diff --git a/rtchallenge/Cargo.toml b/rtchallenge/Cargo.toml
index 32b461b..6962e19 100644
--- a/rtchallenge/Cargo.toml
+++ b/rtchallenge/Cargo.toml
@@ -6,6 +6,9 @@ edition = "2018"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[features]
+disable_inverse_cache = []
+
 [dependencies]
 anyhow = "1.0.41"
 criterion = "0.3.4"
diff --git a/rtchallenge/src/camera.rs b/rtchallenge/src/camera.rs
index a7f3181..bbe9af2 100644
--- a/rtchallenge/src/camera.rs
+++ b/rtchallenge/src/camera.rs
@@ -112,6 +112,7 @@ impl Camera {
     ///     Tuple::vector(2_f32.sqrt() / 2., 0., -2_f32.sqrt() / 2.)
     /// );
     /// ```
+    #[cfg(not(feature = "disable_inverse_cache"))]
     pub fn ray_for_pixel(&self, px: usize, py: usize) -> Ray {
         // The offset from the edge of the canvas to the pixel's corner.
         let xoffset = (px as f32 + 0.5) * self.pixel_size;
@@ -124,16 +125,35 @@ impl Camera {
 
         // Using the camera matrix, transofmrm the canvas point and the origin,
         // and then compute the ray's direction vector.
-        // (Remember that the canves is at z>=-1).
+        // (Remember that the canvas is at z>=-1).
         let pixel = self.inverse_transform * Tuple::point(world_x, world_y, -1.);
         let origin = self.inverse_transform * Tuple::point(0., 0., 0.);
         let direction = (pixel - origin).normalize();
 
         Ray::new(origin, direction)
     }
+    #[cfg(feature = "disable_inverse_cache")]
+    pub fn ray_for_pixel(&self, px: usize, py: usize) -> Ray {
+        // The offset from the edge of the canvas to the pixel's corner.
+        let xoffset = (px as f32 + 0.5) * self.pixel_size;
+        let yoffset = (py as f32 + 0.5) * self.pixel_size;
+
+        // The untransformed coordinates of the pixle in world space.
+        // (Remember that the camera looks toward -z, so +x is to the left.)
+        let world_x = self.half_width - xoffset;
+        let world_y = self.half_height - yoffset;
+
+        // Using the camera matrix, transofmrm the canvas point and the origin,
+        // and then compute the ray's direction vector.
+        // (Remember that the canvas is at z>=-1).
+        let pixel = self.transform.inverse() * Tuple::point(world_x, world_y, -1.);
+        let origin = self.transform.inverse() * Tuple::point(0., 0., 0.);
+        let direction = (pixel - origin).normalize();
+
+        Ray::new(origin, direction)
+    }
 
     /// Use camera to render an image of the given world.
-    ///
     /// # Examples
     /// ```
     /// use std::f32::consts::PI;
@@ -160,7 +180,28 @@ impl Camera {
     }
 
     #[allow(dead_code)]
-    fn render_parallel(&self, w: &World) -> Canvas {
+    fn render_parallel_one_tread_per_core(&self, w: &World) -> Canvas {
+        let image_mu = Mutex::new(Canvas::new(self.hsize, self.vsize, BLACK));
+
+        (0..self.vsize).into_par_iter().for_each(|y| {
+            let mut row_image = Canvas::new(self.hsize, 1, BLACK);
+            for x in 0..self.hsize {
+                let ray = self.ray_for_pixel(x, y);
+                let color = w.color_at(&ray);
+                row_image.set(x, 0, color);
+            }
+            // TODO(wathiede): create a row based setter for memcpying the row as a whole.
+            let mut image = image_mu.lock().expect("failed to lock image mutex");
+            for x in 0..self.hsize {
+                image.set(x, y, row_image.get(x, 0));
+            }
+        });
+        image_mu
+            .into_inner()
+            .expect("failed to get image out of mutex")
+    }
+    #[allow(dead_code)]
+    fn render_parallel_rayon(&self, w: &World) -> Canvas {
         let image_mu = Mutex::new(Canvas::new(self.hsize, self.vsize, BLACK));
 
         (0..self.vsize).into_par_iter().for_each(|y| {
diff --git a/rtchallenge/src/spheres.rs b/rtchallenge/src/spheres.rs
index 9a27f33..64bc3e3 100644
--- a/rtchallenge/src/spheres.rs
+++ b/rtchallenge/src/spheres.rs
@@ -105,6 +105,7 @@ impl Sphere {
     /// let n = s.normal_at(Tuple::point(0., 2_f32.sqrt()/2., -2_f32.sqrt()/2.));
     /// assert_eq!(n, Tuple::vector(0., 0.97014, -0.24254));
     /// ```
+    #[cfg(not(feature = "disable_inverse_cache"))]
     pub fn normal_at(&self, world_point: Tuple) -> Tuple {
         let object_point = self.inverse_transform * world_point;
         let object_normal = object_point - Tuple::point(0., 0., 0.);
@@ -112,6 +113,14 @@ impl Sphere {
         world_normal.w = 0.;
         world_normal.normalize()
     }
+    #[cfg(feature = "disable_inverse_cache")]
+    pub fn normal_at(&self, world_point: Tuple) -> Tuple {
+        let object_point = self.transform.inverse() * world_point;
+        let object_normal = object_point - Tuple::point(0., 0., 0.);
+        let mut world_normal = self.transform.inverse().transpose() * object_normal;
+        world_normal.w = 0.;
+        world_normal.normalize()
+    }
 
     pub fn transform(&self) -> Matrix4x4 {
         self.transform