💿🐜 Antkeeper source code
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383 lines
13 KiB
383 lines
13 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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* Approximates the density of exponentially-distributed atmospheric particles between two points using the trapezoidal rule.
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*
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* @param a Start point.
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* @param b End point.
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* @param r Radius of the planet.
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* @param sh Scale height of the atmospheric particles.
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* @param n Number of samples.
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*/
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template <class T>
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T optical_depth(const math::vector3<T>& a, const math::vector3<T>& b, T r, T sh, std::size_t n)
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{
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T inverse_sh = T(-1) / sh;
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T h = math::length(b - a) / T(n);
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math::vector3<T> dy = (b - a) / T(n);
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math::vector3<T> y = a + dy;
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T f_x = std::exp((length(a) - r) * inverse_sh);
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T f_y = std::exp((length(y) - r) * inverse_sh);
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T sum = (f_x + f_y);
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for (std::size_t i = 1; i < n; ++i)
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{
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f_x = f_y;
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y += dy;
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f_y = std::exp((length(y) - r) * inverse_sh);
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sum += (f_x + f_y);
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}
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return sum / T(2) * h;
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}
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template <class T>
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math::vector3<T> transmittance(T depth_r, T depth_m, T depth_o, const math::vector3<T>& beta_r, const math::vector3<T>& beta_m)
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{
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math::vector3<T> transmittance_r = beta_r * depth_r;
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math::vector3<T> transmittance_m = beta_m * depth_m;
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math::vector3<T> transmittance_o = {0, 0, 0};
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math::vector3<T> t = transmittance_r + transmittance_m + transmittance_o;
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t.x = std::exp(-t.x);
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t.y = std::exp(-t.y);
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t.z = std::exp(-t.z);
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return t;
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}
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double calc_beta_r(double wavelength, double ior, double density)
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{
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double wavelength2 = wavelength * wavelength;
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double ior2m1 = ior * ior - 1.0;
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double num = 8.0 * (math::pi<double> * math::pi<double> * math::pi<double>) * ior2m1 * ior2m1;
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double den = 3.0 * density * (wavelength2 * wavelength2);
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return num / den;
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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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registry.on_construct<ecs::blackbody_component>().connect<&astronomy_system::on_blackbody_construct>(this);
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registry.on_replace<ecs::blackbody_component>().connect<&astronomy_system::on_blackbody_replace>(this);
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registry.on_construct<ecs::atmosphere_component>().connect<&astronomy_system::on_atmosphere_construct>(this);
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registry.on_replace<ecs::atmosphere_component>().connect<&astronomy_system::on_atmosphere_replace>(this);
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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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// Update blackbody lighting
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registry.view<blackbody_component, orbit_component>().each(
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[&](ecs::entity entity, auto& blackbody, auto& orbit)
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{
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// Calculate blackbody inertial basis
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double3 blackbody_forward_inertial = math::normalize(reference_orbit.state.r - orbit.state.r);
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double3 blackbody_up_inertial = {0, 0, 1};
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// Transform blackbody inertial position and basis into topocentric space
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double3 blackbody_position_topocentric = inertial_to_topocentric * orbit.state.r;
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double3 blackbody_forward_topocentric = inertial_to_topocentric.rotation * blackbody_forward_inertial;
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double3 blackbody_up_topocentric = inertial_to_topocentric.rotation * blackbody_up_inertial;
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// Calculate distance from observer to blackbody
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const double meters_per_au = 1.496e+11;
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double blackbody_distance = math::length(blackbody_position_topocentric) * meters_per_au;
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// Calculate blackbody illuminance according to distance
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double blackbody_illuminance = blackbody.luminous_intensity / (blackbody_distance * blackbody_distance);
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// Get blackbody color
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double3 blackbody_color = blackbody.color;
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// Get atmosphere component of reference body, if any
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if (this->registry.has<ecs::atmosphere_component>(reference_body))
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{
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const ecs::atmosphere_component& atmosphere = this->registry.get<ecs::atmosphere_component>(reference_body);
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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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// Altitude of observer in meters
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geom::ray<double> sample_ray;
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sample_ray.origin = {0, observer_location[0] * meters_per_au, 0};
