world coords to screen space (#1258)

Add Camera::world_to_screen to convert world coordinates to screen space

crates/bevy_render/src/camera/camera.rs

@@ -1,8 +1,9 @@
 use super::CameraProjection;
 use bevy_app::prelude::EventReader;
 use bevy_ecs::{Added, Component, Entity, Query, QuerySet, Res};
-use bevy_math::Mat4;
+use bevy_math::{Mat4, Vec2, Vec3};
 use bevy_reflect::{Reflect, ReflectComponent};
+use bevy_transform::components::GlobalTransform;
 use bevy_window::{WindowCreated, WindowId, WindowResized, Windows};
 
 #[derive(Default, Debug, Reflect)]
@@ -28,6 +29,30 @@ impl Default for DepthCalculation {
     }
 }
 
+impl Camera {
+    /// Given a position in world space, use the camera to compute the screen space coordinates.
+    pub fn world_to_screen(
+        &self,
+        windows: &Windows,
+        camera_transform: &GlobalTransform,
+        world_position: Vec3,
+    ) -> Option<Vec2> {
+        let window = windows.get(self.window)?;
+        let window_size = Vec2::new(window.width(), window.height());
+        // Build a transform to convert from world to NDC using camera data
+        let world_to_ndc: Mat4 =
+            self.projection_matrix * camera_transform.compute_matrix().inverse();
+        let ndc_space_coords: Vec3 = world_to_ndc.transform_point3(world_position);
+        // NDC z-values outside of 0 < z < 1 are behind the camera and are thus not in screen space
+        if ndc_space_coords.z < 0.0 || ndc_space_coords.z > 1.0 {
+            return None;
+        }
+        // Once in NDC space, we can discard the z element and rescale x/y to fit the screen
+        let screen_space_coords = (ndc_space_coords.truncate() + Vec2::one()) / 2.0 * window_size;
+        Some(screen_space_coords)
+    }
+}
+
 pub fn camera_system<T: CameraProjection + Component>(
     mut window_resized_events: EventReader<WindowResized>,
     mut window_created_events: EventReader<WindowCreated>,