From 83799a02a9d31cb2c3ded7de870c20a9f8c994f5 Mon Sep 17 00:00:00 2001
From: Bill Thiede <git@xinu.tv>
Date: Fri, 16 Jul 2021 16:38:40 -0700
Subject: [PATCH] matrices: implement Matrix4x4::shearing

---
 rtchallenge/src/matrices.rs | 47 ++++++++++++++++++++++++++++++++++---
 1 file changed, 44 insertions(+), 3 deletions(-)

diff --git a/rtchallenge/src/matrices.rs b/rtchallenge/src/matrices.rs
index 5843ca9..d68fb80 100644
--- a/rtchallenge/src/matrices.rs
+++ b/rtchallenge/src/matrices.rs
@@ -1,9 +1,6 @@
 use std::fmt;
 use std::ops::{Index, IndexMut, Mul};
 
-// Implement a PartialEq that does approx_eq internally.
-//use float_cmp::{ApproxEq, F32Margin};
-
 use crate::tuples::Tuple;
 
 /// Value considered close enough for PartialEq implementations.
@@ -402,6 +399,50 @@ impl Matrix4x4 {
             ],
         }
     }
+    /// Create a transform matrix that will shear (skew) points.
+    /// # Examples
+    ///
+    /// ```
+    /// use rtchallenge::{matrices::Matrix4x4, tuples::Tuple};
+    ///
+    /// // A shearing transform moves x in proportion to y.
+    /// let transform = Matrix4x4::shearing(1.,0.,0.,0.,0.,0.);
+    /// let p = Tuple::point(2.,3.,4.);
+    /// assert_eq!(transform * p, Tuple::point(5.,3.,4.));
+    ///
+    /// // A shearing transform moves x in proportion to z.
+    /// let transform = Matrix4x4::shearing(0.,1.,0.,0.,0.,0.);
+    /// let p = Tuple::point(2.,3.,4.);
+    /// assert_eq!(transform * p, Tuple::point(6.,3.,4.));
+    ///
+    /// // A shearing transform moves y in proportion to x.
+    /// let transform = Matrix4x4::shearing(0.,0.,1.,0.,0.,0.);
+    /// let p = Tuple::point(2.,3.,4.);
+    /// assert_eq!(transform * p, Tuple::point(2.,5.,4.));
+    ///
+    /// // A shearing transform moves y in proportion to z.
+    /// let transform = Matrix4x4::shearing(0.,0.,0.,1.,0.,0.);
+    /// let p = Tuple::point(2.,3.,4.);
+    /// assert_eq!(transform * p, Tuple::point(2.,7.,4.));
+    ///
+    /// // A shearing transform moves z in proportion to x.
+    /// let transform = Matrix4x4::shearing(0.,0.,0.,0.,1.,0.);
+    /// let p = Tuple::point(2.,3.,4.);
+    /// assert_eq!(transform * p, Tuple::point(2.,3.,6.));
+    ///
+    /// // A shearing transform moves z in proportion to y.
+    /// let transform = Matrix4x4::shearing(0.,0.,0.,0.,0.,1.);
+    /// let p = Tuple::point(2.,3.,4.);
+    /// assert_eq!(transform * p, Tuple::point(2.,3.,7.));
+
+    pub fn shearing(xy: f32, xz: f32, yx: f32, yz: f32, zx: f32, zy: f32) -> Matrix4x4 {
+        Matrix4x4::new(
+            [1., xy, xz, 0.],
+            [yx, 1., yz, 0.],
+            [zx, zy, 1., 0.],
+            [0., 0., 0., 1.],
+        )
+    }
 
     /// Returns a new matrix that is the inverse of self. If self is A, inverse returns A<sup>-1</sup>, where
     /// AA<sup>-1</sup> = I.