vanrijn/src/integrators.rs

use crate::math::Vec3;

use super::colour::{ColourRgbF, Photon, Spectrum};
use super::raycasting::{IntersectionInfo, Ray};
use super::sampler::Sampler;
use super::util::algebra_utils::try_change_of_basis_matrix;

pub trait Integrator {
    fn integrate(
        &self,
        sampler: &Sampler,
        info: &IntersectionInfo,
        photon: &Photon,
        recursion_limit: u16,
    ) -> Photon;
}

pub struct DirectionalLight {
    pub direction: Vec3,
    pub spectrum: Spectrum,
}

pub struct WhittedIntegrator {
    pub ambient_light: Spectrum,
    pub lights: Vec<DirectionalLight>,
}

impl Integrator for WhittedIntegrator {
    fn integrate(
        &self,
        sampler: &Sampler,
        info: &IntersectionInfo,
        photon: &Photon,
        recursion_limit: u16,
    ) -> Photon {
        let world_to_bsdf_space =
            try_change_of_basis_matrix(&info.tangent, &info.cotangent, &info.normal)
                .expect("Normal, tangent and cotangent don't for a valid basis.");
        let bsdf_to_world_space = world_to_bsdf_space
            .try_inverse()
            .expect("Expected matrix to be invertable.");
        self.lights
            .iter()
            .map(|light| {
                match sampler.sample(&Ray::new(info.location, light.direction).bias(0.000_000_1)) {
                    Some(_) => self.ambient_light.emit_photon(&photon),
                    None => info.material.bsdf()(
                        &(world_to_bsdf_space * info.retro),
                        &(world_to_bsdf_space * light.direction),
                        &light
                            .spectrum
                            .emit_photon(&photon)
                            .scale_intensity(light.direction.dot(&info.normal).abs()),
                    ),
                }
            })
            .chain(
                [info
                    .material
                    .sample(&(world_to_bsdf_space * info.retro), &photon)]
                .iter()
                .map(|direction| {
                    let world_space_direction = bsdf_to_world_space * direction;
                    match sampler
                        .sample(&Ray::new(info.location, world_space_direction).bias(0.000_000_1))
                    {
                        Some(recursive_hit) => {
                            if recursion_limit > 0 {
                                let photon = info.material.bsdf()(
                                    &(world_to_bsdf_space * info.retro),
                                    direction,
                                    &self.integrate(
                                        &sampler,
                                        &recursive_hit,
                                        &photon,
                                        recursion_limit - 1,
                                    ),
                                );
                                photon
                                    .scale_intensity(world_space_direction.dot(&info.normal).abs())
                            } else {
                                photon.scale_intensity(0.0)
                            }
                        }
                        None => photon.scale_intensity(0.0),
                    }
                }),
            )
            .fold(photon.clone(), |a, b| {
                let mut result = a;
                result.intensity += b.intensity;
                result
            })
    }
}

pub struct SimpleRandomIntegrator {}

impl Integrator for SimpleRandomIntegrator {
    fn integrate(
        &self,
        sampler: &Sampler,
        info: &IntersectionInfo,
        photon: &Photon,
        recursion_limit: u16,
    ) -> Photon {
        if recursion_limit == 0 {
            return Photon {
                wavelength: 0.0,
                intensity: 0.0,
            };
        }
        let world_to_bsdf_space =
            try_change_of_basis_matrix(&info.tangent, &info.cotangent, &info.normal)
                .expect("Normal, tangent and cotangent don't form a valid basis.");
        let bsdf_to_world_space = world_to_bsdf_space
            .try_inverse()
            .expect("Expected matrix to be invertable.");
        let world_space_w_i = info.retro;
        let w_i = world_to_bsdf_space * world_space_w_i;
        let w_o = info.material.sample(&w_i, &photon);
        let world_space_w_o = bsdf_to_world_space * w_o;
        info.material.bsdf()(
            &w_o.normalize(),
            &w_i.normalize(),
            &match sampler.sample(&Ray::new(info.location, world_space_w_o).bias(0.000_000_1)) {
                None => photon.set_intensity(test_lighting_environment(
                    &world_space_w_o,
                    photon.wavelength,
                )),
                Some(recursive_hit) => {
                    self.integrate(&sampler, &recursive_hit, &photon, recursion_limit - 1)
                }
            }
            .scale_intensity(world_space_w_o.dot(&info.normal).abs()),
        )
    }
}

pub fn test_lighting_environment(w_o: &Vec3, wavelength: f64) -> f64 {
    let sun_direction = Vec3::new(1.0, 1.0, -1.0).normalize();
    if w_o.dot(&sun_direction) >= 0.99 {
        300.0
    } else {
        let sky_colour = ColourRgbF::new(w_o.y(), w_o.y(), 1.0) * 0.1;
        Spectrum::reflection_from_linear_rgb(&sky_colour).intensity_at_wavelength(wavelength)
    }
}