diff --git a/.gitignore b/.gitignore
index 657a0dd..fd8c240 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,6 +12,8 @@ test/test_materials
 test/test_spheres
 test/test_world
 test/test_camera
+test/test_shapes
+test/test_planes
 raytracer
 *.ppm
 *.png
diff --git a/camera.h b/camera.h
index 91e49fc..93bb93b 100644
--- a/camera.h
+++ b/camera.h
@@ -50,11 +50,11 @@ struct ray camera_ray_for_pixel(struct camera * camera, float px, float py)
   float world_x = camera->half_width - xoffset;
   float world_y = camera->half_height - yoffset;
 
-  struct mat4x4 t_inverse = mat4x4_inverse(&camera->transform);
+  struct mat4x4 t_inverse = mat4x4_inverse(camera->transform);
   struct tuple world_point = point(world_x, world_y, -1.0f);
-  struct tuple pixel = mat4x4_mul_t(&t_inverse, &world_point);
+  struct tuple pixel = mat4x4_mul_t(t_inverse, world_point);
   struct tuple world_origin = point(0.0f, 0.0f, 0.0f);
-  struct tuple origin = mat4x4_mul_t(&t_inverse, &world_origin);
+  struct tuple origin = mat4x4_mul_t(t_inverse, world_origin);
   struct tuple direction = tuple_normalize(tuple_sub(pixel, origin));
 
   return ray(origin, direction);
diff --git a/intersections.h b/intersections.h
index d4185ab..b93ab63 100644
--- a/intersections.h
+++ b/intersections.h
@@ -1,29 +1,26 @@
 #pragma once
 
 #include <stdarg.h>
-#include <assert.h>
-#include <stdbool.h>
-
-#include "spheres.h"
-#include "rays.h"
 
 struct intersection {
   float t;
-  struct sphere const * object;
+  struct shape const * object;
 };
 
-struct intersection intersection(float t, struct sphere const * const object)
-{
-  return (struct intersection){ t, object };
-}
-
-#define MAX_INTERSECTIONS 1024
+#ifndef INTERSECTIONS_MAX
+#define INTERSECTIONS_MAX 1024
+#endif
 
 struct intersections {
   int count;
-  struct intersection i[MAX_INTERSECTIONS];
+  struct intersection i[INTERSECTIONS_MAX];
 };
 
+struct intersection intersection(float t, struct shape const * const object)
+{
+  return (struct intersection){ t, object };
+}
+
 inline static void intersections(struct intersections * intersections, int count, ...)
 {
   va_list ap;
@@ -35,113 +32,3 @@ inline static void intersections(struct intersections * intersections, int count
 
   intersections->count = count;
 }
-
-inline static void intersect(struct sphere const * const s, struct ray r, struct intersections * intersections)
-{
-  struct mat4x4 m = mat4x4_inverse(&s->transform);
-  struct ray r2 = ray_transform(r, &m);
-  struct tuple sphere_to_ray = tuple_sub(r2.origin, point(0.0f, 0.0f, 0.0f));
-
-  float a = tuple_dot(r2.direction, r2.direction);
-  float b = 2 * tuple_dot(r2.direction, sphere_to_ray);
-  float c = tuple_dot(sphere_to_ray, sphere_to_ray) - 1;
-
-  float discriminant = b * b - 4 * a * c;
-
-  if (discriminant < 0) {
-    return;
-  } else {
-    float root = sqrtf(discriminant);
-    float t1 = (-b - root) / (2 * a);
-    float t2 = (-b + root) / (2 * a);
-
-    intersections->i[intersections->count++] = intersection(t1, s);
-    intersections->i[intersections->count++] = intersection(t2, s);
-    assert(intersections->count < MAX_INTERSECTIONS);
-    return;
-  }
-}
-
-inline static struct intersection * hit(struct intersections * xs)
-{
-  struct intersection * i = NULL;
-  for (int n = 0; n < xs->count; n++) {
-    if (xs->i[n].t >= 0) {
-      if (i == NULL || i->t > xs->i[n].t)
-        i = &xs->i[n];
-    }
-  }
-  return i;
-}
-
-
-inline static void intersections_sort(struct intersections * xs)
-{
-#define left_child(i) (2 * i + 1)
-
-  int start = xs->count / 2;
-  int end = xs->count;
-  while (end > 1) {
-    if (start > 0) {
-      start = start - 1;
-    } else {
-      end = end - 1;
-      struct intersection tmp = xs->i[0];
-      xs->i[0] = xs->i[end];
-      xs->i[end] = tmp;
-    }
-
-    int root = start;
-    while (left_child(root) < end) {
-      int child = left_child(root);
-      if (child + 1 < end && xs->i[child].t < xs->i[child+1].t) {
-        child = child + 1;
-      }
-      if (xs->i[root].t < xs->i[child].t) {
-        struct intersection tmp = xs->i[root];
-        xs->i[root] = xs->i[child];
-        xs->i[child] = tmp;
-        root = child;
-      } else {
-        break;
-      }
-    }
-  }
-
-#undef left_child
-}
-
-struct computations {
-  float t;
-  struct sphere const * object;
-  struct tuple point;
-  struct tuple eyev;
-  struct tuple normalv;
-  bool inside;
-  struct tuple over_point;
-};
-
-inline static struct computations prepare_computations(struct intersection intersection, struct ray ray)
-{
-  struct tuple point = ray_position(ray, intersection.t);
-  struct tuple eyev = tuple_neg(ray.direction);
-  struct tuple normalv = sphere_normal_at(intersection.object, point);
-
-  bool inside = false;
-  if (tuple_dot(normalv, eyev) < 0.0f) {
-    inside = true;
-    normalv = tuple_neg(normalv);
-  }
-
-  struct tuple over_point = tuple_add(point, tuple_mul(normalv, epsilon * 100));
-
-  return (struct computations){
-    intersection.t,
-    intersection.object,
-    point,
-    eyev,
-    normalv,
-    inside,
-    over_point
-  };
-}
diff --git a/intersections_shapes.h b/intersections_shapes.h
new file mode 100644
index 0000000..9464b5d
--- /dev/null
+++ b/intersections_shapes.h
@@ -0,0 +1,153 @@
+#pragma once
+
+#include <assert.h>
+#include <stdbool.h>
+
+#include "rays.h"
+#include "shapes.h"
+#include "intersections.h"
+#include "spheres.h"
+#include "planes.h"
+
+#ifdef TEST
+struct ray intersections_saved_ray;
+#endif
+
+inline static void intersect(struct shape const * const s, struct ray r, struct intersections * intersections)
+{
+  struct mat4x4 m = mat4x4_inverse(s->transform);
+  struct ray local_ray = ray_transform(r, m);
+
+  switch (s->type) {
+#ifdef TEST
+  case SHAPE_TEST:
+    intersections_saved_ray = local_ray;
+    break;
+#endif
+  case SHAPE_SPHERE:
+    sphere_intersect(s, local_ray, intersections);
+    break;
+  case SHAPE_PLANE:
+    plane_intersect(s, local_ray, intersections);
+    break;
+  default:
+    assert(false);
+    break;
+  }
+  assert(intersections->count < INTERSECTIONS_MAX);
+}
+
+inline static struct intersection * hit(struct intersections * xs)
+{
+  struct intersection * i = NULL;
+  for (int n = 0; n < xs->count; n++) {
+    if (xs->i[n].t >= 0) {
+      if (i == NULL || i->t > xs->i[n].t)
+        i = &xs->i[n];
+    }
+  }
+  return i;
+}
+
+
+inline static void intersections_sort(struct intersections * xs)
+{
+#define left_child(i) (2 * i + 1)
+
+  int start = xs->count / 2;
+  int end = xs->count;
+  while (end > 1) {
+    if (start > 0) {
+      start = start - 1;
+    } else {
+      end = end - 1;
+      struct intersection tmp = xs->i[0];
+      xs->i[0] = xs->i[end];
+      xs->i[end] = tmp;
+    }
+
+    int root = start;
+    while (left_child(root) < end) {
+      int child = left_child(root);
+      if (child + 1 < end && xs->i[child].t < xs->i[child+1].t) {
+        child = child + 1;
+      }
+      if (xs->i[root].t < xs->i[child].t) {
+        struct intersection tmp = xs->i[root];
+        xs->i[root] = xs->i[child];
+        xs->i[child] = tmp;
+        root = child;
+      } else {
+        break;
+      }
+    }
+  }
+
+#undef left_child
+}
+
+struct computations {
+  float t;
+  struct shape const * object;
+  struct tuple point;
+  struct tuple eyev;
+  struct tuple normalv;
+  bool inside;
+  struct tuple over_point;
+};
+
+inline static struct tuple normal_at(struct shape const * const s, struct tuple world_point)
+{
+  struct mat4x4 inv = mat4x4_inverse(s->transform);
+  struct mat4x4 inv_t = mat4x4_transpose(inv);
+
+  struct tuple local_point = mat4x4_mul_t(inv, world_point);
+  struct tuple local_normal;
+
+  switch (s->type) {
+#ifdef TEST
+  case SHAPE_TEST:
+    local_normal = vector(local_point.x, local_point.y, local_point.z);
+    break;
+#endif
+  case SHAPE_SPHERE:
+    local_normal = sphere_normal_at(local_point);
+    break;
+  case SHAPE_PLANE:
+    local_normal = plane_normal_at(local_point);
+    break;
+  default:
+    assert(false);
+    break;
+  }
+
+  struct tuple world_normal = mat4x4_mul_t(inv_t, local_normal);
+  world_normal.w = 0.0f;
+
+  return tuple_normalize(world_normal);
+}
+
+inline static struct computations prepare_computations(struct intersection intersection, struct ray ray)
+{
+  struct tuple point = ray_position(ray, intersection.t);
+  struct tuple eyev = tuple_neg(ray.direction);
+  struct tuple normalv = normal_at(intersection.object, point);
+
+  bool inside = false;
+  if (tuple_dot(normalv, eyev) < 0.0f) {
+    inside = true;
+    normalv = tuple_neg(normalv);
+  }
+
+  struct tuple over_point = tuple_add(point, tuple_mul(normalv, epsilon * 100));
+
+  return (struct computations){
+    intersection.t,
+    intersection.object,
+    point,
+    eyev,
+    normalv,
+    inside,
+    over_point
+  };
+}
diff --git a/math.h b/math.h
index 166cd90..698e9d0 100644
--- a/math.h
+++ b/math.h
@@ -6,6 +6,7 @@
 #define sinf __builtin_sinf
 #define powf __builtin_powf
 #define tanf __builtin_tanf
+#define fabsf __builtin_fabsf
 
 static const float tau = 6.283185307179586f;
 static const float pi = tau / 2.0f;
diff --git a/matrices.h b/matrices.h
index 4041818..73a065e 100644
--- a/matrices.h
+++ b/matrices.h
@@ -48,61 +48,61 @@ inline static struct mat2x2 mat2x2(float a1, float a2,
   }};
 }
 
