Added App::clear_color() that apps can use to specify background color

CHANGELOG.md

@@ -25,7 +25,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 * `ui.image` now takes `impl Into<Vec2>` as a `size` argument.
 * Made some more fields of `RawInput` optional.
 * `Slider` and `DragValue` uses fewer decimals by default. See the full precision by hovering over the value.
-* `egui::App`: added `fn name(&self)`
+* `egui::App`: added `fn name(&self)` and `fn clear_color(&self)`
 
 ### Deprecated
 * Deprecated `RawInput::screen_size` - use `RawInput::screen_rect` instead.

egui/src/app.rs

@@ -14,6 +14,11 @@ use crate::Context;
 pub trait App {
     /// The name of your App.
     fn name(&self) -> &str;
+
+    /// Background color for the app.
+    /// e.g. what is sent to `gl.clearColor`
+    fn clear_color(&self) -> crate::Rgba {
+        crate::Srgba::from_rgb(16, 16, 16).into()
     }
 
     /// Called once before the first frame.

egui_glium/src/backend.rs

@@ -111,7 +111,13 @@ pub fn run(mut storage: Box<dyn egui::app::Storage>, mut app: Box<dyn App>) -> !
 
             let frame_time = (Instant::now() - egui_start).as_secs_f64() as f32;
             previous_frame_time = Some(frame_time);
-            painter.paint_jobs(&display, ctx.pixels_per_point(), paint_jobs, &ctx.texture());
+            painter.paint_jobs(
+                &display,
+                ctx.pixels_per_point(),
+                app.clear_color(),
+                paint_jobs,
+                &ctx.texture(),
+            );
 
             {
                 let egui::app::AppOutput {

egui_glium/src/painter.rs

@@ -126,6 +126,7 @@ impl Painter {
         &mut self,
         display: &glium::Display,
         pixels_per_point: f32,
+        clear_color: egui::Rgba,
         jobs: PaintJobs,
         egui_texture: &egui::Texture,
     ) {
@@ -133,7 +134,13 @@ impl Painter {
         self.upload_pending_user_textures(display);
 
         let mut target = display.draw();
-        target.clear_color(0.0, 0.0, 0.0, 0.0);
+        // Verified to be gamma-correct.
+        target.clear_color(
+            clear_color[0],
+            clear_color[1],
+            clear_color[2],
+            clear_color[3],
+        );
         for (clip_rect, triangles) in jobs {
             self.paint_job(
                 &mut target,

egui_web/src/backend.rs

@@ -55,10 +55,13 @@ impl WebBackend {
         Ok((output, paint_jobs))
     }
 
-    pub fn paint(&mut self, paint_jobs: egui::PaintJobs) -> Result<(), JsValue> {
-        let bg_color = egui::color::TRANSPARENT; // Use background css color.
+    pub fn paint(
+        &mut self,
+        clear_color: egui::Rgba,
+        paint_jobs: egui::PaintJobs,
+    ) -> Result<(), JsValue> {
         self.painter.paint_jobs(
-            bg_color,
+            clear_color,
             paint_jobs,
             &self.ctx.texture(),
             self.ctx.pixels_per_point(),
@@ -214,7 +217,7 @@ impl AppRunner {
     }
 
     pub fn paint(&mut self, paint_jobs: egui::PaintJobs) -> Result<(), JsValue> {
-        self.web_backend.paint(paint_jobs)
+        self.web_backend.paint(self.app.clear_color(), paint_jobs)
     }
 }
 

egui_web/src/webgl.rs

@@ -328,11 +328,12 @@ impl Painter {
             self.canvas.width() as i32,
             self.canvas.height() as i32,
         );
+        let clear_color: Srgba = clear_color.into();
         gl.clear_color(
-            clear_color[0],
-            clear_color[1],
-            clear_color[2],
-            clear_color[3],
+            clear_color[0] as f32 / 255.0,
+            clear_color[1] as f32 / 255.0,
+            clear_color[2] as f32 / 255.0,
+            clear_color[3] as f32 / 255.0,
         );
         gl.clear(Gl::COLOR_BUFFER_BIT);