rotation around x axis

features/src/transformations.rs

@@ -1,3 +1,4 @@
+use std::f32::consts::PI;
 use crate::matrix::Matrix;
 use crate::structs::Tuple;
 
@@ -20,6 +21,15 @@ impl Matrix {
             [0.0, 0.0, 0.0, 1.0],
         ])
     }
+
+    pub fn rotation_x(r: f32) -> Self {
+        Matrix::from_array([
+            [1.0 , 0.0, 0.0, 0.0],
+            [0.0, r.cos(), -1.0 * r.sin(), 0.0],
+            [0.0, r.sin(),  r.cos() , 0.0],
+            [0.0, 0.0, 0.0, 1.0],
+        ])
+    }
 }
 
 #[cfg(test)]
@@ -86,4 +96,27 @@ mod tests {
         let p = Tuple::point(2.0, 3.0, 4.0);
         assert_eq!(&transform * &p, Tuple::point(-2.0, 3.0, 4.0));
     }
+
+    fn sqrt_of_2() -> f32 {
+        (2.0 as f32).sqrt()
+    }
+
+    #[test]
+    fn rotating_a_point_around_the_x_axis() {
+        let p = Tuple::point(0.0, 1.0, 0.0);
+        let half_quarter = Matrix::rotation_x(PI / 4.0);
+        let full_quarter = Matrix::rotation_x(PI / 2.0);
+        
+        assert_eq!(&half_quarter * &p, Tuple::point(0.0, sqrt_of_2() / 2.0, sqrt_of_2() / 2.0));
+        assert_eq!(&full_quarter * &p, Tuple::point(0.0, 0.0, 1.0));
+    }
+
+    #[test]
+    fn inverse_of_an_x_rotation_rotates_opposite_direction() {
+        let p = Tuple::point(0.0, 1.0, 0.0);
+        let half_quarter = Matrix::rotation_x(PI / 4.0);
+        let inv = half_quarter.inverse();
+
+        assert_eq!(&inv * &p, Tuple::point(0.0, sqrt_of_2() / 2.0, -1.0 * sqrt_of_2() / 2.0));
+    }
 }