-inline static bool mat4x4_equal(struct mat4x4 const * const a,
-                                struct mat4x4 const * const b)
+inline static bool mat4x4_equal(struct mat4x4 a,
+                                struct mat4x4 b)
 {
   return
-    float_equal(a->e[0][0], b->e[0][0]) &&
-    float_equal(a->e[0][1], b->e[0][1]) &&
-    float_equal(a->e[0][2], b->e[0][2]) &&
-    float_equal(a->e[0][3], b->e[0][3]) &&
-    float_equal(a->e[1][0], b->e[1][0]) &&
-    float_equal(a->e[1][1], b->e[1][1]) &&
-    float_equal(a->e[1][2], b->e[1][2]) &&
-    float_equal(a->e[1][3], b->e[1][3]) &&
-    float_equal(a->e[2][0], b->e[2][0]) &&
-    float_equal(a->e[2][1], b->e[2][1]) &&
-    float_equal(a->e[2][2], b->e[2][2]) &&
-    float_equal(a->e[2][3], b->e[2][3]) &&
-    float_equal(a->e[3][0], b->e[3][0]) &&
-    float_equal(a->e[3][1], b->e[3][1]) &&
-    float_equal(a->e[3][2], b->e[3][2]) &&
-    float_equal(a->e[3][3], b->e[3][3]);
+    float_equal(a.e[0][0], b.e[0][0]) &&
+    float_equal(a.e[0][1], b.e[0][1]) &&
+    float_equal(a.e[0][2], b.e[0][2]) &&
+    float_equal(a.e[0][3], b.e[0][3]) &&
+    float_equal(a.e[1][0], b.e[1][0]) &&
+    float_equal(a.e[1][1], b.e[1][1]) &&
+    float_equal(a.e[1][2], b.e[1][2]) &&
+    float_equal(a.e[1][3], b.e[1][3]) &&
+    float_equal(a.e[2][0], b.e[2][0]) &&
+    float_equal(a.e[2][1], b.e[2][1]) &&
+    float_equal(a.e[2][2], b.e[2][2]) &&
+    float_equal(a.e[2][3], b.e[2][3]) &&
+    float_equal(a.e[3][0], b.e[3][0]) &&
+    float_equal(a.e[3][1], b.e[3][1]) &&
+    float_equal(a.e[3][2], b.e[3][2]) &&
+    float_equal(a.e[3][3], b.e[3][3]);
 }
 
-inline static bool mat3x3_equal(struct mat3x3 * a,
-                                struct mat3x3 * b)
+inline static bool mat3x3_equal(struct mat3x3 a,
+                                struct mat3x3 b)
 {
   return
-    float_equal(a->e[0][0], b->e[0][0]) &&
-    float_equal(a->e[0][1], b->e[0][1]) &&
-    float_equal(a->e[0][2], b->e[0][2]) &&
-    float_equal(a->e[1][0], b->e[1][0]) &&
-    float_equal(a->e[1][1], b->e[1][1]) &&
-    float_equal(a->e[1][2], b->e[1][2]) &&
-    float_equal(a->e[2][0], b->e[2][0]) &&
-    float_equal(a->e[2][1], b->e[2][1]) &&
-    float_equal(a->e[2][2], b->e[2][2]);
+    float_equal(a.e[0][0], b.e[0][0]) &&
+    float_equal(a.e[0][1], b.e[0][1]) &&
+    float_equal(a.e[0][2], b.e[0][2]) &&
+    float_equal(a.e[1][0], b.e[1][0]) &&
+    float_equal(a.e[1][1], b.e[1][1]) &&
+    float_equal(a.e[1][2], b.e[1][2]) &&
+    float_equal(a.e[2][0], b.e[2][0]) &&
+    float_equal(a.e[2][1], b.e[2][1]) &&
+    float_equal(a.e[2][2], b.e[2][2]);
 }
 
-inline static bool mat2x2_equal(struct mat2x2 * a,
-                                struct mat2x2 * b)
+inline static bool mat2x2_equal(struct mat2x2 a,
+                                struct mat2x2 b)
 {
   return
-    float_equal(a->e[0][0], b->e[0][0]) &&
-    float_equal(a->e[0][1], b->e[0][1]) &&
-    float_equal(a->e[1][0], b->e[1][0]) &&
-    float_equal(a->e[1][1], b->e[1][1]);
+    float_equal(a.e[0][0], b.e[0][0]) &&
+    float_equal(a.e[0][1], b.e[0][1]) &&
+    float_equal(a.e[1][0], b.e[1][0]) &&
+    float_equal(a.e[1][1], b.e[1][1]);
 }
 
-inline static struct mat4x4 mat4x4_mul_m(struct mat4x4 const * const a,
-                                         struct mat4x4 const * const b)
+inline static struct mat4x4 mat4x4_mul_m(struct mat4x4 a,
+                                         struct mat4x4 b)
 {
 #define dot(row, col) \
-    a->e[row][0] * b->e[0][col] +  \
-    a->e[row][1] * b->e[1][col] +  \
-    a->e[row][2] * b->e[2][col] +  \
-    a->e[row][3] * b->e[3][col]
+    a.e[row][0] * b.e[0][col] +  \
+    a.e[row][1] * b.e[1][col] +  \
+    a.e[row][2] * b.e[2][col] +  \
+    a.e[row][3] * b.e[3][col]
 
   return mat4x4(dot(0, 0), dot(0, 1), dot(0, 2), dot(0, 3),
                 dot(1, 0), dot(1, 1), dot(1, 2), dot(1, 3),
@@ -112,14 +112,14 @@ inline static struct mat4x4 mat4x4_mul_m(struct mat4x4 const * const a,
 #undef dot
 }
 
-inline static struct tuple mat4x4_mul_t(struct mat4x4 const * const a,
-                                        struct tuple * b)
+inline static struct tuple mat4x4_mul_t(struct mat4x4 a,
+                                        struct tuple b)
 {
 #define dot(row) \
-    a->e[row][0] * b->e[0] +  \
-    a->e[row][1] * b->e[1] +  \
-    a->e[row][2] * b->e[2] +  \
-    a->e[row][3] * b->e[3]
+    a.e[row][0] * b.e[0] +  \
+    a.e[row][1] * b.e[1] +  \
+    a.e[row][2] * b.e[2] +  \
+    a.e[row][3] * b.e[3]
 
   return tuple(dot(0), dot(1), dot(2), dot(3));
 #undef dot
@@ -133,20 +133,20 @@ inline static struct mat4x4 mat4x4_identity()
                 0.0f, 0.0f, 0.0f, 1.0f);
 }
 
-inline static struct mat4x4 mat4x4_transpose(struct mat4x4 const * const a)
+inline static struct mat4x4 mat4x4_transpose(struct mat4x4 a)
 {
-  return mat4x4(a->e[0][0], a->e[1][0], a->e[2][0], a->e[3][0],
-                a->e[0][1], a->e[1][1], a->e[2][1], a->e[3][1],
-                a->e[0][2], a->e[1][2], a->e[2][2], a->e[3][2],
-                a->e[0][3], a->e[1][3], a->e[2][3], a->e[3][3]);
+  return mat4x4(a.e[0][0], a.e[1][0], a.e[2][0], a.e[3][0],
+                a.e[0][1], a.e[1][1], a.e[2][1], a.e[3][1],
+                a.e[0][2], a.e[1][2], a.e[2][2], a.e[3][2],
+                a.e[0][3], a.e[1][3], a.e[2][3], a.e[3][3]);
 }
 
-inline static float mat2x2_determinant(struct mat2x2 * a)
+inline static float mat2x2_determinant(struct mat2x2 a)
 {
-  return a->e[0][0] * a->e[1][1] - a->e[0][1] * a->e[1][0];
+  return a.e[0][0] * a.e[1][1] - a.e[0][1] * a.e[1][0];
 }
 
-inline static struct mat2x2 mat3x3_submatrix(struct mat3x3 * a, int r, int c)
+inline static struct mat2x2 mat3x3_submatrix(struct mat3x3 a, int r, int c)
 {
   struct mat2x2 b;
   int row2 = 0;
@@ -155,7 +155,7 @@ inline static struct mat2x2 mat3x3_submatrix(struct mat3x3 * a, int r, int c)
     int col2 = 0;
     for (int col3 = 0; col3 < 3; col3++) {
       if (col3 == c) continue;
-      b.e[row2][col2] = a->e[row3][col3];
+      b.e[row2][col2] = a.e[row3][col3];
       col2++;
     }
     row2++;
@@ -163,7 +163,7 @@ inline static struct mat2x2 mat3x3_submatrix(struct mat3x3 * a, int r, int c)
   return b;
 }
 
-inline static struct mat3x3 mat4x4_submatrix(struct mat4x4 const * const a, int r, int c)
+inline static struct mat3x3 mat4x4_submatrix(struct mat4x4 a, int r, int c)
 {
   struct mat3x3 b;
   int row3 = 0;
@@ -172,7 +172,7 @@ inline static struct mat3x3 mat4x4_submatrix(struct mat4x4 const * const a, int
     int col3 = 0;
     for (int col4 = 0; col4 < 4; col4++) {
       if (col4 == c) continue;
-      b.e[row3][col3] = a->e[row4][col4];
+      b.e[row3][col3] = a.e[row4][col4];
       col3++;
     }
     row3++;
@@ -180,14 +180,14 @@ inline static struct mat3x3 mat4x4_submatrix(struct mat4x4 const * const a, int
   return b;
 }
 
-inline static float mat3x3_minor(struct mat3x3 * a, int r, int c)
+inline static float mat3x3_minor(struct mat3x3 a, int r, int c)
 {
   struct mat2x2 s = mat3x3_submatrix(a, r, c);
-  float ret = mat2x2_determinant(&s);
+  float ret = mat2x2_determinant(s);
   return ret;
 }
 
-inline static float mat3x3_cofactor(struct mat3x3 * a, int r, int c)
+inline static float mat3x3_cofactor(struct mat3x3 a, int r, int c)
 {
   float minor = mat3x3_minor(a, r, c);
   if ((r + c) & 1)
@@ -196,25 +196,25 @@ inline static float mat3x3_cofactor(struct mat3x3 * a, int r, int c)
     return minor;
 }
 
-inline static float mat3x3_determinant(struct mat3x3 * a)
+inline static float mat3x3_determinant(struct mat3x3 a)
 {
   float f0 = mat3x3_cofactor(a, 0, 0);
   float f1 = mat3x3_cofactor(a, 0, 1);
   float f2 = mat3x3_cofactor(a, 0, 2);
   return
-    a->e[0][0] * f0 +
-    a->e[0][1] * f1 +
-    a->e[0][2] * f2;
+    a.e[0][0] * f0 +
+    a.e[0][1] * f1 +
+    a.e[0][2] * f2;
 }
 
-inline static float mat4x4_minor(struct mat4x4 const * const a, int r, int c)
+inline static float mat4x4_minor(struct mat4x4 a, int r, int c)
 {
   struct mat3x3 s = mat4x4_submatrix(a, r, c);
-  float ret = mat3x3_determinant(&s);
+  float ret = mat3x3_determinant(s);
   return ret;
 }
 
-inline static float mat4x4_cofactor(struct mat4x4 const * const a, int r, int c)
+inline static float mat4x4_cofactor(struct mat4x4 a, int r, int c)
 {
   float minor = mat4x4_minor(a, r, c);
   if ((r + c) & 1)
@@ -223,25 +223,25 @@ inline static float mat4x4_cofactor(struct mat4x4 const * const a, int r, int c)
     return minor;
 }
 
-inline static float mat4x4_determinant(struct mat4x4 const * const a)
+inline static float mat4x4_determinant(struct mat4x4 a)
 {
   float f0 = mat4x4_cofactor(a, 0, 0);
   float f1 = mat4x4_cofactor(a, 0, 1);
   float f2 = mat4x4_cofactor(a, 0, 2);
   float f3 = mat4x4_cofactor(a, 0, 3);
   return
-    a->e[0][0] * f0 +
-    a->e[0][1] * f1 +
-    a->e[0][2] * f2 +
-    a->e[0][3] * f3;
+    a.e[0][0] * f0 +
+    a.e[0][1] * f1 +
+    a.e[0][2] * f2 +
+    a.e[0][3] * f3;
 }
 
-inline static bool mat4x4_is_invertible(struct mat4x4 const * const a)
+inline static bool mat4x4_is_invertible(struct mat4x4 a)
 {
   return !float_equal(mat4x4_determinant(a), 0.f);
 }
 
