Fix DPI scaling with Skia.

src/Skia/Avalonia.Skia/BitmapImpl.cs

@@ -1,9 +1,5 @@
 using System;
-using System.Collections.Generic;
 using System.IO;
-using System.Runtime.InteropServices;
-using System.Text;
-using Avalonia.Media;
 using Avalonia.Platform;
 using Avalonia.Rendering;
 using SkiaSharp;
@@ -12,6 +8,9 @@ namespace Avalonia.Skia
 {
     class BitmapImpl : IRenderTargetBitmapImpl, IWritableBitmapImpl
     {
+        private double _dpiX;
+        private double _dpiY;
+
         public SKBitmap Bitmap { get; private set; }
 
         public BitmapImpl(SKBitmap bm)
@@ -19,12 +18,16 @@ namespace Avalonia.Skia
             Bitmap = bm;
             PixelHeight = bm.Height;
             PixelWidth = bm.Width;
+            _dpiX = 96;
+            _dpiY = 96;
         }
 
-        public BitmapImpl(int width, int height, PixelFormat? fmt = null)
+        public BitmapImpl(int width, int height, double dpiX, double dpiY, PixelFormat? fmt = null)
         {
             PixelHeight = height;
             PixelWidth = width;
+            _dpiX = dpiX;
+            _dpiY = dpiY;
             var colorType = fmt?.ToSkColorType() ?? SKImageInfo.PlatformColorType;
             var runtime = AvaloniaLocator.Current?.GetService<IRuntimePlatform>()?.GetRuntimeInfo();
             if (runtime?.IsDesktop == true && runtime?.OperatingSystem == OperatingSystemType.Linux)
@@ -68,8 +71,8 @@ namespace Avalonia.Skia
         {
             private readonly SKSurface _surface;
 
-            public BitmapDrawingContext(SKBitmap bitmap, IVisualBrushRenderer visualBrushRenderer)
-                : this(CreateSurface(bitmap), visualBrushRenderer)
+            public BitmapDrawingContext(SKBitmap bitmap, double dpiX, double dpiY, IVisualBrushRenderer visualBrushRenderer)
+                : this(CreateSurface(bitmap), dpiX, dpiY, visualBrushRenderer)
             {
 
             }
@@ -83,8 +86,8 @@ namespace Avalonia.Skia
                 return rv;
             }
 
-            public BitmapDrawingContext(SKSurface surface, IVisualBrushRenderer visualBrushRenderer)
-                : base(surface.Canvas, visualBrushRenderer)
+            public BitmapDrawingContext(SKSurface surface, double dpiX, double dpiY, IVisualBrushRenderer visualBrushRenderer)
+                : base(surface.Canvas, dpiX, dpiY, visualBrushRenderer)
             {
                 _surface = surface;
             }
@@ -98,7 +101,7 @@ namespace Avalonia.Skia
 
         public IDrawingContextImpl CreateDrawingContext(IVisualBrushRenderer visualBrushRenderer)
         {
-            return new BitmapDrawingContext(Bitmap, visualBrushRenderer);
+            return new BitmapDrawingContext(Bitmap, _dpiX, _dpiY, visualBrushRenderer);
         }
 
         public void Save(Stream stream)

src/Skia/Avalonia.Skia/DrawingContextImpl.cs

@@ -11,6 +11,8 @@ namespace Avalonia.Skia
 {
     internal class DrawingContextImpl : IDrawingContextImpl
     {
+        private readonly double _dpiX;
+        private readonly double _dpiY;
         private readonly Matrix? _postTransform;
         private readonly IDisposable[] _disposables;
         private readonly IVisualBrushRenderer _visualBrushRenderer;
@@ -20,12 +22,16 @@ namespace Avalonia.Skia
 
         public DrawingContextImpl(
             SKCanvas canvas,
+            double dpiX,
+            double dpiY,
             IVisualBrushRenderer visualBrushRenderer,
-            Matrix? postTransform = null,
             params IDisposable[] disposables)
         {
-            if (postTransform.HasValue && !postTransform.Value.IsIdentity)
-                _postTransform = postTransform;
+            _dpiX = dpiX;
+            _dpiY = dpiY;
+
+            if (dpiX != 96 || dpiY != 96)
+                _postTransform = Matrix.CreateScale(dpiX / 96, dpiY / 96);
             _visualBrushRenderer = visualBrushRenderer;
             _disposables = disposables;
             Canvas = canvas;
@@ -217,7 +223,7 @@ namespace Avalonia.Skia
 
                     if (intermediateSize.Width >= 1 && intermediateSize.Height >= 1)
                     {
-                        var intermediate = new BitmapImpl((int)intermediateSize.Width, (int)intermediateSize.Height);
+                        var intermediate = new BitmapImpl((int)intermediateSize.Width, (int)intermediateSize.Height, _dpiX, _dpiY);
 
                         using (var ctx = intermediate.CreateDrawingContext(_visualBrushRenderer))
                         {
@@ -242,7 +248,7 @@ namespace Avalonia.Skia
             if (tileBrush != null && tileBrushImage != null)
             {
                 var calc = new TileBrushCalculator(tileBrush, new Size(tileBrushImage.PixelWidth, tileBrushImage.PixelHeight), targetSize);
-                var bitmap = new BitmapImpl((int)calc.IntermediateSize.Width, (int)calc.IntermediateSize.Height);
+                var bitmap = new BitmapImpl((int)calc.IntermediateSize.Width, (int)calc.IntermediateSize.Height, _dpiX, _dpiY);
                 rv.AddDisposable(bitmap);
                 using (var context = bitmap.CreateDrawingContext(null))
                 {

src/Skia/Avalonia.Skia/FramebufferRenderTarget.cs

@@ -76,8 +76,7 @@ namespace Avalonia.Skia
             canvas.RestoreToCount(0);
             canvas.Save();
             canvas.ResetMatrix();
-            var scale = Matrix.CreateScale(fb.Dpi.Width / 96, fb.Dpi.Height / 96);
-            return new DrawingContextImpl(canvas, visualBrushRenderer, scale, canvas, surface, shim, fb);
+            return new DrawingContextImpl(canvas, fb.Dpi.Width, fb.Dpi.Height, visualBrushRenderer, canvas, surface, shim, fb);
         }
     }
 }

src/Skia/Avalonia.Skia/PlatformRenderInterface.cs

@@ -77,7 +77,7 @@ namespace Avalonia.Skia
             if (height < 1)
                 throw new ArgumentException("Height can't be less than 1", nameof(height));
 
-            return new BitmapImpl(width, height);
+            return new BitmapImpl(width, height, dpiX, dpiY);
         }
 
         public virtual IRenderTarget CreateRenderTarget(IEnumerable<object> surfaces)
@@ -90,7 +90,7 @@ namespace Avalonia.Skia
 
         public IWritableBitmapImpl CreateWritableBitmap(int width, int height, PixelFormat? format = null)
         {
-            return new BitmapImpl(width, height, format);
+            return new BitmapImpl(width, height, 96, 96, format);
         }
     }
 }