Add a test scene that animates the base color

The test scenes can now supply their own optional base (background)
color. In the with_winit example, we also allow the user to provide a
base color as a CLI option. The precedence is as follows:

   1. Use the color from the CLI options, if any.
   2. Otherwise use the scene provided base color, if any.
   3. Otherwise default to black.

examples/headless/src/main.rs

@@ -96,6 +96,7 @@ async fn render(mut scenes: SceneSet, index: usize, args: &Args) -> Result<()> {
         time: args.time.unwrap_or(0.),
         text: &mut text,
         resolution: None,
+        base_color: None,
     };
     (example_scene.function)(&mut builder, &mut scene_params);
     builder.finish();
@@ -130,6 +131,7 @@ async fn render(mut scenes: SceneSet, index: usize, args: &Args) -> Result<()> {
         base_color: args
             .args
             .get_base_color()?
+            .or(scene_params.base_color)
             .unwrap_or(vello::peniko::Color::BLACK),
         width,
         height,

examples/scenes/src/lib.rs

@@ -19,6 +19,7 @@ pub struct SceneParams<'a> {
     pub time: f64,
     pub text: &'a mut SimpleText,
     pub resolution: Option<Vec2>,
+    pub base_color: Option<vello::peniko::Color>,
 }
 
 pub struct SceneConfig {

examples/scenes/src/test_scenes.rs

@@ -32,6 +32,7 @@ pub fn test_scenes() -> SceneSet {
         scene!(blend_grid),
         scene!(conflation_artifacts),
         scene!(labyrinth),
+        scene!(base_color_test: animated),
     ];
     #[cfg(target_arch = "wasm32")]
     scenes.push(ExampleScene {
@@ -582,6 +583,21 @@ fn labyrinth(sb: &mut SceneBuilder, _: &mut SceneParams) {
     )
 }
 
+fn base_color_test(sb: &mut SceneBuilder, params: &mut SceneParams) {
+    // Cycle through the hue value every 5 seconds (t % 5) * 360/5
+    let color = Color::hlc((params.time % 5.0) * 72.0, 80.0, 80.0);
+    params.base_color = Some(color);
+
+    // Blend a white square over it.
+    sb.fill(
+        Fill::NonZero,
+        Affine::IDENTITY,
+        Color::rgba8(255, 255, 255, 128),
+        None,
+        &Rect::new(50.0, 50.0, 500.0, 500.0),
+    );
+}
+
 fn around_center(xform: Affine, center: Point) -> Affine {
     Affine::translate(center.to_vec2()) * xform * Affine::translate(-center.to_vec2())
 }

examples/with_winit/src/main.rs

@@ -59,7 +59,7 @@ async fn run(
     event_loop: EventLoop<UserEvent>,
     window: Window,
     args: Args,
-    base_color: Color,
+    base_color_arg: Option<Color>,
     mut scenes: SceneSet,
 ) {
     use winit::{event::*, event_loop::ControlFlow};
@@ -163,12 +163,6 @@ async fn run(
             let height = surface.config.height;
             let device_handle = &render_cx.devices[surface.dev_id];
 
-            let render_params = vello::RenderParams {
-                base_color,
-                width,
-                height,
-            };
-
             // Allow looping forever
             scene_ix = scene_ix.rem_euclid(scenes.scenes.len() as i32);
             let example_scene = &mut scenes.scenes[scene_ix as usize];
@@ -182,9 +176,20 @@ async fn run(
                 time: start.elapsed().as_secs_f64(),
                 text: &mut simple_text,
                 resolution: None,
+                base_color: None,
             };
             (example_scene.function)(&mut builder, &mut scene_params);
             builder.finish();
+
+            // If the user specifies a base color in the CLI we use that. Otherwise we use any
+            // color specified by the scene. The default is black.
+            let render_params = vello::RenderParams {
+                base_color: base_color_arg
+                    .or(scene_params.base_color)
+                    .unwrap_or(Color::BLACK),
+                width,
+                height,
+            };
             let mut builder = SceneBuilder::for_scene(&mut scene);
             let mut transform = transform;
             if let Some(resolution) = scene_params.resolution {
@@ -257,7 +262,7 @@ fn main() -> Result<()> {
     env_logger::init();
     let args = Args::parse();
     let scenes = args.args.select_scene_set(|| Args::command())?;
-    let base_color = args.args.get_base_color()?.unwrap_or(Color::BLACK);
+    let base_color = args.args.get_base_color()?;
     if let Some(scenes) = scenes {
         #[cfg(not(target_arch = "wasm32"))]
         {