-inline static struct mat4x4 mat4x4_inverse(struct mat4x4 const * const a)
+inline static struct mat4x4 mat4x4_inverse(struct mat4x4 a)
 {
   struct mat4x4 m;
   float det = mat4x4_determinant(a);
diff --git a/planes.h b/planes.h
new file mode 100644
index 0000000..cfefecc
--- /dev/null
+++ b/planes.h
@@ -0,0 +1,35 @@
+#pragma once
+
+#include "math.h"
+#include "matrices.h"
+#include "materials.h"
+#include "shapes.h"
+#include "rays.h"
+#include "intersections.h"
+
+inline static struct shape plane()
+{
+  return (struct shape){
+    mat4x4_identity(),
+    material(),
+    SHAPE_PLANE,
+  };
+}
+
+inline static struct tuple plane_normal_at(struct tuple local_point)
+{
+  return vector(0.0f, 1.0f, 0.0f);
+}
+
+inline static void plane_intersect(struct shape const * const s, struct ray local_ray, struct intersections * intersections)
+{
+  if (fabsf(local_ray.direction.y) < epsilon) {
+    return;
+  }
+
+  float t = -local_ray.origin.y / local_ray.direction.y;
+
+  intersections->i[intersections->count++] = intersection(t, s);
+
+  return;
+}
diff --git a/rays.h b/rays.h
index 7d814f2..788bee7 100644
--- a/rays.h
+++ b/rays.h
@@ -1,6 +1,5 @@
 #pragma once
 
-#include "spheres.h"
 #include "tuples.h"
 #include "matrices.h"
 
@@ -19,8 +18,8 @@ inline static struct tuple ray_position(struct ray ray, float t)
   return tuple_add(ray.origin, tuple_mul(ray.direction, t));
 }
 
-inline static struct ray ray_transform(struct ray r, struct mat4x4 const * const m)
+inline static struct ray ray_transform(struct ray r, struct mat4x4 m)
 {
-  return ray(mat4x4_mul_t(m, &r.origin),
-             mat4x4_mul_t(m, &r.direction));
+  return ray(mat4x4_mul_t(m, r.origin),
+             mat4x4_mul_t(m, r.direction));
 }
diff --git a/raytracer.c b/raytracer.c
index faaab52..f830ced 100644
--- a/raytracer.c
+++ b/raytracer.c
@@ -22,59 +22,60 @@ int main()
   float pixel_size = wall_size / (float)canvas_pixels;
   float half = wall_size / 2.0f;
 
-  struct sphere floor = sphere();
-  floor.transform = scaling(10.0f, 0.01f, 10.0f);
+  struct shape floor = plane();
   floor.material.color = color(1.0f, 0.9f, 0.9f);
   floor.material.specular = 0.0f;
 
-  struct sphere left_wall = sphere();
+  /*
+  struct shape left_wall = sphere();
   {
     struct mat4x4 _translation = translation(0.0f, 0.0f, 5.0f);
     struct mat4x4 _rotation_y = rotation_y(-pi / 4.0f);
     struct mat4x4 _rotation_x = rotation_x(pi / 2.0f);
     struct mat4x4 _scaling = scaling(10.0f, 0.05f, 10.0f);
 
-    struct mat4x4 t0 = mat4x4_mul_m(&_rotation_x, &_scaling);
-    struct mat4x4 t1 = mat4x4_mul_m(&_rotation_y, &t0);
-    struct mat4x4 t2 = mat4x4_mul_m(&_translation, &t1);
+    struct mat4x4 t0 = mat4x4_mul_m(_rotation_x, _scaling);
+    struct mat4x4 t1 = mat4x4_mul_m(_rotation_y, t0);
+    struct mat4x4 t2 = mat4x4_mul_m(_translation, t1);
 
     left_wall.transform = t2;
   }
   left_wall.material = floor.material;
 
-  struct sphere right_wall = sphere();
+  struct shape right_wall = sphere();
   {
     struct mat4x4 _translation = translation(0.0f, 0.0f, 5.0f);
     struct mat4x4 _rotation_y = rotation_y(pi / 4.0f);
     struct mat4x4 _rotation_x = rotation_x(pi / 2.0f);
     struct mat4x4 _scaling = scaling(10.0f, 0.05f, 10.0f);
 
-    struct mat4x4 t0 = mat4x4_mul_m(&_rotation_x, &_scaling);
-    struct mat4x4 t1 = mat4x4_mul_m(&_rotation_y, &t0);
-    struct mat4x4 t2 = mat4x4_mul_m(&_translation, &t1);
+    struct mat4x4 t0 = mat4x4_mul_m(_rotation_x, _scaling);
+    struct mat4x4 t1 = mat4x4_mul_m(_rotation_y, t0);
+    struct mat4x4 t2 = mat4x4_mul_m(_translation, t1);
 
     right_wall.transform = t2;
   }
   right_wall.material = floor.material;
+  */
 
-  struct sphere middle = sphere();
+  struct shape middle = sphere();
   middle.transform = translation(-0.5f, 1.0f, 0.5f);
   middle.material.color = color((float)0xfc / 255.f , (float)0x6d / 255.f, (float)0x09 / 255.f);
   middle.material.diffuse = 0.7;
   middle.material.specular = 0.3;
 
-  struct sphere right = sphere();
+  struct shape right = sphere();
   struct mat4x4 right_t = translation(1.5f, 0.5f, -0.5f);
   struct mat4x4 right_s = scaling(0.5f, 0.5f, 0.5f);
-  right.transform = mat4x4_mul_m(&right_t, &right_s);
+  right.transform = mat4x4_mul_m(right_t, right_s);
   right.material.color = color(0.5f, 1.0f, 0.1f);
   right.material.diffuse = 0.7;
   right.material.specular = 0.3;
 
-  struct sphere left = sphere();
+  struct shape left = sphere();
   struct mat4x4 left_t = translation(-1.5f, 0.33f, -0.75f);
   struct mat4x4 left_s = scaling(0.33f, 0.33f, 0.33f);
-  left.transform = mat4x4_mul_m(&left_t, &left_s);
+  left.transform = mat4x4_mul_m(left_t, left_s);
   left.material.color = color((float)0x12 / 255.f, (float)0xc9 / 255.f, (float)0xcc / 255.f);
   left.material.diffuse = 0.7;
   left.material.specular = 0.3;
@@ -83,42 +84,104 @@ int main()
   struct tuple light_color = color(1.0f, 1.0f, 1.0f);
   struct light light = point_light(light_position, light_color);
 
+
+  struct shape h1 = plane();
+  struct shape h2 = plane();
+  struct shape h3 = plane();
+  struct shape h4 = plane();
+  struct shape h5 = plane();
+  struct shape h6 = plane();
+  struct mat4x4 h1_t = translation(0, 8, 8);
+  struct mat4x4 h2_t = translation(4, 0, 6.9282);
+  struct mat4x4 h3_t = translation(-4, 0, 6.9282);
+  struct mat4x4 h4_t = translation(0, -8, 8);
+  struct mat4x4 h5_t = translation(-4, 0, -6.9282);
+  struct mat4x4 h6_t = translation(4, 0, -6.9282);
+  struct mat4x4 h1_r = rotation_x(tau / 4.0f);
+  struct mat4x4 h2_r = rotation_x(tau / 4.0f);
+  struct mat4x4 h3_r = rotation_x(tau / 4.0f);
+  struct mat4x4 h4_r = rotation_x(tau / 4.0f);
+  struct mat4x4 h5_r = rotation_x(tau / 4.0f);
+  struct mat4x4 h6_r = rotation_x(tau / 4.0f);
+
+  struct mat4x4 h1_rr = rotation_y(tau / 4.0f * 0);
+  h1.transform = mat4x4_mul_m(h1_t, h1_r);
+  h1.transform = mat4x4_mul_m(h1_rr, h1.transform);
+
+  struct mat4x4 h2_rr = rotation_y(tau / 4.0f * 1);
+  h2.transform = mat4x4_mul_m(h2_t, h2_r);
+  h2.transform = mat4x4_mul_m(h2_rr, h2.transform);
+
+  struct mat4x4 h3_rr = rotation_y(tau / 4.0f * 2);
+  h3.transform = mat4x4_mul_m(h3_t, h3_r);
+  h3.transform = mat4x4_mul_m(h3_rr, h3.transform);
+
+  struct mat4x4 h4_rr = rotation_y(tau / 4.0f * 3);
+  h4.transform = mat4x4_mul_m(h4_t, h4_r);
+  h4.transform = mat4x4_mul_m(h4_rr, h4.transform);
+
+  struct mat4x4 h5_rr = rotation_y(tau / 4.0f);
+  h5.transform = mat4x4_mul_m(h5_rr, h5_r);
+  h5.transform = mat4x4_mul_m(h5_t, h5.transform);
+
+  struct mat4x4 h6_rr = rotation_y(tau / 4.0f);
+  h6.transform = mat4x4_mul_m(h6_rr, h6_r);
+  h6.transform = mat4x4_mul_m(h6_t, h6.transform);
+
+  h1.material = floor.material;
+  h1.material.color = color(1.0f, 0.0f, 0.0f);
+  h2.material = floor.material;
+  h2.material.color = color(0.0f, 1.0f, 0.0f);
+  h3.material = floor.material;
+  h3.material.color = color(0.0f, 0.0f, 1.0f);
+  h4.material = floor.material;
+  h4.material.color = color(1.0f, 1.0f, 0.0f);
+  h5.material = floor.material;
+  h5.material.color = color(0.0f, 1.0f, 1.0f);
+  h6.material = floor.material;
+  h6.material.color = color(1.0f, 0.0f, 1.0f);
+
   struct world world;
   world.light = light;
-  world.object_count = 6;
+  world.object_count = 4 + 4;
   world.objects[0] = floor;
-  world.objects[1] = left_wall;
-  world.objects[2] = right_wall;
-  world.objects[3] = middle;
-  world.objects[4] = right;
-  world.objects[5] = left;
+  world.objects[1] = middle;
+  world.objects[2] = right;
+  world.objects[3] = left;
 
-  struct camera _camera = camera(1600.0f, 800.0f, pi / 3.0f);
+  world.objects[4] = h1;
+  world.objects[5] = h2;
+  world.objects[6] = h3;
+  world.objects[7] = h4;
+  world.objects[8] = h5;
+  world.objects[9] = h6;
+
+  struct camera _camera = camera(1600.0f, 800.0f, pi / 4.f);
   _camera.transform = view_transform(point(0.0f, 1.5f, -5.0f),
                                      point(0.0f, 1.0f, 0.0f),
                                      vector(0.0f, 1.0f, 0.0f));
 
-  struct mat4x4 middle_t = world.objects[3].transform;
+  struct mat4x4 middle_t = world.objects[1].transform;
 
   for (int i = 0; i < 360; i++) {
 
-    _camera.transform = view_transform(point(5.0f * cosf(pi / 360 * (float)i / 10),
-                                             1.5f,
-                                             -5.0f * sinf(pi / 360 * (float)i / 10)),
+    _camera.transform = view_transform(point(2.0f * cosf(pi / 360 * (float)i * 0.5),
+                                             8.5f,
+                                             1.0f * sinf(pi / 360 * (float)i * 0.5)),
                                        point(0.0f, 1.0f, 0.0f),
                                        vector(0.0f, 1.0f, 0.0f));
 
