psychopath/src/light/rectangle_light.rs

use mem_arena::MemArena;

use bbox::BBox;
use boundable::Boundable;
use color::{XYZ, SpectralSample, Color};
use lerp::lerp_slice;
use math::{Vector, Point, Matrix4x4};
use ray::{Ray, AccelRay};
use sampling::{spherical_triangle_solid_angle, uniform_sample_spherical_triangle};
use shading::SurfaceShader;
use surface::{Surface, SurfaceIntersection};

use super::SurfaceLight;


#[derive(Copy, Clone, Debug)]
pub struct RectangleLight<'a> {
    dimensions: &'a [(f32, f32)],
    colors: &'a [XYZ],
    bounds_: &'a [BBox],
}

impl<'a> RectangleLight<'a> {
    pub fn new<'b>(
        arena: &'b MemArena,
        dimensions: Vec<(f32, f32)>,
        colors: Vec<XYZ>,
    ) -> RectangleLight<'b> {
        let bbs: Vec<_> = dimensions
            .iter()
            .map(|d| {
                BBox {
                    min: Point::new(d.0 * -0.5, d.1 * -0.5, 0.0),
                    max: Point::new(d.0 * 0.5, d.1 * 0.5, 0.0),
                }
            })
            .collect();
        RectangleLight {
            dimensions: arena.copy_slice(&dimensions),
            colors: arena.copy_slice(&colors),
            bounds_: arena.copy_slice(&bbs),
        }
    }

    // fn sample_pdf(
    //     &self,
    //     space: &Matrix4x4,
    //     arr: Point,
    //     sample_dir: Vector,
    //     sample_u: f32,
    //     sample_v: f32,
    //     wavelength: f32,
    //     time: f32,
    // ) -> f32 {
    //     // We're not using these, silence warnings
    //     let _ = (sample_dir, sample_u, sample_v, wavelength);

    //     let dim = lerp_slice(self.dimensions, time);

    //     // Get the four corners of the rectangle, transformed into world space
    //     let space_inv = space.inverse();
    //     let p1 = Point::new(dim.0 * 0.5, dim.1 * 0.5, 0.0) * space_inv;
    //     let p2 = Point::new(dim.0 * -0.5, dim.1 * 0.5, 0.0) * space_inv;
    //     let p3 = Point::new(dim.0 * -0.5, dim.1 * -0.5, 0.0) * space_inv;
    //     let p4 = Point::new(dim.0 * 0.5, dim.1 * -0.5, 0.0) * space_inv;

    //     // Get the four corners of the rectangle, projected on to the unit
    //     // sphere centered around arr.
    //     let sp1 = (p1 - arr).normalized();
    //     let sp2 = (p2 - arr).normalized();
    //     let sp3 = (p3 - arr).normalized();
    //     let sp4 = (p4 - arr).normalized();

    //     // Get the solid angles of the rectangle split into two triangles
    //     let area_1 = spherical_triangle_solid_angle(sp2, sp1, sp3);
    //     let area_2 = spherical_triangle_solid_angle(sp4, sp1, sp3);

    //     1.0 / (area_1 + area_2)
    // }

    // fn outgoing(
    //     &self,
    //     space: &Matrix4x4,
    //     dir: Vector,
    //     u: f32,
    //     v: f32,
    //     wavelength: f32,
    //     time: f32,
    // ) -> SpectralSample {
    //     // We're not using these, silence warnings
    //     let _ = (space, dir, u, v);

    //     let dim = lerp_slice(self.dimensions, time);
    //     let col = lerp_slice(self.colors, time);

    //     // TODO: Is this right?  Do we need to get the surface area post-transform?
    //     let surface_area_inv: f64 = 1.0 / (dim.0 as f64 * dim.1 as f64);

