Much better output. Nearly there. Touch #9

Image in issue works perfectly as does rgba test image. rgb 8x8 test
image shows 2 pixel error though and I don't know why.

src/ImageSharp/Formats/Png/Filters/AverageFilter.cs

@@ -5,8 +5,6 @@
 
 namespace ImageSharp.Formats
 {
-    using System;
-
     /// <summary>
     /// The Average filter uses the average of the two neighboring pixels (left and above) to predict
     /// the value of a pixel.

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -440,28 +440,28 @@ namespace ImageSharp.Formats
             where TColor : struct, IPackedPixel<TPacked>
             where TPacked : struct
         {
-            for (int pass = 0; pass < 7; pass++)
-            {
-                int y = Adam7FirstRow[pass];
-                int numColumns = this.ComputeColumnsAdam7(pass);
-
-                if (numColumns == 0)
-                {
-                    // This pass contains no data; skip to next pass
-                    continue;
-                }
+            byte[] previousScanline = ArrayPool<byte>.Shared.Rent(this.bytesPerScanline);
+            byte[] scanline = ArrayPool<byte>.Shared.Rent(this.bytesPerScanline);
 
-                int bytesPerInterlaceScanline = this.CalculateScanlineLength(numColumns) + 1;
+            // Zero out the previousScanline, because the bytes that are rented from the arraypool may not be zero.
+            Array.Clear(previousScanline, 0, this.bytesPerScanline);
 
-                while (y < this.header.Height)
+            try
+            {
+                for (int pass = 0; pass < 7; pass++)
                 {
-                    byte[] previousScanline = ArrayPool<byte>.Shared.Rent(this.bytesPerScanline);
-                    byte[] scanline = ArrayPool<byte>.Shared.Rent(this.bytesPerScanline);
+                    int y = Adam7FirstRow[pass];
+                    int numColumns = this.ComputeColumnsAdam7(pass);
 
-                    // Zero out the previousScanline, because the bytes that are rented from the arraypool may not be zero.
-                    Array.Clear(previousScanline, 0, this.bytesPerScanline);
+                    if (numColumns == 0)
+                    {
+                        // This pass contains no data; skip to next pass
+                        continue;
+                    }
 
-                    try
+                    int bytesPerInterlaceScanline = this.CalculateScanlineLength(numColumns) + 1;
+
+                    while (y < this.header.Height)
                     {
                         compressedStream.Read(scanline, 0, bytesPerInterlaceScanline);
 
@@ -506,16 +506,17 @@ namespace ImageSharp.Formats
                         this.ProcessInterlacedDefilteredScanline(scanline, y, pixels, Adam7FirstColumn[pass], Adam7ColumnIncrement[pass]);
 
                         Swap(ref scanline, ref previousScanline);
-                    }
-                    finally
-                    {
-                        ArrayPool<byte>.Shared.Return(previousScanline);
-                        ArrayPool<byte>.Shared.Return(scanline);
-                    }
 
-                    y += Adam7RowIncrement[pass];
+
+                        y += Adam7RowIncrement[pass];
+                    }
                 }
             }
+            finally
+            {
+                ArrayPool<byte>.Shared.Return(previousScanline);
+                ArrayPool<byte>.Shared.Return(scanline);
+            }
         }
 
         /// <summary>
@@ -765,7 +766,6 @@ namespace ImageSharp.Formats
             }
         }
 
-
         /// <summary>
         /// Reads a text chunk containing image properties from the data.
         /// </summary>

tests/ImageSharp.Tests/FileTestBase.cs

@@ -31,8 +31,9 @@ namespace ImageSharp.Tests
             // new TestFile(TestImages.Bmp.Neg_height), // Perf: Enable for local testing only
             // new TestFile(TestImages.Png.Blur), // Perf: Enable for local testing only
             // new TestFile(TestImages.Png.Indexed), // Perf: Enable for local testing only
-            //new TestFile(TestImages.Png.Splash),
-            new TestFile(TestImages.Png.SplashInterlace),
+            new TestFile(TestImages.Png.Splash),
+            //new TestFile(TestImages.Png.SplashInterlaced),
+            new TestFile(TestImages.Png.Interlaced),
             // new TestFile(TestImages.Png.Filter0), // Perf: Enable for local testing only
             // new TestFile(TestImages.Png.Filter1), // Perf: Enable for local testing only
             // new TestFile(TestImages.Png.Filter2), // Perf: Enable for local testing only

tests/ImageSharp.Tests/TestImages.cs

@@ -20,7 +20,10 @@ namespace ImageSharp.Tests
             public static string Indexed => folder + "indexed.png";
             public static string Splash => folder + "splash.png";
 
-            public static string SplashInterlace => folder + "splash-interlaced.png";
+            public static string SplashInterlaced => folder + "splash-interlaced.png";
+
+            public static string Interlaced => folder + "interlaced.png";
+
             // filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html
             public static string Filter0 => folder + "filter0.png";
             public static string Filter1 => folder + "filter1.png";

tests/ImageSharp.Tests/TestImages/Formats/Png/interlaced.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:7e1c92b1a57b762f736e14d480eb016b551ce0dc67aa588b246d838d00c0fc3f
-size 17372
+oid sha256:21d34b50acf6bf71c68673585a298b8ddce3a908a46c38a6be9530487ebab8dc
+size 17949