-    struct tuple light_position = point(-10.0f * cosf(pi / 360 * (float)i),
+    struct tuple light_position = point(-6.0f * cosf(pi / 360 * (float)i * 3),
                                         10.0f,
-                                        -10.0f * sinf(pi / 360 * (float)i));
+                                        -6.0f * sinf(pi / 360 * (float)i * 3));
     world.light.position = light_position;
 
     struct mat4x4 middle_s = scaling(1.0f, 0.4f, 1.0f);
     struct mat4x4 middle_rz = rotation_z(tau / 360.f * (float)i);
     struct mat4x4 middle_ry = rotation_y(tau / 360.f * (float)i);
-    struct mat4x4 middle_r = mat4x4_mul_m(&middle_rz, &middle_ry);
-    struct mat4x4 middle_transform = mat4x4_mul_m(&middle_r, &middle_s);
-    world.objects[3].transform = mat4x4_mul_m(&middle_t, &middle_transform);
+    struct mat4x4 middle_r = mat4x4_mul_m(middle_rz, middle_ry);
+    struct mat4x4 middle_transform = mat4x4_mul_m(middle_r, middle_s);
+    world.objects[1].transform = mat4x4_mul_m(middle_t, middle_transform);
 
     camera_render(&_camera, &world, &_canvas);
 
diff --git a/shapes.h b/shapes.h
new file mode 100644
index 0000000..61a95d3
--- /dev/null
+++ b/shapes.h
@@ -0,0 +1,25 @@
+#pragma once
+
+#include "matrices.h"
+#include "materials.h"
+
+enum shape_type {
+  SHAPE_TEST,
+  SHAPE_SPHERE,
+  SHAPE_PLANE,
+};
+
+struct shape {
+  struct mat4x4 transform;
+  struct material material;
+  enum shape_type type;
+};
+
+inline static struct shape shape()
+{
+  return (struct shape){
+    mat4x4_identity(),
+    material(),
+    SHAPE_TEST,
+  };
+}
diff --git a/spheres.h b/spheres.h
index d912c29..4428c3d 100644
--- a/spheres.h
+++ b/spheres.h
@@ -2,27 +2,43 @@
 
 #include "matrices.h"
 #include "materials.h"
+#include "shapes.h"
+#include "rays.h"
+#include "intersections.h"
 
-struct sphere {
-  struct mat4x4 transform;
-  struct material material;
-};
-
-inline static struct sphere sphere()
+inline static struct shape sphere()
 {
-  return (struct sphere){
+  return (struct shape){
     mat4x4_identity(),
-    material()
+    material(),
+    SHAPE_SPHERE,
   };
 }
 
-inline static struct tuple sphere_normal_at(struct sphere const * const s, struct tuple world_point)
+inline static struct tuple sphere_normal_at(struct tuple local_point)
 {
-  struct mat4x4 inv = mat4x4_inverse(&s->transform);
-  struct tuple object_point = mat4x4_mul_t(&inv, &world_point);
-  struct tuple object_normal = tuple_sub(object_point, point(0.0f, 0.0f, 0.0f));
-  struct mat4x4 inv_t = mat4x4_transpose(&inv);
-  struct tuple world_normal = mat4x4_mul_t(&inv_t, &object_normal);
-  world_normal.w = 0.0f;
-  return tuple_normalize(world_normal);
+  return tuple_sub(local_point, point(0.0f, 0.0f, 0.0f));
+}
+
+inline static void sphere_intersect(struct shape const * const s, struct ray local_ray, struct intersections * intersections)
+{
+  struct tuple sphere_to_ray = tuple_sub(local_ray.origin, point(0.0f, 0.0f, 0.0f));
+
+  float a = tuple_dot(local_ray.direction, local_ray.direction);
+  float b = 2 * tuple_dot(local_ray.direction, sphere_to_ray);
+  float c = tuple_dot(sphere_to_ray, sphere_to_ray) - 1;
+
+  float discriminant = b * b - 4 * a * c;
+
+  if (discriminant < 0) {
+    return;
+  } else {
+    float root = sqrtf(discriminant);
+    float t1 = (-b - root) / (2 * a);
+    float t2 = (-b + root) / (2 * a);
+
+    intersections->i[intersections->count++] = intersection(t1, s);
+    intersections->i[intersections->count++] = intersection(t2, s);
+    return;
+  }
 }
diff --git a/test/run.sh b/test/run.sh
index ce50151..54db03e 100644
--- a/test/run.sh
+++ b/test/run.sh
@@ -2,10 +2,13 @@
 
 set -eux
 
-for name in tuples canvas matrices transformations rays intersections spheres lights materials world camera; do
+
+for name in tuples canvas matrices transformations rays intersections shapes spheres planes lights materials world camera; do
     gcc -g -gdwarf-5 \
         -Wall -Werror -Wfatal-errors \
+        -Wno-error=unused-variable \
         -I. \
+        -DTEST \
         test/test_${name}.c -o test/test_${name} -O0 -lm
     ./test/test_${name}
 done
diff --git a/test/test_camera.c b/test/test_camera.c
index 9820d9c..af7f402 100644
--- a/test/test_camera.c
+++ b/test/test_camera.c
@@ -21,7 +21,7 @@ static bool camera_test_0(const char ** scenario)
     float_equal(c.hsize, 160.0f) &&
     float_equal(c.vsize, 120.0f) &&
     float_equal(c.field_of_view, pi / 2.0f) &&
-    mat4x4_equal(&c.transform, &identity_matrix);
+    mat4x4_equal(c.transform, identity_matrix);
 }
 
 static bool camera_test_1(const char ** scenario)
@@ -73,7 +73,7 @@ static bool camera_test_5(const char ** scenario)
   struct camera c = camera(201.0f, 101.0f, pi / 2.0f);
   struct mat4x4 rotate = rotation_y(pi / 4.0f);
   struct mat4x4 translate = translation(0.0f, -2.0f, 5.0f);
-  c.transform = mat4x4_mul_m(&rotate, &translate);
+  c.transform = mat4x4_mul_m(rotate, translate);
   struct ray r = camera_ray_for_pixel(&c, 100.0f, 50.0f);
 
   return
diff --git a/test/test_intersections.c b/test/test_intersections.c
index 98025dd..a42ff2a 100644
--- a/test/test_intersections.c
+++ b/test/test_intersections.c
@@ -1,7 +1,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 
-#include "intersections.h"
+#include "intersections_shapes.h"
 #include "transformations.h"
 #include "runner.h"
 
@@ -9,7 +9,7 @@ static bool intersections_test_0(const char ** scenario)
 {
   *scenario = "An intersection encapsulates t and object";
 
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersection i = intersection(3.5f, &s);
 
   return
@@ -21,7 +21,7 @@ static bool intersections_test_1(const char ** scenario)
 {
   *scenario = "Aggregating intersections";
 
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersection i1 = intersection(1.0f, &s);
   struct intersection i2 = intersection(2.0f, &s);
   struct intersections xs;
@@ -37,7 +37,7 @@ static bool intersections_test_2(const char ** scenario)
 {
   *scenario = "The hit, when all intersections have positive t";
 
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersection i1 = intersection(1.0f, &s);
   struct intersection i2 = intersection(2.0f, &s);
   struct intersections xs;
@@ -55,7 +55,7 @@ static bool intersections_test_3(const char ** scenario)
 {
   *scenario = "The hit, when some intersections have negative t";
 
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersection i1 = intersection(-1.0f, &s);
   struct intersection i2 = intersection( 1.0f, &s);
   struct intersections xs;
@@ -73,7 +73,7 @@ static bool intersections_test_4(const char ** scenario)
 {
   *scenario = "The hit, when all intersections have negative t";
 
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersection i1 = intersection(-2.0f, &s);
   struct intersection i2 = intersection(-1.0f, &s);
   struct intersections xs;
@@ -88,7 +88,7 @@ static bool intersections_test_5(const char ** scenario)
 {
   *scenario = "The hit is always the lowest nonnegative intersection";
 
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersection i1 = intersection( 5.0f, &s);
   struct intersection i2 = intersection( 7.0f, &s);
   struct intersection i3 = intersection(-3.0f, &s);
@@ -108,13 +108,13 @@ static bool intersections_test_6(const char ** scenario)
 {
   *scenario = "In-place ascending sort of intersections";
 
-  struct sphere s1 = sphere();
+  struct shape s1 = sphere();
   s1.material.ambient = 1.0f;
-  struct sphere s2 = sphere();
+  struct shape s2 = sphere();
   s2.material.ambient = 2.0f;
-  struct sphere s3 = sphere();
+  struct shape s3 = sphere();
   s3.material.ambient = 3.0f;
-  struct sphere s4 = sphere();
+  struct shape s4 = sphere();
   s4.material.ambient = 4.0f;
   struct intersection i1 = intersection( 5.0f, &s1);
   struct intersection i2 = intersection( 7.0f, &s2);
@@ -141,7 +141,7 @@ static bool intersections_test_7(const char ** scenario)
   *scenario = "Precomputing the state of an intersection";
 
   struct ray r = ray(point(0.0f, 0.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere shape = sphere();
+  struct shape shape = sphere();
   struct intersection i = intersection(4.0f, &shape);
   struct computations comps = prepare_computations(i, r);
 
@@ -158,7 +158,7 @@ static bool intersections_test_8(const char ** scenario)
   *scenario = "The hit, when an intersection occurs on the outside";
 
   struct ray r = ray(point(0.0f, 0.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere shape = sphere();
+  struct shape shape = sphere();
   struct intersection i = intersection(4.0f, &shape);
   struct computations comps = prepare_computations(i, r);
 
@@ -171,7 +171,7 @@ static bool intersections_test_9(const char ** scenario)
   *scenario = "The hit, when an intersection occurs on the inside";
 
   struct ray r = ray(point(0.0f, 0.0f, 0.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere shape = sphere();
+  struct shape shape = sphere();
   struct intersection i = intersection(1.0f, &shape);
   struct computations comps = prepare_computations(i, r);
 
@@ -187,7 +187,7 @@ static bool intersections_test_10(const char ** scenario)
   *scenario = "The hit should offset the point";
 
   struct ray r = ray(point(0.0f, 0.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere shape = sphere();
+  struct shape shape = sphere();
   shape.transform = translation(0.0f, 0.0f, 1.0f);
   struct intersection i = intersection(5.0f, &shape);
   struct computations comps = prepare_computations(i, r);
diff --git a/test/test_matrices.c b/test/test_matrices.c
index 7071abf..fdc31c0 100644
--- a/test/test_matrices.c
+++ b/test/test_matrices.c
@@ -58,7 +58,7 @@ static bool matrices_test_3(const char ** scenario)
                            9.0f, 8.0f, 7.0f, 6.0f,
                            5.0f, 4.0f, 3.0f, 2.0f);
 
-  return mat4x4_equal(&a, &b);
+  return mat4x4_equal(a, b);
 }
 
 static bool matrices_test_4(const char ** scenario)
@@ -75,7 +75,7 @@ static bool matrices_test_4(const char ** scenario)
                            8.0f, 7.0f, 6.0f, 5.0f,
                            4.0f, 3.0f, 2.0f, 1.0f);
 
-  return !mat4x4_equal(&a, &b);
+  return !mat4x4_equal(a, b);
 }
 
 static bool matrices_test_5(const char ** scenario)
@@ -92,12 +92,12 @@ static bool matrices_test_5(const char ** scenario)
                             4.0f, 3.0f, 6.0f,  5.0f,
                             1.0f, 2.0f, 7.0f,  8.0f);
 
-  struct mat4x4 m1 = mat4x4_mul_m(&a, &b);
+  struct mat4x4 m1 = mat4x4_mul_m(a, b);
   struct mat4x4 m2 = mat4x4(20.0f, 22.0f,  50.0f,  48.0f,
                             44.0f, 54.0f, 114.0f, 108.0f,
                             40.0f, 58.0f, 110.0f, 102.0f,
                             16.0f, 26.0f,  46.0f,  42.0f);
-  return mat4x4_equal(&m1, &m2);
+  return mat4x4_equal(m1, m2);
 }
 
 static bool matrices_test_6(const char ** scenario)
@@ -110,7 +110,7 @@ static bool matrices_test_6(const char ** scenario)
                            0.0f, 0.0f, 0.0f, 1.0f);
   struct tuple b = tuple(1.0f, 2.0f, 3.0f, 1.0f);
 
-  struct tuple c = mat4x4_mul_t(&a, &b);
+  struct tuple c = mat4x4_mul_t(a, b);
   return tuple_equal(c, tuple(18, 24, 33, 1));
 }
 
