Headless Mode fixes for the Backend (#65)

This PR brings some fixes that are needed for the backend to render
scenes.

studio/src/main-headless.rs

@@ -1,4 +1,7 @@
-use bevy::render::render_resource::Extent3d;
+use bevy::{
+    prelude::*, render::render_resource::Extent3d, transform::components::Transform,
+    window::PrimaryWindow,
+};
 use clap::{Args, Parser, Subcommand};
 use studio::headless::{start_headless, HeadlessMode};
 
@@ -19,6 +22,8 @@ struct StudioArgs {
 enum Subcommands {
     Scene {
         scene_dir: String,
+        #[arg(long)]
+        orbit: bool,
         #[arg(long, short)]
         output_dir: Option<String>,
         #[arg(long)]
@@ -67,6 +72,7 @@ fn main() {
     let mut app = match args.subcommand {
         Subcommands::Scene {
             scene_dir,
+            orbit,
             skybox,
             output_dir,
             output_resolution,
@@ -82,13 +88,20 @@ fn main() {
             let fps = fps.unwrap_or(24);
             let frames = frames.unwrap_or(30);
             let skybox = skybox.unwrap_or("gum_trees_4k".to_string());
-            start_headless(
+            let mut app = start_headless(
                 HeadlessMode::Scene(scene_dir, skybox),
                 fps,
                 &output_dir,
                 frames,
                 output_res,
-            )
+            );
+
+            if orbit {
+                app.insert_resource(OrbitSettings { radius: 10.0 })
+                    .add_systems(Update, orbit_scene);
+            }
+
+            app
         }
         Subcommands::Bvh {
             bvh_dir,
@@ -145,6 +158,28 @@ fn main() {
     app.run();
 }
 
+#[derive(Resource)]
+struct OrbitSettings {
+    radius: f32,
+}
+
+fn orbit_scene(
+    mut camera: Query<&mut Transform, With<PrimaryWindow>>,
+    orbit_settings: Res<OrbitSettings>,
+    time: Res<Time>,
+) {
+    let Ok(mut main_cam_transform) = camera.get_single_mut() else {
+        return;
+    };
+
+    main_cam_transform.translation = Vec3::new(
+        orbit_settings.radius * time.elapsed_seconds().cos() * 10.0,
+        orbit_settings.radius * time.elapsed_seconds().sin(),
+        6.0,
+    );
+    main_cam_transform.look_at(Vec3::ZERO, Vec3::Y);
+}
+
 //==============================================================================
 //                        Parsers
 //==============================================================================