    //     (col * surface_area_inv as f32 * 0.5).to_spectral_sample(wavelength)
    // }
}

impl<'a> SurfaceLight for RectangleLight<'a> {
    fn sample_from_point(
        &self,
        space: &Matrix4x4,
        arr: Point,
        u: f32,
        v: f32,
        wavelength: f32,
        time: f32,
    ) -> (SpectralSample, Vector, f32) {
        // Calculate time interpolated values
        let dim = lerp_slice(self.dimensions, time);
        let col = lerp_slice(self.colors, time);

        // TODO: Is this right?  Do we need to get the surface area post-transform?
        let surface_area_inv: f64 = 1.0 / (dim.0 as f64 * dim.1 as f64);

        // Get the four corners of the rectangle, transformed into world space
        let space_inv = space.inverse();
        let p1 = Point::new(dim.0 * 0.5, dim.1 * 0.5, 0.0) * space_inv;
        let p2 = Point::new(dim.0 * -0.5, dim.1 * 0.5, 0.0) * space_inv;
        let p3 = Point::new(dim.0 * -0.5, dim.1 * -0.5, 0.0) * space_inv;
        let p4 = Point::new(dim.0 * 0.5, dim.1 * -0.5, 0.0) * space_inv;

        // Get the four corners of the rectangle, projected on to the unit
        // sphere centered around arr.
        let sp1 = (p1 - arr).normalized();
        let sp2 = (p2 - arr).normalized();
        let sp3 = (p3 - arr).normalized();
        let sp4 = (p4 - arr).normalized();

        // Get the solid angles of the rectangle split into two triangles
        let area_1 = spherical_triangle_solid_angle(sp2, sp1, sp3);
        let area_2 = spherical_triangle_solid_angle(sp4, sp1, sp3);

        // Normalize the solid angles for selection purposes
        let prob_1 = area_1 / (area_1 + area_2);
        let prob_2 = 1.0 - prob_1;

        // Select one of the triangles and sample it
        let shadow_vec = if u < prob_1 {
            uniform_sample_spherical_triangle(sp2, sp1, sp3, v, u / prob_1)
        } else {
            uniform_sample_spherical_triangle(sp4, sp1, sp3, v, 1.0 - ((u - prob_1) / prob_2))
        };

        // Project shadow_vec back onto the light's surface
        let arr_local = arr * *space;
        let shadow_vec_local = shadow_vec * *space;
        let shadow_vec_local = shadow_vec_local * (-arr_local.z() / shadow_vec_local.z());
        let mut sample_point_local = arr_local + shadow_vec_local;
        {
            let x = sample_point_local.x().max(dim.0 * -0.5).min(dim.0 * 0.5);
            let y = sample_point_local.y().max(dim.1 * -0.5).min(dim.1 * 0.5);
            sample_point_local.set_x(x);
            sample_point_local.set_y(y);
            sample_point_local.set_z(0.0);
        }
        let sample_point = sample_point_local * space_inv;
        let shadow_vec = sample_point - arr;

        // Calculate pdf and light energy
        let pdf = 1.0 / (area_1 + area_2); // PDF of the ray direction being sampled
        let spectral_sample = (col * surface_area_inv as f32 * 0.5).to_spectral_sample(wavelength);

        (spectral_sample, shadow_vec, pdf as f32)
    }

    fn is_delta(&self) -> bool {
        false
    }

    fn approximate_energy(&self) -> f32 {
        let color: XYZ = self.colors.iter().fold(
            XYZ::new(0.0, 0.0, 0.0),
            |a, &b| a + b,
        ) / self.colors.len() as f32;
        color.y
    }
}


impl<'a> Surface for RectangleLight<'a> {
    fn intersect_rays(
        &self,
        accel_rays: &mut [AccelRay],
        wrays: &[Ray],
        isects: &mut [SurfaceIntersection],
        shader: &SurfaceShader,
        space: &[Matrix4x4],
    ) {
        let _ = (accel_rays, wrays, isects, shader, space);
        unimplemented!()
    }
}

impl<'a> Boundable for RectangleLight<'a> {
    fn bounds(&self) -> &[BBox] {
        self.bounds_
    }
}