From 125c96c25fb9d84d7cd07648d105e0efbaa661e6 Mon Sep 17 00:00:00 2001
From: Bill Thiede <git@xinu.tv>
Date: Sat, 17 Jul 2021 17:40:35 -0700
Subject: [PATCH] transformations: implement view_transform.

---
 rtchallenge/src/lib.rs             |  1 +
 rtchallenge/src/transformations.rs | 59 ++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 rtchallenge/src/transformations.rs

diff --git a/rtchallenge/src/lib.rs b/rtchallenge/src/lib.rs
index d69dec1..f4ff091 100644
--- a/rtchallenge/src/lib.rs
+++ b/rtchallenge/src/lib.rs
@@ -5,6 +5,7 @@ pub mod materials;
 pub mod matrices;
 pub mod rays;
 pub mod spheres;
+pub mod transformations;
 pub mod tuples;
 pub mod world;
 
diff --git a/rtchallenge/src/transformations.rs b/rtchallenge/src/transformations.rs
new file mode 100644
index 0000000..b0963da
--- /dev/null
+++ b/rtchallenge/src/transformations.rs
@@ -0,0 +1,59 @@
+use crate::{
+    matrices::Matrix4x4,
+    tuples::{cross, Tuple},
+};
+
+/// Create a matrix representing a eye at `from` looking at `to`, with an `up`
+/// as the up vector.
+///
+/// # Examples
+/// ```
+/// use rtchallenge::{matrices::Matrix4x4, transformations::view_transform, tuples::Tuple};
+///
+/// // The transofrmation matrix for the default orientation.
+/// let from = Tuple::point(0., 0., 0.);
+/// let to = Tuple::point(0., 0., -1.);
+/// let up = Tuple::vector(0., 1., 0.);
+/// let t = view_transform(from, to, up);
+/// assert_eq!(t, Matrix4x4::identity());
+///
+/// // A view transformation matrix looking in positive z direction.
+/// let from = Tuple::point(0., 0., 0.);
+/// let to = Tuple::point(0., 0., 1.);
+/// let up = Tuple::vector(0., 1., 0.);
+/// let t = view_transform(from, to, up);
+/// assert_eq!(t, Matrix4x4::scaling(-1., 1., -1.));
+///
+/// // The view tranformation moves the world.
+/// let from = Tuple::point(0., 0., 8.);
+/// let to = Tuple::point(0., 0., 0.);
+/// let up = Tuple::vector(0., 1., 0.);
+/// let t = view_transform(from, to, up);
+/// assert_eq!(t, Matrix4x4::translation(0., 0., -8.));
+///
+/// // An arbitrary view transformation.
+/// let from = Tuple::point(1., 3., 2.);
+/// let to = Tuple::point(4., -2., 8.);
+/// let up = Tuple::vector(1., 1., 0.);
+/// let t = view_transform(from, to, up);
+/// assert_eq!(
+///     t,
+///     Matrix4x4::new(
+///         [-0.50709, 0.50709, 0.67612, -2.36643],
+///         [0.76772, 0.60609, 0.12122, -2.82843],
+///         [-0.35857, 0.59761, -0.71714, 0.],
+///         [0., 0., 0., 1.],
+///     )
+/// );
+/// ```
+pub fn view_transform(from: Tuple, to: Tuple, up: Tuple) -> Matrix4x4 {
+    let forward = (to - from).normalize();
+    let left = cross(forward, up.normalize());
+    let true_up = cross(left, forward);
+    Matrix4x4::new(
+        [left.x, left.y, left.z, 0.],
+        [true_up.x, true_up.y, true_up.z, 0.],
+        [-forward.x, -forward.y, -forward.z, 0.],
+        [0., 0., 0., 1.],
+    ) * Matrix4x4::translation(-from.x, -from.y, -from.z)
+}