Correctly handle x,y offset for Bitmap.CopyPixels

src/Avalonia.Base/Media/Imaging/Bitmap.cs

@@ -182,20 +182,23 @@ public void Save(Stream stream, int? quality = null)
         private protected unsafe void CopyPixelsCore(PixelRect sourceRect, IntPtr buffer, int bufferSize, int stride,
             ILockedFramebuffer fb)
         {
+            if (Format == null)
+                throw new NotSupportedException("CopyPixels is not supported for this bitmap type");
+
             if ((sourceRect.Width <= 0 || sourceRect.Height <= 0) && (sourceRect.X != 0 || sourceRect.Y != 0))
                 throw new ArgumentOutOfRangeException(nameof(sourceRect));
 
             if (sourceRect.X < 0 || sourceRect.Y < 0)
                 throw new ArgumentOutOfRangeException(nameof(sourceRect));
-            
+
             if (sourceRect.Width <= 0)
                 sourceRect = sourceRect.WithWidth(PixelSize.Width);
             if (sourceRect.Height <= 0)
                 sourceRect = sourceRect.WithHeight(PixelSize.Height);
 
             if (sourceRect.Right > PixelSize.Width || sourceRect.Bottom > PixelSize.Height)
                 throw new ArgumentOutOfRangeException(nameof(sourceRect));
-            
+
             int minStride = checked(((sourceRect.Width * fb.Format.BitsPerPixel) + 7) / 8);
             if (stride < minStride)
                 throw new ArgumentOutOfRangeException(nameof(stride));
@@ -204,14 +207,16 @@ private protected unsafe void CopyPixelsCore(PixelRect sourceRect, IntPtr buffer
             if (minBufferSize > bufferSize)
                 throw new ArgumentOutOfRangeException(nameof(bufferSize));
 
+            var offsetX = checked(((sourceRect.X * Format.Value.BitsPerPixel) + 7) / 8);
+
             for (var y = 0; y < sourceRect.Height; y++)
             {
-                var srcAddress = fb.Address + fb.RowBytes * y;
+                var srcAddress = fb.Address + fb.RowBytes * (sourceRect.Y + y) + offsetX;
                 var dstAddress = buffer + stride * y;
                 Unsafe.CopyBlock(dstAddress.ToPointer(), srcAddress.ToPointer(), (uint)minStride);
             }
         }
-        
+
         public virtual void CopyPixels(PixelRect sourceRect, IntPtr buffer, int bufferSize, int stride)
         {
             if (

tests/Avalonia.RenderTests/Media/BitmapTests.cs

@@ -246,5 +246,34 @@ public unsafe void CopyPixelsShouldWorkForNonTranscodedBitmaps()
                 bmp.CopyPixels(default, new IntPtr(pCopyTo), data.Length, stride);
             Assert.Equal(data, copyTo);
         }
+
+        [Fact]
+        public unsafe void Should_CopyPixels_With_Source_Rect()
+        {
+            var size = 80;
+            var partSize = 20;
+            var bitmap = new RenderTargetBitmap(new PixelSize(size, size));
+
+            using (var context = bitmap.CreateDrawingContext())
+            {
+                context.FillRectangle(Brushes.Black,
+                    new Rect(0, 0, bitmap.PixelSize.Width, bitmap.PixelSize.Height));
+                context.FillRectangle(Brushes.White, new Rect(partSize, partSize, partSize, partSize));
+            }
+
+            var bpp = bitmap.Format!.Value.BitsPerPixel / 8;
+            var buffer = new byte[partSize * partSize * bpp];
+
+            fixed (byte* pointer = buffer)
+            {
+                bitmap.CopyPixels(new PixelRect(partSize, partSize, partSize, partSize), (IntPtr)pointer,
+                    buffer.Length, partSize * bpp);
+            }
+
+            foreach (var t in buffer)
+            {
+                Assert.Equal(byte.MaxValue, t);
+            }
+        }
     }
 }