@@ -124,8 +124,8 @@ static bool matrices_test_7(const char ** scenario)
                            4.0f, 8.0f, 16.0f, 32.0f);
 
   struct mat4x4 id = mat4x4_identity();
-  struct mat4x4 c = mat4x4_mul_m(&a, &id);
-  return mat4x4_equal(&a, &c);
+  struct mat4x4 c = mat4x4_mul_m(a, id);
+  return mat4x4_equal(a, c);
 }
 
 static bool matrices_test_8(const char ** scenario)
@@ -135,7 +135,7 @@ static bool matrices_test_8(const char ** scenario)
   struct tuple a = tuple(1.0f, 2.0f, 3.0f, 4.0f);
 
   struct mat4x4 id = mat4x4_identity();
-  struct tuple c = mat4x4_mul_t(&id, &a);
+  struct tuple c = mat4x4_mul_t(id, a);
   return tuple_equal(a, c);
 }
 
@@ -153,9 +153,9 @@ static bool matrices_test_9(const char ** scenario)
                            3.0f, 0.0f, 5.0f, 5.0f,
                            0.0f, 8.0f, 3.0f, 8.0f);
 
-  struct mat4x4 c = mat4x4_transpose(&a);
+  struct mat4x4 c = mat4x4_transpose(a);
 
-  return mat4x4_equal(&c, &b);
+  return mat4x4_equal(c, b);
 }
 
 static bool matrices_test_10(const char ** scenario)
@@ -164,9 +164,9 @@ static bool matrices_test_10(const char ** scenario)
 
   struct mat4x4 a = mat4x4_identity();
 
-  struct mat4x4 c = mat4x4_transpose(&a);
+  struct mat4x4 c = mat4x4_transpose(a);
 
-  return mat4x4_equal(&a, &c);
+  return mat4x4_equal(a, c);
 }
 
 static bool matrices_test_11(const char ** scenario)
@@ -176,7 +176,7 @@ static bool matrices_test_11(const char ** scenario)
   struct mat2x2 a = mat2x2( 1.0f, 5.0f,
                            -3.0f, 2.0f);
 
-  return float_equal(mat2x2_determinant(&a), 17);
+  return float_equal(mat2x2_determinant(a), 17);
 }
 
 static bool matrices_test_12(const char ** scenario)
@@ -187,11 +187,11 @@ static bool matrices_test_12(const char ** scenario)
                            -3.0f, 2.0f,  7.0f,
                             0.0f, 6.0f, -3.0f);
 
-  struct mat2x2 b = mat3x3_submatrix(&a, 0, 2);
+  struct mat2x2 b = mat3x3_submatrix(a, 0, 2);
   struct mat2x2 c = mat2x2(-3.0f, 2.0f,
                             0.0f, 6.0f);
 
-  return mat2x2_equal(&b, &c);
+  return mat2x2_equal(b, c);
 }
 
 static bool matrices_test_13(const char ** scenario)
@@ -203,12 +203,12 @@ static bool matrices_test_13(const char ** scenario)
                            -1.0f, 0.0f,  8.0f, 2.0f,
                            -7.0f, 1.0f, -1.0f, 1.0f);
 
-  struct mat3x3 b = mat4x4_submatrix(&a, 2, 1);
+  struct mat3x3 b = mat4x4_submatrix(a, 2, 1);
   struct mat3x3 c = mat3x3(-6.0f,  1.0f, 6.0f,
                            -8.0f,  8.0f, 6.0f,
                            -7.0f, -1.0f, 1.0f);
 
-  return mat3x3_equal(&b, &c);
+  return mat3x3_equal(b, c);
 }
 
 static bool matrices_test_14(const char ** scenario)
@@ -218,11 +218,11 @@ static bool matrices_test_14(const char ** scenario)
   struct mat3x3 a = mat3x3(3,  5,  0,
                            2, -1, -7,
                            6, -1,  5);
-  struct mat2x2 b = mat3x3_submatrix(&a, 1, 0);
+  struct mat2x2 b = mat3x3_submatrix(a, 1, 0);
 
   return
-    float_equal(mat2x2_determinant(&b), 25.0f) &&
-    float_equal(mat3x3_minor(&a, 1, 0), 25.0f);
+    float_equal(mat2x2_determinant(b), 25.0f) &&
+    float_equal(mat3x3_minor(a, 1, 0), 25.0f);
 }
 
 static bool matrices_test_15(const char ** scenario)
@@ -234,11 +234,11 @@ static bool matrices_test_15(const char ** scenario)
                            6, -1,  5);
 
   return
-    float_equal(mat3x3_minor(&a, 0, 0), -12.0f) &&
-    float_equal(mat3x3_cofactor(&a, 0, 0), -12.0f) &&
-    float_equal(mat3x3_minor(&a, 1, 0),  25.0f) &&
-    float_equal(mat3x3_cofactor(&a, 1, 0), -25.0f) &&
-    float_equal(mat3x3_cofactor(&a, 2, 1), 21.0f);
+    float_equal(mat3x3_minor(a, 0, 0), -12.0f) &&
+    float_equal(mat3x3_cofactor(a, 0, 0), -12.0f) &&
+    float_equal(mat3x3_minor(a, 1, 0),  25.0f) &&
+    float_equal(mat3x3_cofactor(a, 1, 0), -25.0f) &&
+    float_equal(mat3x3_cofactor(a, 2, 1), 21.0f);
 
 }
 
@@ -251,10 +251,10 @@ static bool matrices_test_16(const char ** scenario)
                             2.0f, 6.0f,  4.0f);
 
   return
-    float_equal(mat3x3_cofactor(&a, 0, 0), 56.0f) &&
-    float_equal(mat3x3_cofactor(&a, 0, 1), 12.0f) &&
-    float_equal(mat3x3_cofactor(&a, 0, 2), -46.0f) &&
-    float_equal(mat3x3_determinant(&a), -196.0f);
+    float_equal(mat3x3_cofactor(a, 0, 0), 56.0f) &&
+    float_equal(mat3x3_cofactor(a, 0, 1), 12.0f) &&
+    float_equal(mat3x3_cofactor(a, 0, 2), -46.0f) &&
+    float_equal(mat3x3_determinant(a), -196.0f);
 }
 
 static bool matrices_test_17(const char ** scenario)
@@ -267,11 +267,11 @@ static bool matrices_test_17(const char ** scenario)
                            -6.0f,  7.0f,  7.0f, -9.0f);
 
   return
-    float_equal(mat4x4_cofactor(&a, 0, 0), 690.0f) &&
-    float_equal(mat4x4_cofactor(&a, 0, 1), 447.0f) &&
-    float_equal(mat4x4_cofactor(&a, 0, 2), 210.0f) &&
-    float_equal(mat4x4_cofactor(&a, 0, 3), 51.0f) &&
-    float_equal(mat4x4_determinant(&a), -4071.0f);
+    float_equal(mat4x4_cofactor(a, 0, 0), 690.0f) &&
+    float_equal(mat4x4_cofactor(a, 0, 1), 447.0f) &&
+    float_equal(mat4x4_cofactor(a, 0, 2), 210.0f) &&
+    float_equal(mat4x4_cofactor(a, 0, 3), 51.0f) &&
+    float_equal(mat4x4_determinant(a), -4071.0f);
 }
 
 static bool matrices_test_18(const char ** scenario)
@@ -284,8 +284,8 @@ static bool matrices_test_18(const char ** scenario)
                            9.0f,  1.0f, 7.0f, -6.0f);
 
   return
-    float_equal(mat4x4_determinant(&a), -2120.0f) &&
-    mat4x4_is_invertible(&a);
+    float_equal(mat4x4_determinant(a), -2120.0f) &&
+    mat4x4_is_invertible(a);
 }
 
 static bool matrices_test_19(const char ** scenario)
@@ -298,8 +298,8 @@ static bool matrices_test_19(const char ** scenario)
                             0.0f,  0.0f,  0.0f,  0.0f);
 
   return
-    float_equal(mat4x4_determinant(&a), 0.0f) &&
-    !mat4x4_is_invertible(&a);
+    float_equal(mat4x4_determinant(a), 0.0f) &&
+    !mat4x4_is_invertible(a);
 }
 
 static bool matrices_test_20(const char ** scenario)
@@ -310,7 +310,7 @@ static bool matrices_test_20(const char ** scenario)
                             1.0, -5.0,  1.0,  8.0,
                             7.0,  7.0, -6.0, -7.0,
                             1.0, -3.0,  7.0,  4.0);
-  struct mat4x4 b = mat4x4_inverse(&a);
+  struct mat4x4 b = mat4x4_inverse(a);
 
   struct mat4x4 c = mat4x4( 0.218045f,  0.451128f,  0.240602f, -0.045113f,
                            -0.808271f, -1.456767f, -0.443609f,  0.520677f,
@@ -318,12 +318,12 @@ static bool matrices_test_20(const char ** scenario)
                            -0.522564f, -0.813910f, -0.300752f,  0.306391f);
 
   return
-    float_equal(mat4x4_determinant(&a), 532.0f) &&
-    float_equal(mat4x4_cofactor(&a, 2, 3), -160.0f) &&
+    float_equal(mat4x4_determinant(a), 532.0f) &&
+    float_equal(mat4x4_cofactor(a, 2, 3), -160.0f) &&
     float_equal(b.e[3][2], -160.0f/532.0f) &&
-    float_equal(mat4x4_cofactor(&a, 3, 2), 105.0f) &&
+    float_equal(mat4x4_cofactor(a, 3, 2), 105.0f) &&
     float_equal(b.e[2][3], 105.0f/532.0f) &&
-    mat4x4_equal(&b, &c);
+    mat4x4_equal(b, c);
 }
 
 static bool matrices_test_21(const char ** scenario)
@@ -335,14 +335,14 @@ static bool matrices_test_21(const char ** scenario)
                            -6.0f,  0.0f,  9.0f,  6.0f,
                            -3.0f,  0.0f, -9.0f, -4.0f);
 
-  struct mat4x4 b = mat4x4_inverse(&a);
+  struct mat4x4 b = mat4x4_inverse(a);
 
   struct mat4x4 c = mat4x4(-0.15385f, -0.15385f, -0.28205f, -0.53846f,
                            -0.07692f,  0.12308f,  0.02564f,  0.03077f,
                             0.35897f,  0.35897f,  0.43590f,  0.92308f,
                            -0.69231f, -0.69231f, -0.76923f, -1.92308f);
 
-  return mat4x4_equal(&b, &c);
+  return mat4x4_equal(b, c);
 }
 
 static bool matrices_test_22(const char ** scenario)
@@ -354,14 +354,14 @@ static bool matrices_test_22(const char ** scenario)
                            -4.0f,  9.0f,  6.0f,  4.0f,
                            -7.0f,  6.0f,  6.0f,  2.0f);
 
-  struct mat4x4 b = mat4x4_inverse(&a);
+  struct mat4x4 b = mat4x4_inverse(a);
 
   struct mat4x4 c = mat4x4(-0.04074f, -0.07778f,  0.14444f, -0.22222f,
                            -0.07778f,  0.03333f,  0.36667f, -0.33333f,
                            -0.02901f, -0.14630f, -0.10926f,  0.12963f,
                             0.17778f,  0.06667f, -0.26667f,  0.33333f);
 
-  return mat4x4_equal(&b, &c);
+  return mat4x4_equal(b, c);
 }
 
 static bool matrices_test_23(const char ** scenario)
@@ -378,12 +378,12 @@ static bool matrices_test_23(const char ** scenario)
                             7.0f,  0.0f,  5.0f,  4.0f,
                             6.0f, -2.0f,  0.0f,  5.0f);
 
-  struct mat4x4 c = mat4x4_mul_m(&a, &b);
+  struct mat4x4 c = mat4x4_mul_m(a, b);
 
