enhance: new image alpha blend implementation

src/Views/ImageDiffView.axaml

@@ -21,8 +21,8 @@
         <TextBlock Text="SIDE-BY-SIDE" FontSize="11"/>
       </TabItem.Header>
 
-      <UniformGrid Rows="1" Margin="8,16" HorizontalAlignment="Center" VerticalAlignment="Center">
+      <UniformGrid Rows="1" Margin="0,16" HorizontalAlignment="Center" VerticalAlignment="Center">
-        <Grid RowDefinitions="Auto,*" IsVisible="{Binding Old, Converter={x:Static ObjectConverters.IsNotNull}}">
+        <Grid RowDefinitions="Auto,*" Margin="16,0" IsVisible="{Binding Old, Converter={x:Static ObjectConverters.IsNotNull}}">
           <StackPanel Grid.Row="0" Orientation="Horizontal" HorizontalAlignment="Center">
             <Border Height="16" Background="{DynamicResource Brush.Badge}" CornerRadius="8" VerticalAlignment="Center">
               <TextBlock Classes="monospace" Text="{DynamicResource Text.Diff.Binary.Old}" Margin="8,0" FontSize="10"/>
@@ -36,15 +36,16 @@
           <Border Grid.Row="1" Margin="0,12,0,0" Effect="drop-shadow(0 0 8 #A0000000)">
             <Border Background="{DynamicResource Brush.Popup}" HorizontalAlignment="Center" Padding="8">
               <Border BorderThickness="1" BorderBrush="{DynamicResource Brush.Border1}">
-                <v:ImageContainer>
+                <Grid>
+                  <v:ImageContainer/>
                   <Image Source="{Binding Old}" Stretch="Uniform" VerticalAlignment="Center"/>
-                </v:ImageContainer>
+                </Grid>
               </Border>
             </Border>
           </Border>
         </Grid>
         
-        <Grid RowDefinitions="Auto,*" Margin="32,0,0,0" IsVisible="{Binding New, Converter={x:Static ObjectConverters.IsNotNull}}">
+        <Grid RowDefinitions="Auto,*" Margin="16,0" IsVisible="{Binding New, Converter={x:Static ObjectConverters.IsNotNull}}">
           <StackPanel Grid.Row="0" Orientation="Horizontal" HorizontalAlignment="Center">
             <Border Height="16" Background="Green" CornerRadius="8" VerticalAlignment="Center">
               <TextBlock Classes="monospace" Text="{DynamicResource Text.Diff.Binary.New}" Margin="8,0" FontSize="10"/>
@@ -58,9 +59,10 @@
           <Border Grid.Row="1" Margin="0,12,0,0" Effect="drop-shadow(0 0 8 #A0000000)">
             <Border Background="{DynamicResource Brush.Popup}" HorizontalAlignment="Center" Padding="8">
               <Border BorderThickness="1" BorderBrush="{DynamicResource Brush.Border1}">
-                <v:ImageContainer>
+                <Grid>
+                  <v:ImageContainer/>
                   <Image Source="{Binding New}" Stretch="Uniform" VerticalAlignment="Center"/>
-                </v:ImageContainer>
+                </Grid>
               </Border>
             </Border>
           </Border>
@@ -95,11 +97,9 @@
         <Border Grid.Row="1" Margin="0,12,0,0" Effect="drop-shadow(0 0 8 #A0000000)">
           <Border HorizontalAlignment="Center" Background="{DynamicResource Brush.Window}">
             <Border BorderThickness="1" BorderBrush="{DynamicResource Brush.Border1}" Margin="8">
-              <v:ImageContainer>
               <v:ImagesSwipeControl OldImage="{Binding Old}"
                                     NewImage="{Binding New}"
                                     RenderOptions.BitmapInterpolationMode="HighQuality"/>             
-              </v:ImageContainer>              
             </Border>
           </Border>
         </Border>
@@ -133,12 +133,9 @@
         <Border Grid.Row="1" Margin="0,12,0,0" Effect="drop-shadow(0 0 8 #A0000000)">
           <Border HorizontalAlignment="Center" Background="{DynamicResource Brush.Window}">
             <Border BorderThickness="1" BorderBrush="{DynamicResource Brush.Border1}" Margin="8">
-              <v:ImageContainer>
               <v:ImageBlendControl Alpha="{Binding #ImageBlendSlider.Value}"
                                    OldImage="{Binding Old}"
-                                     NewImage="{Binding New}"
+                                   NewImage="{Binding New}"/>
-                                     RenderOptions.BitmapInterpolationMode="HighQuality"/>
-              </v:ImageContainer>
             </Border>
           </Border>
         </Border>

