src/material/dielectric.rs


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use super::Material;
use crate::{hittable::HitRecord, vec3::Vec3};
use crate::vec3::Color;
use crate::ray::Ray;

pub struct Dielectric {
    pub index_of_refraction: f64,
}

impl Dielectric {
    fn reflectance(cosine: f64, ref_idx: f64) -> f64 {
        // Using Schlick's Approximation:
        let mut r0 = (1.0 - ref_idx) / (1.0 + ref_idx);
        r0 *= r0;
        r0 + (1.0 - r0) * (1.0 - cosine).powi(5)
    }
}

impl Material for Dielectric {
    fn scatter(&self, ray_in: &Ray, hit_record: &HitRecord, attenuation: &mut Color, scattered: &mut Ray) -> bool {
        *attenuation = Color { x: 1.0, y: 1.0, z: 1.0 };
        let refraction_ratio = if hit_record.front_face { 1.0 / self.index_of_refraction } else { self.index_of_refraction };
        let unit_direction = ray_in.direction.unit_vector();
        let cos_theta = hit_record.normal.dot(&-&unit_direction).min(1.0);
        let sin_theta = (1.0 - cos_theta * cos_theta).sqrt();

        let cannot_refract = refraction_ratio * sin_theta > 1.0;
        let direction: Vec3;

        if cannot_refract || Self::reflectance(cos_theta, refraction_ratio) > rand::random::<f64>() {
            direction = unit_direction.reflect(&hit_record.normal)
        } else {
            direction = unit_direction.refract(&hit_record.normal, refraction_ratio)
        }

        *scattered = Ray { origin: hit_record.p.clone(), direction, time: ray_in.time };
        true
    }
}