diff --git a/src/camera.rs b/src/camera.rs
index 6eb9425..dc15680 100644
--- a/src/camera.rs
+++ b/src/camera.rs
@@ -369,7 +369,7 @@ impl Camera {
         } else if focal > 0.0 && z_far > focal {
             z_far = focal - 0.001;
         }
-        self.projection = cgmath::planar(
+        self.projection = planar(
             field_of_view_y,
             self.viewport.aspect(),
             height,
diff --git a/src/prelude/math.rs b/src/prelude/math.rs
index 13e6197..b925331 100644
--- a/src/prelude/math.rs
+++ b/src/prelude/math.rs
@@ -3,8 +3,8 @@
 //!
 
 pub use cgmath::{
-    dot, frustum, ortho, perspective, planar, vec2, vec3, vec4, Deg, Matrix2, Matrix3, Matrix4,
-    Point2, Point3, Quaternion, Rad, Vector2, Vector3, Vector4,
+    dot, frustum, ortho, perspective, vec2, vec3, vec4, Deg, Matrix2, Matrix3, Matrix4, Point2,
+    Point3, Quaternion, Rad, Vector2, Vector3, Vector4,
 };
 pub use cgmath::{
     Angle, EuclideanSpace, InnerSpace, Matrix, MetricSpace, One, Rotation, Rotation2, Rotation3,
@@ -78,3 +78,109 @@ pub fn rotation_matrix_from_dir_to_dir(source_dir: Vec3, target_dir: Vec3) -> Ma
         source_dir, target_dir,
     )))
 }
+
+/// Create a planar projection matrix, which can be either perspective or orthographic.
+///
+/// The projection frustum is always `height` units high at the origin along the view direction,
+/// making the focal point located at `(0.0, 0.0, cot(fovy / 2.0)) * height / 2.0`. Unlike
+/// a standard perspective projection, this allows `fovy` to be zero or negative.
+pub fn planar<S: cgmath::BaseFloat, A: Into<Rad<S>>>(
+    fovy: A,
+    aspect: S,
+    height: S,
+    near: S,
+    far: S,
+) -> Matrix4<S> {
+    PlanarFov {
+        fovy: fovy.into(),
+        aspect,
+        height,
+        near,
+        far,
+    }
+    .into()
+}
+
+/// A planar projection based on a vertical field-of-view angle.
+#[derive(Copy, Clone, Debug, PartialEq)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+struct PlanarFov<S> {
+    pub fovy: Rad<S>,
+    pub aspect: S,
+    pub height: S,
+    pub near: S,
+    pub far: S,
+}
+
+impl<S: cgmath::BaseFloat> From<PlanarFov<S>> for Matrix4<S> {
+    fn from(persp: PlanarFov<S>) -> Matrix4<S> {
+        assert!(
+            persp.fovy > -Rad::turn_div_2(),
+            "The vertical field of view cannot be less than a negative half turn, found: {:?}",
+            persp.fovy
+        );
+        assert!(
+            persp.fovy < Rad::turn_div_2(),
+            "The vertical field of view cannot be greater than a half turn, found: {:?}",
+            persp.fovy
+        );
+        assert! {
+            persp.height >= S::zero(),
+            "The projection plane height cannot be negative, found: {:?}",
+            persp.height
+        }
+
+        let two: S = cgmath::num_traits::cast(2).unwrap();
+        let inv_f = Rad::tan(persp.fovy / two) * two / persp.height;
+
+        let focal_point = -inv_f.recip();
+
+        assert!(
+            cgmath::abs_diff_ne!(persp.aspect.abs(), S::zero()),
+            "The absolute aspect ratio cannot be zero, found: {:?}",
+            persp.aspect.abs()
+        );
+        assert!(
+            cgmath::abs_diff_ne!(persp.far, persp.near),
+            "The far plane and near plane are too close, found: far: {:?}, near: {:?}",
+            persp.far,
+            persp.near
+        );
+        assert!(
+            focal_point < S::min(persp.far, persp.near) || focal_point > S::max(persp.far, persp.near),
+            "The focal point cannot be between the far and near planes, found: focal: {:?}, far: {:?}, near: {:?}",
+            focal_point,
+            persp.far,
+            persp.near,
+        );
+
+        let c0r0 = two / (persp.aspect * persp.height);
+        let c0r1 = S::zero();
+        let c0r2 = S::zero();
+        let c0r3 = S::zero();
+
+        let c1r0 = S::zero();
+        let c1r1 = two / persp.height;
+        let c1r2 = S::zero();
+        let c1r3 = S::zero();
+
+        let c2r0 = S::zero();
+        let c2r1 = S::zero();
+        let c2r2 = ((persp.far + persp.near) * inv_f + two) / (persp.near - persp.far);
+        let c2r3 = -inv_f;
+
+        let c3r0 = S::zero();
+        let c3r1 = S::zero();
+        let c3r2 = (two * persp.far * persp.near * inv_f + (persp.far + persp.near))
+            / (persp.near - persp.far);
+        let c3r3 = S::one();
+
+        #[cfg_attr(rustfmt, rustfmt_skip)]
+        Matrix4::new(
+            c0r0, c0r1, c0r2, c0r3,
+            c1r0, c1r1, c1r2, c1r3,
+            c2r0, c2r1, c2r2, c2r3,
+            c3r0, c3r1, c3r2, c3r3,
+        )
+    }
+}