Add custom window background/clear color (incl. transparency) support, fixes #272

wgpu would currently ignore the alpha: https://github.com/gfx-rs/wgpu/issues/687

glow (and naively patched wgpu) requires premultiplied alpha, so if you don't
multiply the RGB by the A right now, the semi-transparent color would be wrong
(too bright).

winit with_transparent doesn't seem necessary.

glow/src/settings.rs

@@ -1,10 +1,11 @@
 //! Configure a renderer.
 pub use iced_graphics::Antialiasing;
+pub use iced_native::Color;
 
 /// The settings of a [`Renderer`].
 ///
 /// [`Renderer`]: ../struct.Renderer.html
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Settings {
     /// The bytes of the font that will be used by default.
     ///
@@ -13,6 +14,9 @@ pub struct Settings {
 
     /// The antialiasing strategy that will be used for triangle primitives.
     pub antialiasing: Option<Antialiasing>,
+
+    /// The color that will be used to clear the window surface.
+    pub background_color: Color,
 }
 
 impl Default for Settings {
@@ -20,6 +24,7 @@ impl Default for Settings {
         Settings {
             default_font: None,
             antialiasing: None,
+            background_color: Color::WHITE,
         }
     }
 }

glow/src/window/compositor.rs

@@ -21,7 +21,12 @@ impl iced_graphics::window::GLCompositor for Compositor {
     ) -> (Self, Self::Renderer) {
         let gl = glow::Context::from_loader_function(loader_function);
 
-        gl.clear_color(1.0, 1.0, 1.0, 1.0);
+        gl.clear_color(
+            settings.background_color.r,
+            settings.background_color.g,
+            settings.background_color.b,
+            settings.background_color.a,
+        );
 
         // Enable auto-conversion from/to sRGB
         gl.enable(glow::FRAMEBUFFER_SRGB);

src/application.rs

@@ -195,6 +195,7 @@ pub trait Application: Sized {
                 } else {
                     None
                 },
+                background_color: settings.background_color,
                 ..crate::renderer::Settings::default()
             };
 

src/settings.rs

@@ -1,8 +1,8 @@
 //! Configure your application.
-use crate::window;
+use crate::{window, Color};
 
 /// The settings of an application.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Settings<Flags> {
     /// The window settings.
     ///
@@ -32,6 +32,29 @@ pub struct Settings<Flags> {
     ///
     /// [`Canvas`]: ../widget/canvas/struct.Canvas.html
     pub antialiasing: bool,
+
+    /// The background color of the window.
+    ///
+    /// On supported backends, this makes it possible to have
+    /// (semi-)transparent windows.
+    ///
+    /// By default, it is white.
+    pub background_color: Color,
+}
+
+impl<Flags> Default for Settings<Flags>
+where
+    Flags: Default,
+{
+    fn default() -> Self {
+        Self {
+            flags: Default::default(),
+            antialiasing: Default::default(),
+            default_font: Default::default(),
+            window: Default::default(),
+            background_color: Color::WHITE,
+        }
+    }
 }
 
 impl<Flags> Settings<Flags> {
@@ -46,6 +69,7 @@ impl<Flags> Settings<Flags> {
             antialiasing: Default::default(),
             default_font: Default::default(),
             window: Default::default(),
+            background_color: Color::WHITE,
         }
     }
 }

wgpu/src/settings.rs

@@ -1,10 +1,11 @@
 //! Configure a renderer.
 pub use crate::Antialiasing;
+pub use iced_native::Color;
 
 /// The settings of a [`Renderer`].
 ///
 /// [`Renderer`]: ../struct.Renderer.html
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Settings {
     /// The output format of the [`Renderer`].
     ///
@@ -18,6 +19,9 @@ pub struct Settings {
 
     /// The antialiasing strategy that will be used for triangle primitives.
     pub antialiasing: Option<Antialiasing>,
+
+    /// The color that will be used to clear the window surface.
+    pub background_color: Color,
 }
 
 impl Default for Settings {
@@ -26,6 +30,7 @@ impl Default for Settings {
             format: wgpu::TextureFormat::Bgra8UnormSrgb,
             default_font: None,
             antialiasing: None,
+            background_color: Color::WHITE,
         }
     }
 }

wgpu/src/window/compositor.rs

@@ -119,10 +119,10 @@ impl iced_graphics::window::Compositor for Compositor {
                 load_op: wgpu::LoadOp::Clear,
                 store_op: wgpu::StoreOp::Store,
                 clear_color: wgpu::Color {
-                    r: 1.0,
+                    r: self.settings.background_color.r.into(),
-                    g: 1.0,
+                    g: self.settings.background_color.g.into(),
-                    b: 1.0,
+                    b: self.settings.background_color.b.into(),
-                    a: 1.0,
+                    a: self.settings.background_color.a.into(),
                 },
             }],
             depth_stencil_attachment: None,