working version of the frustum based calculation for the perspective matrix

src/offaxis.rs

@@ -3,8 +3,10 @@ use bevy::prelude::*;
 use bevy::render::camera::{Camera, CameraProjection};
 use bevy::render::primitives::Frustum;
 use bevy::render::view::VisibleEntities;
+use bevy::math::Mat4;
 
 use crate::screeninfo::ScreenInfo;
+use crate::viewer::*;
 
 #[derive(Component, Debug, Clone, Reflect)]
 #[reflect(Component, Default)]
@@ -19,6 +21,8 @@ impl CameraProjection for OffAxisProjection {
     
         println!("Here we go! {:?}",self);
 
+        //Mat4::perspective_rh_gl(fov_y_radians, aspect_ratio, z_near, z_far)
+
         Mat4::orthographic_rh(-self.aspect, self.aspect, -1.0, 1.0, self.near, self.far)
     }
 
@@ -42,6 +46,23 @@ impl Default for OffAxisProjection {
     }
 }
 
+
+pub fn make_projection_rh_from_frustum(left: f32, right: f32, bottom: f32, top: f32, near:f32, far:f32) -> Mat4
+{
+    // based on OpenSceneGraph / glFrustum implementation
+    let a = (right + left) / (right - left);
+    let b: f32 = (top + bottom) / (top - bottom);
+    let c= if far.abs() > f32::MAX  { -1.0 } else { -(far + near) / (far - near)};
+    let d = if far.abs() > f32::MAX { -2.0 * near } else { -2.0 * far * near / (far - near) };
+
+    Mat4::from_cols( 
+        Vec4::new(2.0 * near/(right-left),0.0,0.0,0.0), 
+        Vec4::new(0.0,2.0*near/(top-bottom), 0.0,0.0),
+        Vec4::new(a, b, c, -1.0),
+        Vec4::new(0.0, 0.0, d, 0.0))
+}
+
+
 pub fn offaxis_camera_setup(mut commands: Commands) {
 
     let projection = OffAxisProjection::default();
@@ -68,6 +89,48 @@ pub fn offaxis_camera_setup(mut commands: Commands) {
         VisibleEntities::default(),
         Camera::default(),
         Camera3d::default(),
-        ScreenInfo::new("Test")
+        ScreenInfo::new("Test"),
+        Viewer::new(transform.translation)
     ));
 }
+
+
+
+#[cfg(test)]
+mod tests {
+    use bevy::prelude::Mat4;
+
+    use super::make_projection_rh_from_frustum;
+
+
+    #[test]
+    fn compare_projections() {
+
+        // build an on-axis frustum
+        let fovy = 33.0_f32;
+        let aspect_ratio = 1.6666_f32;
+        let z_near = 1.0_f32;
+        let z_far = 500.0_f32;
+
+        let tan_fovy = (fovy * 0.5).to_radians().tan(); // use half angle beta
+        let right = tan_fovy * aspect_ratio * z_near;
+        let left = -right;
+        let top = tan_fovy * z_near;
+        let bottom = -top;
+
+        let mat_frust = make_projection_rh_from_frustum(left, right, bottom, top, z_near, z_far);
+
+        println!("mat 1 {:?}",mat_frust);
+
+        let mat_pers = Mat4::perspective_rh_gl(fovy.to_radians(),aspect_ratio,z_near,z_far);
+
+        println!("mat 2 {:?}",mat_pers);
+
+        assert!(mat_frust.abs_diff_eq(mat_pers, f32::EPSILON));
+
+    }
+
+
+
+
+}