Add white background for png image. (#367)

src/Common/ChartDrawing/IO/CopyImageAsCommand.cs

@@ -6,7 +6,7 @@ namespace CompMs.Graphics.IO
 {
     public class CopyImageAsCommand : ICommand
     {
-        public static CopyImageAsCommand BitmapInstance { get; } = new CopyImageAsCommand(ImageFormat.Png);
+        public static CopyImageAsCommand BitmapInstance { get; } = new CopyImageAsCommand(ImageFormat.Png) { Converter = SetBackgroundConverter.White };
 
         public static CopyImageAsCommand EmfInstance { get; } = new CopyImageAsCommand(ImageFormat.Emf);
 

src/Common/ChartDrawing/IO/SaveImageAsCommand.cs

@@ -11,7 +11,7 @@ public class SaveImageAsCommand : ICommand
     {
         public static SaveImageAsCommand Instance { get; } = new SaveImageAsCommand();
 
-        public static SaveImageAsCommand PngInstance { get; } = new SaveImageAsCommand(ImageFormat.Png);
+        public static SaveImageAsCommand PngInstance { get; } = new SaveImageAsCommand(ImageFormat.Png) { Converter = SetBackgroundConverter.White };
 
         public static SaveImageAsCommand EmfInstance { get; } = new SaveImageAsCommand(ImageFormat.Emf);
 

src/Common/ChartDrawing/IO/SetBackgroundConverter.cs

@@ -0,0 +1,50 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Media;
+using System.Windows.Shapes;
+
+namespace CompMs.Graphics.IO;
+
+public sealed class SetBackgroundConverter : DependencyObject, IVisualConverter
+{
+    public static readonly DependencyProperty BackgroundProperty =
+        DependencyProperty.Register(
+            nameof(Background),
+            typeof(Brush),
+            typeof(SetBackgroundConverter),
+            new PropertyMetadata(Brushes.Transparent));
+
+    public Brush Background {
+        get => (Brush)GetValue(BackgroundProperty);
+        set => SetValue(BackgroundProperty, value);
+    }
+
+    public FrameworkElement Convert(FrameworkElement element) {
+        var brush = new VisualBrush(element);
+        var width = element.ActualWidth;
+        var height = element.ActualHeight;
+        var canvas = new Canvas
+        {
+            Width = width,
+            Height = height,
+            Background = Background,
+        };
+
+        var rectangle = new Rectangle
+        {
+            Width = width,
+            Height = height,
+            Fill = brush,
+        };
+        Canvas.SetTop(rectangle, 0d);
+        Canvas.SetLeft(rectangle, 0d);
+
+        canvas.Children.Add(rectangle);
+        canvas.Measure(new Size(width, height));
+        canvas.Arrange(new Rect(0, 0, width, height));
+
+        return canvas;
+    }
+
+    public static SetBackgroundConverter White { get; } = new SetBackgroundConverter { Background = Brushes.White };
+}