Support transparent windows in vger

Author: jrmoulton

tiny_skia/src/lib.rs

@@ -374,7 +374,7 @@ impl<W: raw_window_handle::HasWindowHandle + raw_window_handle::HasDisplayHandle
 {
     fn begin(&mut self, _capture: bool) {
         self.transform = Affine::IDENTITY;
-        self.pixmap.fill(tiny_skia::Color::WHITE);
+        self.pixmap.fill(tiny_skia::Color::TRANSPARENT);
         self.clip = None;
     }
 
@@ -613,7 +613,8 @@ impl<W: raw_window_handle::HasWindowHandle + raw_window_handle::HasDisplayHandle
         for (out_pixel, pixel) in (buffer.iter_mut()).zip(self.pixmap.pixels().iter()) {
             *out_pixel = ((pixel.red() as u32) << 16)
                 | ((pixel.green() as u32) << 8)
-                | (pixel.blue() as u32);
+                | (pixel.blue() as u32)
+                | ((pixel.alpha() as u32) << 24);
         }
 
         buffer

vger/src/lib.rs

@@ -16,7 +16,10 @@ use peniko::{
     kurbo::{Affine, Point, Rect, Shape},
     BrushRef, Color, GradientKind,
 };
-use wgpu::{Device, DeviceType, Queue, StoreOp, Surface, SurfaceConfiguration, TextureFormat};
+use wgpu::{
+    CompositeAlphaMode, Device, DeviceType, Queue, StoreOp, Surface, SurfaceConfiguration,
+    TextureFormat,
+};
 
 pub struct VgerRenderer {
     device: Arc<Device>,
@@ -74,14 +77,27 @@ impl VgerRenderer {
             .into_iter()
             .find(|it| matches!(it, TextureFormat::Rgba8Unorm | TextureFormat::Bgra8Unorm))
             .ok_or_else(|| anyhow::anyhow!("surface should support Rgba8Unorm or Bgra8Unorm"))?;
+        let alpha_mode = if surface_caps
+            .alpha_modes
+            .contains(&CompositeAlphaMode::PreMultiplied)
+        {
+            CompositeAlphaMode::PreMultiplied
+        } else if surface_caps
+            .alpha_modes
+            .contains(&CompositeAlphaMode::PostMultiplied)
+        {
+            CompositeAlphaMode::PostMultiplied
+        } else {
+            CompositeAlphaMode::Auto
+        };
 
         let config = wgpu::SurfaceConfiguration {
             usage: wgpu::TextureUsages::RENDER_ATTACHMENT,
             format: texture_format,
             width,
             height,
             present_mode: wgpu::PresentMode::Fifo,
-            alpha_mode: wgpu::CompositeAlphaMode::Auto,
+            alpha_mode,
             view_formats: vec![],
             desired_maximum_frame_latency: 2,
         };