trying out custom projection code

src/offaxis.rs

@@ -0,0 +1,91 @@
+use bevy::prelude::*;
+
+use bevy::render::primitives::Frustum;
+use bevy::render::camera::{Camera, Camera2d, CameraProjection, DepthCalculation};
+use bevy::render::view::VisibleEntities;
+
+#[derive(Component)]
+pub struct OffAxisProjection {
+    near: f32,
+    far: f32,
+    aspect: f32,
+}
+
+impl CameraProjection for OffAxisProjection {
+    fn get_projection_matrix(&self) -> Mat4 {
+        Mat4::orthographic_rh(
+            -self.aspect, self.aspect, -1.0, 1.0, self.near, self.far
+        )
+    }
+
+    // what to do on window resize
+    fn update(&mut self, width: f32, height: f32) {
+        self.aspect = width / height;
+    }
+
+    fn depth_calculation(&self) -> DepthCalculation {
+        // for 2D (camera doesn't rotate)
+        DepthCalculation::ZDifference
+
+        // otherwise
+        //DepthCalculation::Distance
+    }
+
+    fn far(&self) -> f32 {
+        self.far
+    }
+}
+
+impl Default for OffAxisProjection {
+    fn default() -> Self {
+        Self { near: 0.0, far: 1000.0, aspect: 1.0 }
+    }
+}
+
+pub fn camera_setup(mut commands: Commands) {
+    // We need all the components that Bevy's built-in camera bundles would add
+
+    let projection = OffAxisProjection::default();
+    let camera = Camera {
+        near: projection.near,
+        far: projection.far,
+        ..default()
+    };
+    // position the camera like bevy would do by default for 2D:
+    let transform = Transform::from_xyz(0.0, 0.0, projection.far - 0.1);
+    // frustum construction code copied from Bevy
+    let view_projection =
+        projection.get_projection_matrix() * transform.compute_matrix().inverse();
+    let frustum = Frustum::from_view_projection(
+        &view_projection,
+        &transform.translation,
+        &transform.back(),
+        projection.far,
+    );
+
+    commands.spawn_bundle((
+        camera,
+        projection,
+        frustum,
+        VisibleEntities::default(),
+        transform,
+        GlobalTransform::default(),
+        Camera2d,
+    ));
+}
+
+// fn main() {
+//     // need to add a bevy-internal camera system to update
+//     // the projection on window resizing
+
+//     use bevy::render::camera::camera_system;
+
+//     App::new()
+//         .add_plugins(DefaultPlugins)
+//         .add_startup_system(setup)
+//         .add_system_to_stage(
+//             CoreStage::PostUpdate,
+//             camera_system::<SimpleOrthoProjection>,
+//         )
+//         .run();
+// }