-  struct mat4x4 b_i = mat4x4_inverse(&b);
-  struct mat4x4 d = mat4x4_mul_m(&c, &b_i);
+  struct mat4x4 b_i = mat4x4_inverse(b);
+  struct mat4x4 d = mat4x4_mul_m(c, b_i);
 
-  return mat4x4_equal(&d, &a);
+  return mat4x4_equal(d, a);
 }
 
 test_t matrices_tests[] = {
diff --git a/test/test_planes.c b/test/test_planes.c
new file mode 100644
index 0000000..a6e5863
--- /dev/null
+++ b/test/test_planes.c
@@ -0,0 +1,86 @@
+#include <stdbool.h>
+#include <stdio.h>
+
+#include "rays.h"
+#include "planes.h"
+#include "intersections_shapes.h"
+#include "runner.h"
+#include "matrices.h"
+#include "transformations.h"
+
+static bool planes_test_0(const char ** scenario)
+{
+  *scenario = "The normal of a plane is constant everywhere";
+
+  struct tuple n1 = plane_normal_at(point(0.0f, 0.0f, 0.0f));
+  struct tuple n2 = plane_normal_at(point(10.0f, 0.0f, -10.0f));
+  struct tuple n3 = plane_normal_at(point(-5.0f, 0.0f, 150.0f));
+  return
+    tuple_equal(n1, vector(0.0f, 1.0f, 0.0f)) &&
+    tuple_equal(n2, vector(0.0f, 1.0f, 0.0f)) &&
+    tuple_equal(n3, vector(0.0f, 1.0f, 0.0f));
+}
+
+static bool planes_test_1(const char ** scenario)
+{
+  *scenario = "Intersect with a ray parallel to the plane";
+
+  struct shape p = plane();
+  struct ray r = ray(point(0.0f, 10.0f, 0.0f), vector(0.0f, 0.0f, 1.0f));
+  struct intersections xs;
+  xs.count = 0;
+  plane_intersect(&p, r, &xs);
+  return xs.count == 0;
+}
+
+static bool planes_test_2(const char ** scenario)
+{
+  *scenario = "Intersect with a coplanar ray";
+
+  struct shape p = plane();
+  struct ray r = ray(point(0.0f, 0.0f, 0.0f), vector(0.0f, 0.0f, 1.0f));
+  struct intersections xs;
+  xs.count = 0;
+  plane_intersect(&p, r, &xs);
+  return xs.count == 0;
+}
+
+static bool planes_test_3(const char ** scenario)
+{
+  *scenario = "A ray intersecting a plane from above";
+
+  struct shape p = plane();
+  struct ray r = ray(point(0.0f, 1.0f, 0.0f), vector(0.0f, -1.0f, 0.0f));
+  struct intersections xs;
+  xs.count = 0;
+  plane_intersect(&p, r, &xs);
+  return
+    xs.count == 1 &&
+    float_equal(xs.i[0].t, 1.0f) &&
+    xs.i[0].object == &p;
+}
+
+static bool planes_test_4(const char ** scenario)
+{
+  *scenario = "A ray intersecting a plane from below";
+
+  struct shape p = plane();
+  struct ray r = ray(point(0.0f, -1.0f, 0.0f), vector(0.0f, 1.0f, 1.0f));
+  struct intersections xs;
+  xs.count = 0;
+  plane_intersect(&p, r, &xs);
+  return
+    xs.count == 1 &&
+    float_equal(xs.i[0].t, 1.0f) &&
+    xs.i[0].object == &p;
+}
+
+test_t planes_tests[] = {
+  planes_test_0,
+  planes_test_1,
+  planes_test_2,
+  planes_test_3,
+  planes_test_4,
+};
+
+RUNNER(planes_tests);
diff --git a/test/test_rays.c b/test/test_rays.c
index d861a42..49740a5 100644
--- a/test/test_rays.c
+++ b/test/test_rays.c
@@ -38,7 +38,7 @@ static bool rays_test_2(const char ** scenario)
 
   struct ray r = ray(point(1.0f, 2.0f, 3.0f), vector(0.0f, 1.0f, 0.0f));
   struct mat4x4 m = translation(3.0f, 4.0f, 5.0f);
-  struct ray r2 = ray_transform(r, &m);
+  struct ray r2 = ray_transform(r, m);
 
   return
     tuple_equal(r2.origin, point(4.0f, 6.0f, 8.0f)) &&
@@ -51,7 +51,7 @@ static bool rays_test_3(const char ** scenario)
 
   struct ray r = ray(point(1.0f, 2.0f, 3.0f), vector(0.0f, 1.0f, 0.0f));
   struct mat4x4 m = scaling(2.0f, 3.0f, 4.0f);
-  struct ray r2 = ray_transform(r, &m);
+  struct ray r2 = ray_transform(r, m);
 
   return
     tuple_equal(r2.origin, point(2.0f, 6.0f, 12.0f)) &&
diff --git a/test/test_shapes.c b/test/test_shapes.c
new file mode 100644
index 0000000..09a13b4
--- /dev/null
+++ b/test/test_shapes.c
@@ -0,0 +1,121 @@
+#include <stdbool.h>
+#include <stdio.h>
+
+#include "shapes.h"
+#include "intersections_shapes.h"
+#include "transformations.h"
+#include "runner.h"
+#include "rays.h"
+#include "spheres.h"
+
+static bool shapes_test_0(const char ** scenario)
+{
+  *scenario = "A shape's default transformation";
+
+  struct shape s = shape();
+  struct mat4x4 identity = mat4x4_identity();
+
+  return mat4x4_equal(s.transform, identity);
+}
+
+static bool shapes_test_1(const char ** scenario)
+{
+  *scenario = "Changing a shape's transformation";
+
+  struct shape s = shape();
+  struct mat4x4 t = translation(2.0f, 3.0f, 4.0f);
+  s.transform = t;
+
+  return mat4x4_equal(s.transform, t);
+}
+
+static bool shapes_test_2(const char ** scenario)
+{
+  *scenario = "A shape has a default material";
+
+  struct shape s = shape();
+  struct material m = s.material;
+  return material_equal(m, material());
+}
+
+static bool shapes_test_3(const char ** scenario)
+{
+  *scenario = "A shape may be assigned a material";
+
+  struct shape s = shape();
+  struct material m = material();
+  m.ambient = 1.0f;
+  s.material = m;
+  return material_equal(s.material, m);
+}
+
+static bool shapes_test_4(const char ** scenario)
+{
+  *scenario = "Intersecting a scaled shape with a ray";
+
+  struct ray r = ray(point(0.0f, 0.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
+  struct shape s = shape();
+  s.transform = scaling(2.0f, 2.0f, 2.0f);
+  struct intersections xs;
+  xs.count = 0;
+  intersect(&s, r, &xs);
+
+  return
+    tuple_equal(intersections_saved_ray.origin, point(0.0f, 0.0f, -2.5f)) &&
+    tuple_equal(intersections_saved_ray.direction, vector(0.0f, 0.0f, 0.5f));
+}
+
+static bool shapes_test_5(const char ** scenario)
+{
+  *scenario = "Intersecting a translated shape with a ray";
+
+  struct ray r = ray(point(0.0f, 0.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
+  struct shape s = shape();
+  s.transform = translation(5.0f, 0.0f, 0.0f);
+  struct intersections xs;
+  xs.count = 0;
+  intersect(&s, r, &xs);
+
+  return
+    tuple_equal(intersections_saved_ray.origin, point(-5.0f, 0.0f, -5.0f)) &&
+    tuple_equal(intersections_saved_ray.direction, vector(0.0f, 0.0f, 1.0f));
+}
+
+static bool shapes_test_6(const char ** scenario)
+{
+  *scenario = "Computing the normal on a translated shape";
+
+  struct shape s = shape();
+  s.transform = translation(0.0f, 1.0f, 0.0f);
+
+  struct tuple n = normal_at(&s, point(0.0f, 1.70711f, -0.70711));
+
+  return tuple_equal(n, vector(0.0f, 0.70711f, -0.70711f));
+}
+
+static bool shapes_test_7(const char ** scenario)
+{
+  *scenario = "Computing the normal on a transformed shape";
+  struct shape s = shape();
+  struct mat4x4 scale = scaling(1.0f, 0.5f, 1.0f);
+  struct mat4x4 rotate = rotation_z(pi / 5.0f);
+  struct mat4x4 m = mat4x4_mul_m(scale, rotate);
+  s.transform = m;
+
+  struct tuple n = normal_at(&s, point(0.0f, 0.7071067811865476, -0.7071067811865476));
+
+  return tuple_equal(n, vector(0.0f, 0.97014f, -0.24254f));
+}
+
+test_t shape_tests[] = {
+  shapes_test_0,
+  shapes_test_1,
+  shapes_test_2,
+  shapes_test_3,
+  shapes_test_4,
+  shapes_test_5,
+  shapes_test_6,
+  shapes_test_7,
+};
+
+RUNNER(shape_tests)
diff --git a/test/test_spheres.c b/test/test_spheres.c
index 0a0afb5..2495806 100644
--- a/test/test_spheres.c
+++ b/test/test_spheres.c
@@ -3,7 +3,7 @@
 
 #include "rays.h"
 #include "spheres.h"
-#include "intersections.h"
+#include "intersections_shapes.h"
 #include "runner.h"
 #include "matrices.h"
 #include "transformations.h"
@@ -13,7 +13,7 @@ static bool spheres_test_0(const char ** scenario)
   *scenario = "A ray intersects a sphere at two points";
 
   struct ray r = ray(point(0.0f, 0.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersections xs;
   xs.count = 0;
   intersect(&s, r, &xs);
@@ -29,7 +29,7 @@ static bool spheres_test_1(const char ** scenario)
   *scenario = "A ray intersects a sphere at a tangent";
 
   struct ray r = ray(point(0.0f, 1.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersections xs;
   xs.count = 0;
   intersect(&s, r, &xs);
@@ -45,7 +45,7 @@ static bool spheres_test_2(const char ** scenario)
   *scenario = "A ray misses a sphere";
 
   struct ray r = ray(point(0.0f, 2.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersections xs;
   xs.count = 0;
   intersect(&s, r, &xs);
@@ -58,7 +58,7 @@ static bool spheres_test_3(const char ** scenario)
   *scenario = "A ray originates in a sphere";
 
   struct ray r = ray(point(0.0f, 0.0f, 0.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersections xs;
   xs.count = 0;
   intersect(&s, r, &xs);
@@ -74,7 +74,7 @@ static bool spheres_test_4(const char ** scenario)
   *scenario = "A sphere is behind a ray";
 
   struct ray r = ray(point(0.0f, 0.0f, 5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersections xs;
   xs.count = 0;
   intersect(&s, r, &xs);
@@ -90,7 +90,7 @@ static bool spheres_test_5(const char ** scenario)
   *scenario = "Intersect sets the object on the intersection";
 
   struct ray r = ray(point(0.0f, 0.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct intersections xs;
   xs.count = 0;
   intersect(&s, r, &xs);
@@ -102,32 +102,11 @@ static bool spheres_test_5(const char ** scenario)
 }
 
 static bool spheres_test_6(const char ** scenario)
-{
-  *scenario = "A sphere's default transformation";
-
-  struct sphere s = sphere();
-  struct mat4x4 identity = mat4x4_identity();
-
-  return mat4x4_equal(&s.transform, &identity);
-}
-
-static bool spheres_test_7(const char ** scenario)
-{
-  *scenario = "Changing a sphere's transformation";
-
-  struct sphere s = sphere();
-  struct mat4x4 t = translation(2.0f, 3.0f, 4.0f);
-  s.transform = t;
-
-  return mat4x4_equal(&s.transform, &t);
-}
-
-static bool spheres_test_8(const char ** scenario)
 {
   *scenario = "Intersecting a scaled sphere with a ray";
 
   struct ray r = ray(point(0.0f, 0.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere s = sphere();
+  struct shape s = sphere();
   s.transform = scaling(2.0f, 2.0f, 2.0f);
   struct intersections xs;
   xs.count = 0;
@@ -139,12 +118,12 @@ static bool spheres_test_8(const char ** scenario)
     float_equal(xs.i[1].t, 7.0f);
 }
 