src/Views/ImageDiffView.axaml.cs

@@ -9,10 +9,8 @@ using Avalonia.Styling;
 
 namespace SourceGit.Views
 {
-    public class ImageContainer : ContentControl
+    public class ImageContainer : Control
     {
-        protected override Type StyleKeyOverride => typeof(ContentControl);
-
         public override void Render(DrawingContext context)
         {
             if (_bgBrush == null)
@@ -54,7 +52,7 @@ namespace SourceGit.Views
         private DrawingBrush _bgBrush = null;
     }
 
-    public class ImagesSwipeControl : Control
+    public class ImagesSwipeControl : ImageContainer
     {
         public static readonly StyledProperty<double> AlphaProperty =
             AvaloniaProperty.Register<ImagesSwipeControl, double>(nameof(Alpha), 0.5);
@@ -91,6 +89,8 @@ namespace SourceGit.Views
 
         public override void Render(DrawingContext context)
         {
+            base.Render(context);
+
             var alpha = Alpha;
             var w = Bounds.Width;
             var h = Bounds.Height;
@@ -199,7 +199,7 @@ namespace SourceGit.Views
         private bool _lastInSlider = false;
     }
 
-    public class ImageBlendControl : Control
+    public class ImageBlendControl : ImageContainer
     {
         public static readonly StyledProperty<double> AlphaProperty =
             AvaloniaProperty.Register<ImageBlendControl, double>(nameof(Alpha), 1.0);
@@ -236,30 +236,50 @@ namespace SourceGit.Views
 
         public override void Render(DrawingContext context)
         {
+            base.Render(context);
+
             var rect = new Rect(0, 0, Bounds.Width, Bounds.Height);
             var alpha = Alpha;
             var left = OldImage;
             var right = NewImage;
-
             var drawLeft = left != null && alpha < 1.0;
             var drawRight = right != null && alpha > 0;
+            var psize = left == null ? right.PixelSize : left.PixelSize;
+            var dpi = left == null ? right.Dpi : left.Dpi;
+
+            using (var rt = new RenderTargetBitmap(psize, dpi))
+            {
+                var rtRect = new Rect(rt.Size);
+                using (var dc = rt.CreateDrawingContext())
+                {
                     if (drawLeft)
                     {
-                using (context.PushRenderOptions(new RenderOptions() { BitmapBlendingMode = BitmapBlendingMode.Source }))
-                using (context.PushOpacity(1 - alpha))
-                    context.DrawImage(left, rect);
-
                         if (drawRight)
                         {
-                    using (context.PushRenderOptions(new RenderOptions() { BitmapBlendingMode = BitmapBlendingMode.Plus }))
+                            using (dc.PushRenderOptions(RO_SRC))
-                    using (context.PushOpacity(alpha))
+                            using (dc.PushOpacity(1 - alpha))
-                        context.DrawImage(right, rect);
+                                dc.DrawImage(left, rtRect);
+
+                            using (dc.PushRenderOptions(RO_DST))
+                            using (dc.PushOpacity(alpha))
+                                dc.DrawImage(right, rtRect);
+                        }
+                        else
+                        {
+                            using (dc.PushRenderOptions(RO_SRC))
+                            using (dc.PushOpacity(1 - alpha))
+                                dc.DrawImage(left, rtRect);
                         }
                     }
                     else if (drawRight)
                     {
-                using (context.PushOpacity(alpha))
+                        using (dc.PushRenderOptions(RO_SRC))
-                    context.DrawImage(right, rect);
+                        using (dc.PushOpacity(alpha))
+                            dc.DrawImage(right, rtRect);
+                    }
+                }
+
+                context.DrawImage(rt, rtRect, rect);
             }
         }
 
@@ -290,6 +310,9 @@ namespace SourceGit.Views
 
             return new Size(scale * img.Width, scale * img.Height);
         }
+
+        private static readonly RenderOptions RO_SRC = new RenderOptions() { BitmapBlendingMode = BitmapBlendingMode.Source, BitmapInterpolationMode = BitmapInterpolationMode.HighQuality };
+        private static readonly RenderOptions RO_DST = new RenderOptions() { BitmapBlendingMode = BitmapBlendingMode.Plus, BitmapInterpolationMode = BitmapInterpolationMode.HighQuality };
     }
 
     public partial class ImageDiffView : UserControl