Support for transparent backbuffer in wgpu winit binding

Choose best fitting composite alpha mode on the fly.

Author: Wumpf

crates/eframe/src/native/run.rs

@@ -958,6 +958,7 @@ mod wgpu_integration {
                     self.native_options.wgpu_options.clone(),
                     self.native_options.multisampling.max(1) as _,
                     self.native_options.depth_buffer,
+                    self.native_options.transparent,
                 );
                 pollster::block_on(painter.set_window(Some(&window)))?;
                 painter

crates/egui-wgpu/CHANGELOG.md

@@ -7,6 +7,7 @@ All notable changes to the `egui-wgpu` integration will be noted in this file.
 * Return `Err` instead of panic if we can't find a device ([#2428](https://github.com/emilk/egui/pull/2428)).
 * `winit::Painter::set_window` is now `async` ([#2434](https://github.com/emilk/egui/pull/2434)).
 * `egui-wgpu` now only depends on `epaint` instead of the entire `egui` ([#2438](https://github.com/emilk/egui/pull/2438)).
+* `winit::Painter` now supports transparent backbuffer ([#2684](https://github.com/emilk/egui/pull/2684)).
 
 
 ## 0.20.0 - 2022-12-08 - web support

crates/egui-wgpu/src/winit.rs

@@ -1,14 +1,14 @@
 use std::sync::Arc;
 
-use tracing::error;
-use wgpu::{Adapter, Instance, Surface};
-
 use epaint::mutex::RwLock;
 
+use tracing::error;
+
 use crate::{renderer, RenderState, Renderer, SurfaceErrorAction, WgpuConfiguration};
 
 struct SurfaceState {
-    surface: Surface,
+    surface: wgpu::Surface,
+    alpha_mode: wgpu::CompositeAlphaMode,
     width: u32,
     height: u32,
 }
@@ -19,11 +19,12 @@ struct SurfaceState {
 pub struct Painter {
     configuration: WgpuConfiguration,
     msaa_samples: u32,
+    support_transparent_backbuffer: bool,
     depth_format: Option<wgpu::TextureFormat>,
     depth_texture_view: Option<wgpu::TextureView>,
 
-    instance: Instance,
-    adapter: Option<Adapter>,
+    instance: wgpu::Instance,
+    adapter: Option<wgpu::Adapter>,
     render_state: Option<RenderState>,
     surface_state: Option<SurfaceState>,
 }
@@ -41,7 +42,12 @@ impl Painter {
     /// [`set_window()`](Self::set_window) once you have
     /// a [`winit::window::Window`] with a valid `.raw_window_handle()`
     /// associated.
-    pub fn new(configuration: WgpuConfiguration, msaa_samples: u32, depth_bits: u8) -> Self {
+    pub fn new(
+        configuration: WgpuConfiguration,
+        msaa_samples: u32,
+        depth_bits: u8,
+        support_transparent_backbuffer: bool,
+    ) -> Self {
         let instance = wgpu::Instance::new(wgpu::InstanceDescriptor {
             backends: configuration.backends,
             dx12_shader_compiler: Default::default(), //
@@ -50,6 +56,7 @@ impl Painter {
         Self {
             configuration,
             msaa_samples,
+            support_transparent_backbuffer,
             depth_format: (depth_bits > 0).then_some(wgpu::TextureFormat::Depth32Float),
             depth_texture_view: None,
 
@@ -69,7 +76,7 @@ impl Painter {
 
     async fn init_render_state(
         &self,
-        adapter: &Adapter,
+        adapter: &wgpu::Adapter,
         target_format: wgpu::TextureFormat,
     ) -> Result<RenderState, wgpu::RequestDeviceError> {
         adapter
@@ -94,7 +101,7 @@ impl Painter {
     // will have the same format and so this render state will remain valid.
     async fn ensure_render_state_for_surface(
         &mut self,
-        surface: &Surface,
+        surface: &wgpu::Surface,
     ) -> Result<(), wgpu::RequestDeviceError> {
         if self.adapter.is_none() {
             self.adapter = self
@@ -121,34 +128,23 @@ impl Painter {
         Ok(())
     }
 