-static bool spheres_test_9(const char ** scenario)
+static bool spheres_test_7(const char ** scenario)
 {
   *scenario = "Intersecting a translated sphere with a ray";
 
   struct ray r = ray(point(0.0f, 0.0f, 5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere s = sphere();
+  struct shape s = sphere();
   s.transform = translation(5.0f, 0.0f, 0.0f);
   struct intersections xs;
   xs.count = 0;
@@ -153,103 +132,83 @@ static bool spheres_test_9(const char ** scenario)
   return xs.count == 0;
 }
 
-static bool spheres_test_10(const char ** scenario)
+static bool spheres_test_8(const char ** scenario)
 {
   *scenario = "The normal on a sphere at a point on the x axis";
 
-  struct sphere s = sphere();
-  struct tuple n = sphere_normal_at(&s, point(1.0f, 0.0f, 0.0f));
+  struct shape s = sphere();
+  struct tuple n = normal_at(&s, point(1.0f, 0.0f, 0.0f));
   return tuple_equal(n, vector(1.0f, 0.0f, 0.0f));
 }
 
+static bool spheres_test_9(const char ** scenario)
+{
+  *scenario = "The normal on a sphere at a point on the y axis";
+
+  struct shape s = sphere();
+  struct tuple n = normal_at(&s, point(0.0f, 1.0f, 0.0f));
+  return tuple_equal(n, vector(0.0f, 1.0f, 0.0f));
+}
+
+static bool spheres_test_10(const char ** scenario)
+{
+  *scenario = "The normal on a sphere at a point on the z axis";
+
+  struct shape s = sphere();
+  struct tuple n = normal_at(&s, point(0.0f, 0.0f, 1.0f));
+  return tuple_equal(n, vector(0.0f, 0.0f, 1.0f));
+}
+
 static bool spheres_test_11(const char ** scenario)
-{
-  *scenario = "The normal on a sphere at a point on the y axis";
-
-  struct sphere s = sphere();
-  struct tuple n = sphere_normal_at(&s, point(0.0f, 1.0f, 0.0f));
-  return tuple_equal(n, vector(0.0f, 1.0f, 0.0f));
-}
-
-static bool spheres_test_12(const char ** scenario)
-{
-  *scenario = "The normal on a sphere at a point on the z axis";
-
-  struct sphere s = sphere();
-  struct tuple n = sphere_normal_at(&s, point(0.0f, 0.0f, 1.0f));
-  return tuple_equal(n, vector(0.0f, 0.0f, 1.0f));
-}
-
-static bool spheres_test_13(const char ** scenario)
 {
   *scenario = "The normal on a sphere at a nonaxial point";
 
-  struct sphere s = sphere();
-  struct tuple n = sphere_normal_at(&s, point(0.5773502691896257,
-                                              0.5773502691896257,
-                                              0.5773502691896257));
+  struct shape s = sphere();
+  struct tuple n = normal_at(&s, point(0.5773502691896257,
+                                       0.5773502691896257,
+                                       0.5773502691896257));
   return tuple_equal(n, vector(0.5773502691896257,
                                0.5773502691896257,
                                0.5773502691896257));
 }
 
-static bool spheres_test_14(const char ** scenario)
+static bool spheres_test_12(const char ** scenario)
 {
   *scenario = "The normal is a normalized vector";
 
-  struct sphere s = sphere();
-  struct tuple n = sphere_normal_at(&s, point(0.5773502691896257,
-                                              0.5773502691896257,
-                                              0.5773502691896257));
+  struct shape s = sphere();
+  struct tuple n = normal_at(&s, point(0.5773502691896257,
+                                       0.5773502691896257,
+                                       0.5773502691896257));
   return tuple_equal(n, tuple_normalize(n));
 }
 
-static bool spheres_test_15(const char ** scenario)
+static bool spheres_test_13(const char ** scenario)
 {
   *scenario = "Computing the normal on a translated sphere";
 
-  struct sphere s = sphere();
+  struct shape s = sphere();
   s.transform = translation(0.0f, 1.0f, 0.0f);
-  struct tuple n = sphere_normal_at(&s, point(0.0f, 1.70711f, -0.70711));
+  struct tuple n = normal_at(&s, point(0.0f, 1.70711f, -0.70711));
 
   return tuple_equal(n, vector(0.0f, 0.70711f, -0.70711f));
 }
 
-static bool spheres_test_16(const char ** scenario)
+static bool spheres_test_14(const char ** scenario)
 {
   *scenario = "Computing the normal on a transformed sphere";
 
-  struct sphere s = sphere();
+  struct shape s = sphere();
   struct mat4x4 scale = scaling(1.0f, 0.5f, 1.0f);
   struct mat4x4 rotate = rotation_z(pi / 5.0f);
-  struct mat4x4 m = mat4x4_mul_m(&scale, &rotate);
+  struct mat4x4 m = mat4x4_mul_m(scale, rotate);
   s.transform = m;
 
-  struct tuple n = sphere_normal_at(&s, point(0.0f, 0.7071067811865476, -0.7071067811865476));
+  struct tuple n = normal_at(&s, point(0.0f, 0.7071067811865476, -0.7071067811865476));
 
   return tuple_equal(n, vector(0.0f, 0.97014f, -0.24254f));
 }
 
-static bool spheres_test_17(const char ** scenario)
-{
-  *scenario = "A sphere has a default material";
-
-  struct sphere s = sphere();
-  struct material m = s.material;
-  return material_equal(m, material());
-}
-
-static bool spheres_test_18(const char ** scenario)
-{
-  *scenario = "A sphere may be assigned a material";
-
-  struct sphere s = sphere();
-  struct material m = material();
-  m.ambient = 1.0f;
-  s.material = m;
-  return material_equal(s.material, m);
-}
-
 test_t spheres_tests[] = {
   spheres_test_0,
   spheres_test_1,
@@ -266,10 +225,6 @@ test_t spheres_tests[] = {
   spheres_test_12,
   spheres_test_13,
   spheres_test_14,
-  spheres_test_15,
-  spheres_test_16,
-  spheres_test_17,
-  spheres_test_18,
 };
 
 RUNNER(spheres_tests);
diff --git a/test/test_transformations.c b/test/test_transformations.c
index e0442e0..b1aec7e 100644
--- a/test/test_transformations.c
+++ b/test/test_transformations.c
@@ -11,7 +11,7 @@ static bool transformations_test_0(const char ** scenario)
   struct mat4x4 transform = translation(5.0f, -3.0f, 2.0f);
   struct tuple p = point(-3.0f, 4.0f, 5.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &p), point(2.0f, 1.0f, 7.0f));
+  return tuple_equal(mat4x4_mul_t(transform, p), point(2.0f, 1.0f, 7.0f));
 }
 
 static bool transformations_test_1(const char ** scenario)
@@ -19,10 +19,10 @@ static bool transformations_test_1(const char ** scenario)
   *scenario = "Multiplying by the inverse of a translation matrix";
 
   struct mat4x4 transform = translation(5.0f, -3.0f, 2.0f);
-  struct mat4x4 inv = mat4x4_inverse(&transform);
+  struct mat4x4 inv = mat4x4_inverse(transform);
   struct tuple p = point(-3.0f, 4.0f, 5.0f);
 
-  return tuple_equal(mat4x4_mul_t(&inv, &p), point(-8.0f, 7.0f, 3.0f));
+  return tuple_equal(mat4x4_mul_t(inv, p), point(-8.0f, 7.0f, 3.0f));
 }
 
 static bool transformations_test_2(const char ** scenario)
@@ -32,7 +32,7 @@ static bool transformations_test_2(const char ** scenario)
   struct mat4x4 transform = translation(5.0f, -3.0f, 2.0f);
   struct tuple v = vector(-3.0f, 4.0f, 5.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &v), v);
+  return tuple_equal(mat4x4_mul_t(transform, v), v);
 }
 
 static bool transformations_test_3(const char ** scenario)
@@ -42,7 +42,7 @@ static bool transformations_test_3(const char ** scenario)
   struct mat4x4 transform = scaling(2.0f, 3.0f, 4.0f);
   struct tuple p = point(-4.0f, 6.0f, 8.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &p), point(-8.0f, 18.0f, 32.0f));
+  return tuple_equal(mat4x4_mul_t(transform, p), point(-8.0f, 18.0f, 32.0f));
 }
 
 static bool transformations_test_4(const char ** scenario)
@@ -52,7 +52,7 @@ static bool transformations_test_4(const char ** scenario)
   struct mat4x4 transform = scaling(2.0f, 3.0f, 4.0f);
   struct tuple v = vector(-4.0f, 6.0f, 8.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &v), vector(-8.0f, 18.0f, 32.0f));
+  return tuple_equal(mat4x4_mul_t(transform, v), vector(-8.0f, 18.0f, 32.0f));
 }
 
 static bool transformations_test_5(const char ** scenario)
@@ -60,10 +60,10 @@ static bool transformations_test_5(const char ** scenario)
   *scenario = "Multiplying by the inverse of a scaling matrix";
 
   struct mat4x4 transform = scaling(2.0f, 3.0f, 4.0f);
-  struct mat4x4 inv = mat4x4_inverse(&transform);
+  struct mat4x4 inv = mat4x4_inverse(transform);
   struct tuple v = vector(-4.0f, 6.0f, 8.0f);
 
-  return tuple_equal(mat4x4_mul_t(&inv, &v), vector(-2.0f, 2.0f, 2.0f));
+  return tuple_equal(mat4x4_mul_t(inv, v), vector(-2.0f, 2.0f, 2.0f));
 }
 
 static bool transformations_test_6(const char ** scenario)
@@ -73,7 +73,7 @@ static bool transformations_test_6(const char ** scenario)
   struct mat4x4 transform = scaling(-1.0f, 1.0f, 1.0f);
   struct tuple p = point(2.0f, 3.0f, 4.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &p), point(-2.0f, 3.0f, 4.0f));
+  return tuple_equal(mat4x4_mul_t(transform, p), point(-2.0f, 3.0f, 4.0f));
 }
 
 static bool transformations_test_7(const char ** scenario)
@@ -85,8 +85,8 @@ static bool transformations_test_7(const char ** scenario)
   struct mat4x4 full_quarter = rotation_x(tau / 4);
 
   return
-    tuple_equal(mat4x4_mul_t(&half_quarter, &p), point(0.0f, 0.7071067811865476, 0.7071067811865476)) &&
-    tuple_equal(mat4x4_mul_t(&full_quarter, &p), point(0.0f, 0.0f, 1.0f));
+    tuple_equal(mat4x4_mul_t(half_quarter, p), point(0.0f, 0.7071067811865476, 0.7071067811865476)) &&
+    tuple_equal(mat4x4_mul_t(full_quarter, p), point(0.0f, 0.0f, 1.0f));
 }
 
 static bool transformations_test_8(const char ** scenario)
@@ -98,8 +98,8 @@ static bool transformations_test_8(const char ** scenario)
   struct mat4x4 full_quarter = rotation_y(tau / 4);
 
   return
-    tuple_equal(mat4x4_mul_t(&half_quarter, &p), point(0.7071067811865476, 0.0f, 0.7071067811865476)) &&
-    tuple_equal(mat4x4_mul_t(&full_quarter, &p), point(1.0f, 0.0f, 0.0f));
+    tuple_equal(mat4x4_mul_t(half_quarter, p), point(0.7071067811865476, 0.0f, 0.7071067811865476)) &&
+    tuple_equal(mat4x4_mul_t(full_quarter, p), point(1.0f, 0.0f, 0.0f));
 }
 
