::pi<double> * sun_radius * sun_radius;
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// Calculate distance attenuation
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double sun_distance_m = math::length(sun_position_topocentric) * meters_per_au;
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double distance_attenuation = 1.0 / (sun_distance_m * sun_distance_m);
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double sun_luminous_flux = blackbody_luminous_flux(sun_temperature, sun_radius);
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double sun_luminous_intensity = sun_luminous_flux / (4.0 * math::pi<double>);
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double sun_illuminance = sun_luminous_intensity / (sun_distance_m * sun_distance_m);
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std::cout << "distance atten: " << distance_attenuation << std::endl;
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std::cout << "scatter atten: " << scattering_attenuation << std::endl;
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std::cout << "luminous flux: " << sun_luminous_flux << std::endl;
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std::cout << "luminous intensity: " << sun_luminous_intensity << std::endl;
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std::cout << "illuminance: " << sun_illuminance * scattering_mean << std::endl;
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// Calculate sun color
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double3 color_xyz = color::cct::to_xyz(sun_temperature);
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double3 color_acescg = color::xyz::to_acescg(color_xyz);
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sun_light->set_color(math::type_cast<float>(color_acescg * scattering_attenuation));
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sun_light->set_intensity(sun_illuminance);
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}
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}
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}
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if (sky_pass != nullptr)
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{
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sky_pass->set_topocentric_frame
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(
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physics::frame<float>
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{
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math::type_cast<float>(inertial_to_topocentric.translation),
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math::type_cast<float>(inertial_to_topocentric.rotation)
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}
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);
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sky_pass->set_sun_object(sun_light);
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}
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}
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void astronomy_system::set_universal_time(double time)
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{
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universal_time = time;
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}
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void astronomy_system::set_time_scale(double scale)
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{
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time_scale = scale;
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}
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void astronomy_system::set_reference_body(ecs::entity entity)
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{
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reference_body = entity;
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}
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void astronomy_system::set_reference_body_axial_tilt(double angle)
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{
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reference_body_axial_tilt = angle;
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}
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void astronomy_system::set_observer_location(const double3& location)
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{
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observer_location = location;
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// Construct reference frame which transforms coordinates from SEZ to EZS
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sez_to_ezs = physics::frame<double>
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{
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{0, 0, 0},
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math::normalize
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(
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math::quaternion<double>::rotate_x(-math::half_pi<double>) *
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math::quaternion<double>::rotate_z(-math::half_pi<double>)
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)
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};
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// Construct reference frame which transforms coordinates from EZS to SEZ
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ezs_to_sez = sez_to_ezs.inverse();
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// Construct reference frame which transforms coordinates from BCBF space to topocentric space
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bcbf_to_topocentric = physics::orbit::bcbf::to_topocentric
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(
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observer_location[0], // Radial distance
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observer_location[1], // Latitude
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observer_location[2] // Longitude
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) * sez_to_ezs;
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}
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void astronomy_system::set_sun_light(scene::directional_light* light)
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{
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sun_light = light;
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}
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void astronomy_system::set_sky_pass(::sky_pass* pass)
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{
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this->sky_pass = pass;
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}
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double astronomy_system::blackbody_luminous_flux(double t, double r)
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{
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// Blackbody spectral power distribution function
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auto spd = [t](double x) -> double
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{
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// Convert nanometers to meters
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x *= double(1e-9);
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return physics::light::blackbody::spectral_radiance<double>(t, x, physics::constants::speed_of_light<double>);
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};
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// Luminous efficiency function (photopic)
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auto lef = [](double x) -> double
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{
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return physics::light::luminosity::photopic<double>(x);
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};
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// Construct range of spectral sample points
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std::vector<double> samples(10000);
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std::iota(samples.begin(), samples.end(), 10);
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// Calculate luminous efficiency
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const double efficiency = physics::light::luminous_efficiency<double>(spd, lef, samples.begin(), samples.end());
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// Calculate surface area of spherical blackbody
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const double a = double(4) * math::pi<double> * r * r;
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// Calculate radiant flux
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const double radiant_flux = physics::light::blackbody::radiant_flux(t, a);
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// Convert radiant flux to luminous flux
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const double luminous_flux = physics::light::watts_to_lumens<double>(radiant_flux, efficiency);
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return luminous_flux;
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}
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} // namespace ecs
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