-    fn configure_surface(&mut self, width_in_pixels: u32, height_in_pixels: u32) {
-        crate::profile_function!();
-
-        let render_state = self
-            .render_state
-            .as_ref()
-            .expect("Render state should exist before surface configuration");
-        let format = render_state.target_format;
-
-        let config = wgpu::SurfaceConfiguration {
-            usage: wgpu::TextureUsages::RENDER_ATTACHMENT,
-            format,
-            width: width_in_pixels,
-            height: height_in_pixels,
-            present_mode: self.configuration.present_mode,
-            alpha_mode: wgpu::CompositeAlphaMode::Auto,
-            view_formats: vec![format],
-        };
-
-        let surface_state = self
-            .surface_state
-            .as_mut()
-            .expect("Surface state should exist before surface configuration");
-        surface_state
-            .surface
-            .configure(&render_state.device, &config);
-        surface_state.width = width_in_pixels;
-        surface_state.height = height_in_pixels;
+    fn configure_surface(
+        surface_state: &SurfaceState,
+        render_state: &RenderState,
+        present_mode: wgpu::PresentMode,
+    ) {
+        surface_state.surface.configure(
+            &render_state.device,
+            &wgpu::SurfaceConfiguration {
+                usage: wgpu::TextureUsages::RENDER_ATTACHMENT,
+                format: render_state.target_format,
+                width: surface_state.width,
+                height: surface_state.height,
+                present_mode,
+                alpha_mode: surface_state.alpha_mode,
+                view_formats: vec![render_state.target_format],
+            },
+        );
     }
 
     /// Updates (or clears) the [`winit::window::Window`] associated with the [`Painter`]
@@ -188,15 +184,34 @@ impl Painter {
 
                 self.ensure_render_state_for_surface(&surface).await?;
 
+                let alpha_mode = if self.support_transparent_backbuffer {
+                    let supported_alpha_modes = surface
+                        .get_capabilities(self.adapter.as_ref().unwrap())
+                        .alpha_modes;
+
+                    // Prefer pre multiplied over post multiplied!
+                    if supported_alpha_modes.contains(&wgpu::CompositeAlphaMode::PreMultiplied) {
+                        wgpu::CompositeAlphaMode::PreMultiplied
+                    } else if supported_alpha_modes
+                        .contains(&wgpu::CompositeAlphaMode::PostMultiplied)
+                    {
+                        wgpu::CompositeAlphaMode::PostMultiplied
+                    } else {
+                        warn!("Transparent window was requested, but the active wgpu surface does not support a `CompositeAlphaMode` with transparency.");
+                        wgpu::CompositeAlphaMode::Auto
+                    }
+                } else {
+                    wgpu::CompositeAlphaMode::Auto
+                };
+
                 let size = window.inner_size();
-                let width = size.width;
-                let height = size.height;
                 self.surface_state = Some(SurfaceState {
                     surface,
-                    width,
-                    height,
+                    width: size.width,
+                    height: size.height,
+                    alpha_mode,
                 });
-                self.resize_and_generate_depth_texture_view(width, height);
+                self.resize_and_generate_depth_texture_view(size.width, size.height);
             }
             None => {
                 self.surface_state = None;
@@ -221,10 +236,17 @@ impl Painter {
         width_in_pixels: u32,
         height_in_pixels: u32,
     ) {
-        self.configure_surface(width_in_pixels, height_in_pixels);
-        let device = &self.render_state.as_ref().unwrap().device;
+        let render_state = self.render_state.as_ref().unwrap();
+        let surface_state = self.surface_state.as_mut().unwrap();
+
+        surface_state.width = width_in_pixels;
+        surface_state.height = height_in_pixels;
+
+        Self::configure_surface(surface_state, render_state, self.configuration.present_mode);
+
         self.depth_texture_view = self.depth_format.map(|depth_format| {
-            device
+            render_state
+                .device
                 .create_texture(&wgpu::TextureDescriptor {
                     label: Some("egui_depth_texture"),
                     size: wgpu::Extent3d {
@@ -269,7 +291,6 @@ impl Painter {
             Some(rs) => rs,
             None => return,
         };
-        let (width, height) = (surface_state.width, surface_state.height);
 
         let output_frame = {
             crate::profile_scope!("get_current_texture");
@@ -282,7 +303,11 @@ impl Painter {
             #[allow(clippy::single_match_else)]
             Err(e) => match (*self.configuration.on_surface_error)(e) {
                 SurfaceErrorAction::RecreateSurface => {
-                    self.configure_surface(width, height);
+                    Self::configure_surface(
+                        surface_state,
+                        render_state,
+                        self.configuration.present_mode,
+                    );
                     return;
                 }
                 SurfaceErrorAction::SkipFrame => {
@@ -300,7 +325,7 @@ impl Painter {
 
         // Upload all resources for the GPU.
         let screen_descriptor = renderer::ScreenDescriptor {
-            size_in_pixels: [width, height],
+            size_in_pixels: [surface_state.width, surface_state.height],
             pixels_per_point,
         };