 static bool transformations_test_9(const char ** scenario)
@@ -111,8 +111,8 @@ static bool transformations_test_9(const char ** scenario)
   struct mat4x4 full_quarter = rotation_z(tau / 4);
 
   return
-    tuple_equal(mat4x4_mul_t(&half_quarter, &p), point(-0.7071067811865476, 0.7071067811865476, 0.0f)) &&
-    tuple_equal(mat4x4_mul_t(&full_quarter, &p), point(-1.0f, 0.0f, 0.0f));
+    tuple_equal(mat4x4_mul_t(half_quarter, p), point(-0.7071067811865476, 0.7071067811865476, 0.0f)) &&
+    tuple_equal(mat4x4_mul_t(full_quarter, p), point(-1.0f, 0.0f, 0.0f));
 }
 
 static bool transformations_test_10(const char ** scenario)
@@ -122,7 +122,7 @@ static bool transformations_test_10(const char ** scenario)
   struct mat4x4 transform = shearing(1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f);
   struct tuple p = point(2.0f, 3.0f, 4.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &p), point(5.0f, 3.0f, 4.0f));
+  return tuple_equal(mat4x4_mul_t(transform, p), point(5.0f, 3.0f, 4.0f));
 }
 
 static bool transformations_test_11(const char ** scenario)
@@ -132,7 +132,7 @@ static bool transformations_test_11(const char ** scenario)
   struct mat4x4 transform = shearing(0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f);
   struct tuple p = point(2.0f, 3.0f, 4.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &p), point(6.0f, 3.0f, 4.0f));
+  return tuple_equal(mat4x4_mul_t(transform, p), point(6.0f, 3.0f, 4.0f));
 }
 
 static bool transformations_test_12(const char ** scenario)
@@ -142,7 +142,7 @@ static bool transformations_test_12(const char ** scenario)
   struct mat4x4 transform = shearing(0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f);
   struct tuple p = point(2.0f, 3.0f, 4.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &p), point(2.0f, 5.0f, 4.0f));
+  return tuple_equal(mat4x4_mul_t(transform, p), point(2.0f, 5.0f, 4.0f));
 }
 
 static bool transformations_test_13(const char ** scenario)
@@ -152,7 +152,7 @@ static bool transformations_test_13(const char ** scenario)
   struct mat4x4 transform = shearing(0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f);
   struct tuple p = point(2.0f, 3.0f, 4.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &p), point(2.0f, 7.0f, 4.0f));
+  return tuple_equal(mat4x4_mul_t(transform, p), point(2.0f, 7.0f, 4.0f));
 }
 
 static bool transformations_test_14(const char ** scenario)
@@ -162,7 +162,7 @@ static bool transformations_test_14(const char ** scenario)
   struct mat4x4 transform = shearing(0.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f);
   struct tuple p = point(2.0f, 3.0f, 4.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &p), point(2.0f, 3.0f, 6.0f));
+  return tuple_equal(mat4x4_mul_t(transform, p), point(2.0f, 3.0f, 6.0f));
 }
 
 static bool transformations_test_15(const char ** scenario)
@@ -172,7 +172,7 @@ static bool transformations_test_15(const char ** scenario)
   struct mat4x4 transform = shearing(0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 1.0f);
   struct tuple p = point(2.0f, 3.0f, 4.0f);
 
-  return tuple_equal(mat4x4_mul_t(&transform, &p), point(2.0f, 3.0f, 7.0f));
+  return tuple_equal(mat4x4_mul_t(transform, p), point(2.0f, 3.0f, 7.0f));
 }
 
 static bool transformations_test_16(const char ** scenario)
@@ -184,9 +184,9 @@ static bool transformations_test_16(const char ** scenario)
   struct mat4x4 b = scaling(5.0f, 5.0f, 5.0f);
   struct mat4x4 c = translation(10.0f, 5.0f, 7.0f);
 
-  struct tuple p2 = mat4x4_mul_t(&a, &p);
-  struct tuple p3 = mat4x4_mul_t(&b, &p2);
-  struct tuple p4 = mat4x4_mul_t(&c, &p3);
+  struct tuple p2 = mat4x4_mul_t(a, p);
+  struct tuple p3 = mat4x4_mul_t(b, p2);
+  struct tuple p4 = mat4x4_mul_t(c, p3);
 
   return
     tuple_equal(p2, point(1.0f, -1.0f, 0.0f)) &&
@@ -203,9 +203,9 @@ static bool transformations_test_17(const char ** scenario)
   struct mat4x4 b = scaling(5.0f, 5.0f, 5.0f);
   struct mat4x4 c = translation(10.0f, 5.0f, 7.0f);
 
-  struct mat4x4 t1 = mat4x4_mul_m(&c, &b);
-  struct mat4x4 t2 = mat4x4_mul_m(&t1, &a);
-  struct tuple p1 = mat4x4_mul_t(&t2, &p);
+  struct mat4x4 t1 = mat4x4_mul_m(c, b);
+  struct mat4x4 t2 = mat4x4_mul_m(t1, a);
+  struct tuple p1 = mat4x4_mul_t(t2, p);
 
   return tuple_equal(p1, point(15.0f, 0.0f, 7.0f));
 }
@@ -220,7 +220,7 @@ static bool transformations_test_18(const char ** scenario)
   struct mat4x4 t = view_transform(from, to, up);
 
   struct mat4x4 identity_matrix = mat4x4_identity();
-  return mat4x4_equal(&t, &identity_matrix);
+  return mat4x4_equal(t, identity_matrix);
 }
 
 static bool transformations_test_19(const char ** scenario)
@@ -233,7 +233,7 @@ static bool transformations_test_19(const char ** scenario)
   struct mat4x4 t = view_transform(from, to, up);
 
   struct mat4x4 scaled = scaling(-1.0f, 1.0f, -1.0f);
-  return mat4x4_equal(&t, &scaled);
+  return mat4x4_equal(t, scaled);
 }
 
 static bool transformations_test_20(const char ** scenario)
@@ -246,7 +246,7 @@ static bool transformations_test_20(const char ** scenario)
   struct mat4x4 t = view_transform(from, to, up);
 
   struct mat4x4 translated = translation(0.0f, 0.0f, -8.0f);
-  return mat4x4_equal(&t, &translated);
+  return mat4x4_equal(t, translated);
 }
 
 static bool transformations_test_21(const char ** scenario)
@@ -263,7 +263,7 @@ static bool transformations_test_21(const char ** scenario)
                                   -0.35857f, 0.59761f, -0.71714f,  0.00000f,
                                    0.00000f, 0.00000f,  0.00000f,  1.00000f
                                   );
-  return mat4x4_equal(&t, &expected);
+  return mat4x4_equal(t, expected);
 }
 
 test_t transformations_tests[] = {
diff --git a/test/test_world.c b/test/test_world.c
index c099367..76f5cc6 100644
--- a/test/test_world.c
+++ b/test/test_world.c
@@ -20,11 +20,11 @@ static bool world_test_1(const char ** scenario)
   *scenario = "The default world";
 
   struct light light = point_light(point(-10.0f, 10.0f, -10.0f), color(1.0f, 1.0f, 1.0f));
-  struct sphere s1 = sphere();
+  struct shape s1 = sphere();
   s1.material.color = color(0.8f, 1.0f, 0.6f);
   s1.material.diffuse = 0.7f;
   s1.material.specular = 0.2f;
-  struct sphere s2 = sphere();
+  struct shape s2 = sphere();
   s2.transform = scaling(0.5f, 0.5f, 0.5f);
   struct world w = world_default();
   return
@@ -34,7 +34,7 @@ static bool world_test_1(const char ** scenario)
     tuple_equal(w.objects[0].material.color, s1.material.color) &&
     float_equal(w.objects[0].material.diffuse, s1.material.diffuse) &&
     float_equal(w.objects[0].material.specular, s1.material.specular) &&
-    mat4x4_equal(&w.objects[1].transform, &s2.transform);
+    mat4x4_equal(w.objects[1].transform, s2.transform);
 }
 
 static bool world_test_2(const char ** scenario)
@@ -61,7 +61,7 @@ static bool world_test_3(const char ** scenario)
 
   struct world w = world_default();
   struct ray r = ray(point(0.0f, 0.0f, -5.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere * shape = &w.objects[0];
+  struct shape * shape = &w.objects[0];
   struct intersection i = intersection(4.0f, shape);
   struct computations comps = prepare_computations(i, r);
   struct tuple c = world_shade_hit(&w, &comps);
@@ -76,7 +76,7 @@ static bool world_test_4(const char ** scenario)
   struct world w = world_default();
   w.light = point_light(point(0.0f, 0.25f, 0.0f), color(1.0f, 1.0f, 1.0f));
   struct ray r = ray(point(0.0f, 0.0f, 0.0f), vector(0.0f, 0.0f, 1.0f));
-  struct sphere * shape = &w.objects[1];
+  struct shape * shape = &w.objects[1];
   struct intersection i = intersection(0.5f, shape);
   struct computations comps = prepare_computations(i, r);
   struct tuple c = world_shade_hit(&w, &comps);
@@ -111,9 +111,9 @@ static bool world_test_7(const char ** scenario)
   *scenario = "The color with an intersection behind the ray";
 
   struct world w = world_default();
-  struct sphere * outer = &w.objects[0];
+  struct shape * outer = &w.objects[0];
   outer->material.ambient = 1.0f;
-  struct sphere * inner = &w.objects[1];
+  struct shape * inner = &w.objects[1];
   inner->material.ambient = 1.0f;
   struct ray r = ray(point(0.0f, 0.0f, 0.75f), vector(0.0f, 0.0f, -1.0f));
   struct tuple c = world_color_at(&w, r);
@@ -167,9 +167,9 @@ static bool world_test_12(const char ** scenario)
 
   struct world w = world();
   w.light = point_light(point(0.0f, 0.0f, -10.0f), color(1.0f, 1.0f, 1.0f));
-  struct sphere s1 = sphere();
+  struct shape s1 = sphere();
   w.objects[0] = s1;
-  struct sphere s2 = sphere();
+  struct shape s2 = sphere();
   s2.transform = translation(0.0f, 0.0f, 10.0f);
   w.objects[1] = s2;
   w.object_count = 2;
diff --git a/transformations.h b/transformations.h
index c07c9ec..1d4802b 100644
--- a/transformations.h
+++ b/transformations.h
@@ -70,5 +70,5 @@ inline static struct mat4x4 view_transform(struct tuple from,
                                      0.0f,        0.0f,       0.0f,      1.0f);
 
   struct mat4x4 translate = translation(-from.x, -from.y, -from.z);
-  return mat4x4_mul_m(&orientation, &translate);
+  return mat4x4_mul_m(orientation, translate);
 }
diff --git a/world.h b/world.h
index 700ffe9..39922fe 100644
--- a/world.h
+++ b/world.h
@@ -3,7 +3,7 @@
 #include "lights.h"
 #include "spheres.h"
 #include "transformations.h"
-#include "intersections.h"
+#include "intersections_shapes.h"
 #include "rays.h"
 #include "materials.h"
 
@@ -12,7 +12,7 @@
 struct world {
   struct light light;
   int object_count;
-  struct sphere objects[WORLD_MAX_OBJECTS];
+  struct shape objects[WORLD_MAX_OBJECTS];
 };
 
 inline static struct world world()
@@ -25,11 +25,11 @@ inline static struct world world()
 
 inline static struct world world_default()
 {
-  struct sphere s1 = sphere();
+  struct shape s1 = sphere();
   s1.material.color = color(0.8f, 1.0f, 0.6f);
   s1.material.diffuse = 0.7f;
   s1.material.specular = 0.2f;
-  struct sphere s2 = sphere();
+  struct shape s2 = sphere();
   s2.transform = scaling(0.5f, 0.5f, 0.5f);
 
   return (struct world){