retrain untouched pixels when applying filter

src/ImageSharp/Processing/Processors/Convolution/Convolution2PassProcessor.cs

@@ -45,16 +45,11 @@ namespace ImageSharp.Processing.Processors
             int width = source.Width;
             int height = source.Height;
 
-            using (PixelAccessor<TPixel> targetPixels = new PixelAccessor<TPixel>(width, height))
+            using (PixelAccessor<TPixel> firstPassPixels = new PixelAccessor<TPixel>(width, height))
+            using (PixelAccessor<TPixel> sourcePixels = source.Lock())
             {
-                using (PixelAccessor<TPixel> firstPassPixels = new PixelAccessor<TPixel>(width, height))
-                using (PixelAccessor<TPixel> sourcePixels = source.Lock())
-                {
-                    this.ApplyConvolution(firstPassPixels, sourcePixels, sourceRectangle, this.KernelX);
-                    this.ApplyConvolution(targetPixels, firstPassPixels, sourceRectangle, this.KernelY);
-                }
-
-                source.SwapPixelsBuffers(targetPixels);
+                this.ApplyConvolution(firstPassPixels, sourcePixels, source.Bounds, this.KernelX);
+                this.ApplyConvolution(sourcePixels, firstPassPixels, sourceRectangle, this.KernelY);
             }
         }
 

tests/ImageSharp.Tests/Processors/Filters/GaussianBlurTest.cs

@@ -6,10 +6,10 @@
 namespace ImageSharp.Tests
 {
     using System.IO;
-
+    using ImageSharp.PixelFormats;
     using Xunit;
 
-    public class GaussianBlurTest : FileTestBase
+    public class GaussianBlurTest
     {
         public static readonly TheoryData<int> GaussianBlurValues
         = new TheoryData<int>
@@ -19,19 +19,33 @@ namespace ImageSharp.Tests
         };
 
         [Theory]
-        [MemberData(nameof(GaussianBlurValues))]
-        public void ImageShouldApplyGaussianBlurFilter(int value)
+        [WithTestPatternImages(nameof(GaussianBlurValues), 320, 240, PixelTypes.StandardImageClass)]
+        public void ImageShouldApplyGaussianBlurFilter<TPixel>(TestImageProvider<TPixel> provider, int value)
+            where TPixel : struct, IPixel<TPixel>
         {
-            string path = this.CreateOutputDirectory("GaussianBlur");
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                image.GaussianBlur(value)
+                    .DebugSave(provider);
+            }
+        }
 
-            foreach (TestFile file in Files)
+        [Theory]
+        [WithTestPatternImages(nameof(GaussianBlurValues), 320, 240, PixelTypes.StandardImageClass)]
+        public void ImageShouldApplyGaussianBlurFilterInBox<TPixel>(TestImageProvider<TPixel> provider, int value)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> source = provider.GetImage())
+            using (Image<TPixel> image = new Image<TPixel>(source))
             {
-                string filename = file.GetFileName(value);
-                using (Image image = file.CreateImage())
-                using (FileStream output = File.OpenWrite($"{path}/{filename}"))
-                {
-                    image.GaussianBlur(value).Save(output);
-                }
+                Rectangle rect = new Rectangle(image.Width / 4, image.Height / 4, image.Width / 2, image.Height / 2);
+                image.GaussianBlur(value, rect)
+                    .DebugSave(provider);
+
+                // lets draw identical shapes over the blured areas and ensure that it didn't change the outer area
+                image.Fill(NamedColors<TPixel>.HotPink, rect);
+                source.Fill(NamedColors<TPixel>.HotPink, rect);
+                ImageComparer.CheckSimilarity(image, source);
             }
         }
     }