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sample_ray.direction = math::normalize(blackbody_position_topocentric);
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geom::sphere<double> exosphere;
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exosphere.center = {0, 0, 0};
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exosphere.radius = earth_radius_m + atmosphere.exosphere_altitude;
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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 optical_depth_r = optical_depth(sample_start, sample_end, earth_radius_m, atmosphere.rayleigh_scale_height, 32);
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double optical_depth_k = optical_depth(sample_start, sample_end, earth_radius_m, atmosphere.mie_scale_height, 32);
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double optical_depth_o = 0.0;
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double3 attenuation = transmittance(optical_depth_r, optical_depth_k, optical_depth_o, atmosphere.rayleigh_scattering_coefficients, atmosphere.mie_scattering_coefficients);
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// Attenuate blackbody color
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blackbody_color *= attenuation;
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}
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}
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if (sun_light != nullptr)
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{
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// Update blackbody light transform
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sun_light->set_translation(math::type_cast<float>(blackbody_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>(blackbody_forward_topocentric),
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math::type_cast<float>(blackbody_up_topocentric)
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)
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);
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// Update blackbody light color and intensity
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sun_light->set_color(math::type_cast<float>(blackbody_color));
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sun_light->set_intensity(static_cast<float>(blackbody_illuminance));
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// Pass blackbody params to sky pas
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if (this->sky_pass)
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{
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this->sky_pass->set_sun_object(sun_light);
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this->sky_pass->set_sun_color(math::type_cast<float>(blackbody.color * blackbody_illuminance));
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}
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}
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});
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// Update sky pass topocentric frame
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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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}
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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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void astronomy_system::on_blackbody_construct(ecs::registry& registry, ecs::entity entity, ecs::blackbody_component& blackbody)
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{
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on_blackbody_replace(registry, entity, blackbody);
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}
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void astronomy_system::on_blackbody_replace(ecs::registry& registry, ecs::entity entity, ecs::blackbody_component& blackbody)
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{
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// Calculate surface area of spherical blackbody
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const double surface_area = double(4) * math::pi<double> * blackbody.radius * blackbody.radius;
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// Calculate radiant flux
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blackbody.radiant_flux = physics::light::blackbody::radiant_flux(blackbody.temperature, surface_area);
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// Blackbody spectral power distribution function
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auto spd = [blackbody](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>(blackbody.temperature, 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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// Convert radiant flux to luminous flux
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blackbody.luminous_flux = physics::light::watts_to_lumens<double>(blackbody.radiant_flux, efficiency);
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// Calculate luminous intensity from luminous flux
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blackbody.luminous_intensity = blackbody.luminous_flux / (4.0 * math::pi<double>);
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// Calculate blackbody color from temperature
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double3 color_xyz = color::cct::to_xyz(blackbody.temperature);
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blackbody.color = color::xyz::to_acescg(color_xyz);
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}
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void astronomy_system::on_atmosphere_construct(ecs::registry& registry, ecs::entity entity, ecs::atmosphere_component& atmosphere)
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{
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on_atmosphere_replace(registry, entity, atmosphere);
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}
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void astronomy_system::on_atmosphere_replace(ecs::registry& registry, ecs::entity entity, ecs::atmosphere_component& atmosphere)
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{
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// ACEScg wavelengths determined by matching wavelengths to XYZ, transforming XYZ to ACEScg, then selecting the max wavelengths for R, G, and B.
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const double3 acescg_wavelengths_nm = {600.0, 540.0, 450.0};
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const double3 acescg_wavelengths_m = acescg_wavelengths_nm * 1.0e-9;
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// Calculate Rayleigh scattering coefficients
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const double air_ior = 1.0003;
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const double molecular_density = 2.545e25;
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double3 beta_r;
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atmosphere.rayleigh_scattering_coefficients =
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{
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calc_beta_r(acescg_wavelengths_m.x, air_ior, molecular_density),
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calc_beta_r(acescg_wavelengths_m.y, air_ior, molecular_density),
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calc_beta_r(acescg_wavelengths_m.z, air_ior, molecular_density)
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};
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// Calculate Mie scattering coefficients
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atmosphere.mie_scattering_coefficients = {2.0e-6, 2.0e-6, 2.0e-6};
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}
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} // namespace ecs
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