From 5c0fc9689eeb4a8df214dcd9ce831674cf7c09d1 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <brunobelanyi@gmail.com>
Date: Mon, 23 Mar 2020 16:33:02 +0100
Subject: [PATCH] library: shape: add InterpolatedTriangle type

This is a triangle with added normal interpolation at its edges. This is
particularly useful when rendering mesh objects.
---
 pathtracer/src/shape/interpolated_triangle.rs | 153 ++++++++++++++++++
 pathtracer/src/shape/mod.rs                   |   4 +
 pathtracer/src/shape/triangle.rs              |   2 +-
 3 files changed, 158 insertions(+), 1 deletion(-)
 create mode 100644 pathtracer/src/shape/interpolated_triangle.rs

diff --git a/pathtracer/src/shape/interpolated_triangle.rs b/pathtracer/src/shape/interpolated_triangle.rs
new file mode 100644
index 0000000..77af206
--- /dev/null
+++ b/pathtracer/src/shape/interpolated_triangle.rs
@@ -0,0 +1,153 @@
+use super::triangle::Triangle;
+use super::Shape;
+use crate::{Point, Point2D, Vector};
+use beevee::aabb::AABB;
+use beevee::ray::Ray;
+use nalgebra::Unit;
+use serde::Deserialize;
+
+/// Represent a triangle with interpolated normals inside the scene.
+#[derive(Clone, Debug, PartialEq, Deserialize)]
+pub struct InterpolatedTriangle {
+    #[serde(flatten)]
+    tri: Triangle,
+    // FIXME: serialize with unit
+    normals: [Unit<Vector>; 3],
+}
+
+impl InterpolatedTriangle {
+    /// Creates a new `InterpolatedTriangle` from 3 [`Point`]s and 3 [`Vector`]s.
+    ///
+    /// [`Point`]: ../../type.Point.html
+    /// [`Point`]: ../../type.Vector.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use pathtracer::shape::InterpolatedTriangle;
+    /// # use pathtracer::{Point, Vector};
+    /// #
+    /// let t = InterpolatedTriangle::new(
+    ///     Point::new(1.0, 0.0, 0.0),
+    ///     Point::new(0.0, 1.0, 0.0),
+    ///     Point::new(0.0, 0.0, 1.0),
+    ///     Vector::x_axis(),
+    ///     Vector::y_axis(),
+    ///     Vector::z_axis(),
+    /// );
+    /// ```
+    pub fn new(
+        c0: Point,
+        c1: Point,
+        c2: Point,
+        n0: Unit<Vector>,
+        n1: Unit<Vector>,
+        n2: Unit<Vector>,
+    ) -> Self {
+        InterpolatedTriangle {
+            tri: Triangle::new(c0, c1, c2),
+            normals: [n0, n1, n2],
+        }
+    }
+}
+
+impl Shape for InterpolatedTriangle {
+    fn intersect(&self, ray: &Ray) -> Option<f32> {
+        self.tri.intersect(ray)
+    }
+
+    fn normal(&self, point: &Point) -> Unit<Vector> {
+        let (u, v) = {
+            let c = self.tri.barycentric(point);
+            (c.x, c.y)
+        };
+        let interpol = self.normals[0].as_ref() * (1. - u - v)
+            + self.normals[1].as_ref() * u
+            + self.normals[2].as_ref() * v;
+        Unit::new_normalize(interpol)
+    }
+
+    fn project_texel(&self, point: &Point) -> Point2D {
+        self.tri.project_texel(point)
+    }
+
+    fn aabb(&self) -> AABB {
+        self.tri.aabb()
+    }
+
+    fn centroid(&self) -> Point {
+        self.tri.centroid()
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    fn simple_triangle() -> InterpolatedTriangle {
+        InterpolatedTriangle::new(
+            Point::origin(),
+            Point::new(0., 1., 1.),
+            Point::new(0., 1., 0.),
+            Vector::x_axis(),
+            Vector::y_axis(),
+            Vector::z_axis(),
+        )
+    }
+
+    #[test]
+    fn normal_interpolation_at_c0_works() {
+        let triangle = simple_triangle();
+        let normal = triangle.normal(&Point::origin());
+        assert_eq!(normal, Vector::x_axis());
+    }
+
+    #[test]
+    fn normal_interpolation_at_c1_works() {
+        let triangle = simple_triangle();
+        let normal = triangle.normal(&Point::new(0., 1., 1.));
+        assert_eq!(normal, Vector::y_axis());
+    }
+
+    #[test]
+    fn normal_interpolation_at_c2_works() {
+        let triangle = simple_triangle();
+        let normal = triangle.normal(&Point::new(0., 1., 0.));
+        assert_eq!(normal, Vector::z_axis());
+    }
+
+    #[test]
+    fn normal_interpolation_at_center_works() {
+        let triangle = simple_triangle();
+        let center = Point::new(0., 2. / 3., 1. / 3.);
+        let normal = triangle.normal(&center);
+        let expected = Unit::new_normalize(Vector::new(1., 1., 1.));
+        assert!((normal.as_ref() - expected.as_ref()).magnitude() < 1e-5)
+    }
+
+    #[test]
+    fn deserialization_works() {
+        let yaml = r#"
+            corners:
+              - [0.0, 0.0, 0.0]
+              - [0.0, 1.0, 1.0]
+              - [0.0, 1.0, 0.0]
+            normals:
+              - [1.0, 0.0, 0.0]
+              - [0.0, 1.0, 0.0]
+              - [0.0, 0.0, 1.0]
+        "#;
+        let triangle: InterpolatedTriangle = serde_yaml::from_str(yaml).unwrap();
+        assert_eq!(
+            triangle,
+            InterpolatedTriangle::new(
+                Point::origin(),
+                Point::new(0., 1., 1.),
+                Point::new(0., 1., 0.),
+                Vector::x_axis(),
+                Vector::y_axis(),
+                Vector::z_axis(),
+            )
+        )
+    }
+}
diff --git a/pathtracer/src/shape/mod.rs b/pathtracer/src/shape/mod.rs
index e68a59a..e510e93 100644
--- a/pathtracer/src/shape/mod.rs
+++ b/pathtracer/src/shape/mod.rs
@@ -18,6 +18,7 @@ use serde::Deserialize;
 pub enum ShapeEnum {
     Sphere,
     Triangle,
+    InterpolatedTriangle,
 }
 
 /// Represent an abstract shape inside the scene.
@@ -51,6 +52,9 @@ impl Intersected for dyn Shape {
     }
 }
 
+mod interpolated_triangle;
+pub use interpolated_triangle::*;
+
 mod sphere;
 pub use sphere::*;
 
diff --git a/pathtracer/src/shape/triangle.rs b/pathtracer/src/shape/triangle.rs
index 1fe5d25..b539d01 100644
--- a/pathtracer/src/shape/triangle.rs
+++ b/pathtracer/src/shape/triangle.rs
@@ -38,7 +38,7 @@ impl Triangle {
         }
     }
 
-    fn barycentric(&self, point: &Point) -> Point2D {
+    pub(crate) fn barycentric(&self, point: &Point) -> Point2D {
         let c0_pos = point - self.c0;
         // P - A  =  u * (B - A) + v * (C - A)
         // (C - A) = v0 is c0c2