Support transparency on Vello and Skia backends

crates/anyrender_skia/src/image_renderer.rs

@@ -64,6 +64,7 @@ impl ImageRenderer for SkiaImageRenderer {
         draw_fn(&mut SkiaScenePainter {
             inner: surface.canvas(),
             cache: &mut self.scene_cache,
+            base_color: Color::WHITE,
         });
         timer.record_time("render");
 
@@ -89,6 +90,7 @@ impl ImageRenderer for SkiaImageRenderer {
         draw_fn(&mut SkiaScenePainter {
             inner: surface.canvas(),
             cache: &mut self.scene_cache,
+            base_color: Color::WHITE,
         });
         timer.record_time("render");
 

crates/anyrender_skia/src/scene.rs

@@ -54,6 +54,7 @@ impl Default for SkiaSceneCache {
 pub struct SkiaScenePainter<'a> {
     pub(crate) inner: &'a Canvas,
     pub(crate) cache: &'a mut SkiaSceneCache,
+    pub(crate) base_color: Color,
 }
 
 impl SkiaScenePainter<'_> {
@@ -364,7 +365,7 @@ impl SkiaScenePainter<'_> {
 
 impl PaintScene for SkiaScenePainter<'_> {
     fn reset(&mut self) {
-        self.inner.clear(Color::WHITE);
+        self.inner.clear(self.base_color);
     }
 
     fn push_layer(

crates/anyrender_skia/src/window_renderer.rs

@@ -25,6 +25,7 @@ struct ActiveRenderState {
 
 pub struct SkiaWindowRenderer {
     render_state: RenderState,
+    base_color: Color,
 }
 
 impl Default for SkiaWindowRenderer {
@@ -37,6 +38,14 @@ impl SkiaWindowRenderer {
     pub fn new() -> Self {
         Self {
             render_state: RenderState::Suspended,
+            base_color: Color::WHITE,
+        }
+    }
+    pub fn with_base_color(base_color: peniko::Color) -> Self {
+        let base_color = base_color.to_rgba8();
+        Self {
+            render_state: RenderState::Suspended,
+            base_color: Color::from_argb(base_color.a, base_color.r, base_color.g, base_color.b),
         }
     }
 }
@@ -92,11 +101,12 @@ impl WindowRenderer for SkiaWindowRenderer {
         };
 
         surface.canvas().restore_to_count(1);
-        surface.canvas().clear(Color::WHITE);
+        surface.canvas().clear(self.base_color);
 
         draw_fn(&mut SkiaScenePainter {
             inner: surface.canvas(),
             cache: &mut state.scene_cache,
+            base_color: self.base_color,
         });
         timer.record_time("cmd");
 

crates/anyrender_vello/src/window_renderer.rs

@@ -128,6 +128,12 @@ impl WindowRenderer for VelloWindowRenderer {
     }
 
     fn resume(&mut self, window_handle: Arc<dyn WindowHandle>, width: u32, height: u32) {
+        let alpha_mode = if self.config.base_color.components[3] < 1.0 {
+            wgpu::CompositeAlphaMode::PreMultiplied
+        } else {
+            wgpu::CompositeAlphaMode::Auto
+        };
+
         // Create wgpu_context::SurfaceRenderer
         let render_surface = pollster::block_on(self.wgpu_context.create_surface(
             window_handle.clone(),
@@ -138,7 +144,7 @@ impl WindowRenderer for VelloWindowRenderer {
                 height,
                 present_mode: PresentMode::AutoVsync,
                 desired_maximum_frame_latency: 2,
-                alpha_mode: wgpu::CompositeAlphaMode::Auto,
+                alpha_mode,
                 view_formats: vec![],
             },
             Some(TextureConfiguration {

crates/wgpu_context/src/surface_renderer.rs

@@ -2,7 +2,8 @@ use crate::{DeviceHandle, WgpuContextError, util::create_texture};
 use wgpu::{
     CommandEncoderDescriptor, CompositeAlphaMode, Device, PresentMode, Queue, Surface,
     SurfaceConfiguration, SurfaceTexture, TextureFormat, TextureUsages, TextureView,
-    TextureViewDescriptor, util::TextureBlitter,
+    TextureViewDescriptor,
+    util::{TextureBlitter, TextureBlitterBuilder},
 };
 
 #[derive(Clone)]
@@ -126,6 +127,22 @@ impl<'s> SurfaceRenderer<'s> {
             view_formats: surface_renderer_config.view_formats,
         };
 
+        // `TextureBlitter::new` only does post-multiplied alpha
+        let blitter = if surface_renderer_config.alpha_mode == CompositeAlphaMode::PreMultiplied {
+            TextureBlitterBuilder::new(&device_handle.device, surface_config.format)
+                .blend_state(wgpu::BlendState {
+                    alpha: wgpu::BlendComponent::REPLACE,
+                    color: wgpu::BlendComponent {
+                        src_factor: wgpu::BlendFactor::SrcAlpha,
+                        dst_factor: wgpu::BlendFactor::Zero,
+                        operation: wgpu::BlendOperation::Add,
+                    },
+                })
+                .build()
+        } else {
+            TextureBlitter::new(&device_handle.device, surface_config.format)
+        };
+
         let intermediate_texture = intermediate_texture_config.map(|texture_config| {
             Box::new(IntermediateTextureStuff {
                 config: texture_config.clone(),
@@ -136,7 +153,7 @@ impl<'s> SurfaceRenderer<'s> {
                     texture_config.usage,
                     &device_handle.device,
                 ),
-                blitter: TextureBlitter::new(&device_handle.device, surface_config.format),
+                blitter,
             })
         });