Don't allocate memory for png decode filters, reuse scanline buffers

- Added png test images with different filters for local tests

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

@@ -26,22 +26,20 @@ namespace ImageSharp.Formats
         public static byte[] Decode(byte[] scanline, byte[] previousScanline, int bytesPerPixel)
         {
             // Average(x) + floor((Raw(x-bpp)+Prior(x))/2)
-            byte[] result = new byte[scanline.Length];
 
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
-            fixed (byte* res = result)
             {
                 for (int x = 1; x < scanline.Length; x++)
                 {
-                    byte left = (x - bytesPerPixel < 1) ? (byte)0 : res[x - bytesPerPixel];
+                    byte left = (x - bytesPerPixel < 1) ? (byte)0 : scan[x - bytesPerPixel];
                     byte above = prev[x];
 
-                    res[x] = (byte)((scan[x] + Average(left, above)) % 256);
+                    scan[x] = (byte)((scan[x] + Average(left, above)) % 256);
                 }
             }
 
-            return result;
+            return scanline;
         }
 
         /// <summary>

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

@@ -25,23 +25,21 @@ namespace ImageSharp.Formats
         public static byte[] Decode(byte[] scanline, byte[] previousScanline, int bytesPerPixel)
         {
             // Paeth(x) + PaethPredictor(Raw(x-bpp), Prior(x), Prior(x-bpp))
-            byte[] result = new byte[scanline.Length];
 
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
-            fixed (byte* res = result)
             {
                 for (int x = 1; x < scanline.Length; x++)
                 {
-                    byte left = (x - bytesPerPixel < 1) ? (byte)0 : res[x - bytesPerPixel];
+                    byte left = (x - bytesPerPixel < 1) ? (byte)0 : scan[x - bytesPerPixel];
                     byte above = prev[x];
                     byte upperLeft = (x - bytesPerPixel < 1) ? (byte)0 : prev[x - bytesPerPixel];
 
-                    res[x] = (byte)((scan[x] + PaethPredicator(left, above, upperLeft)) % 256);
+                    scan[x] = (byte)((scan[x] + PaethPredicator(left, above, upperLeft)) % 256);
                 }
             }
 
-            return result;
+            return scanline;
         }
 
         /// <summary>

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

@@ -21,20 +21,17 @@ namespace ImageSharp.Formats
         public static byte[] Decode(byte[] scanline, int bytesPerPixel)
         {
             // Sub(x) + Raw(x-bpp)
-            byte[] result = new byte[scanline.Length];
 
             fixed (byte* scan = scanline)
-            fixed (byte* res = result)
             {
                 for (int x = 1; x < scanline.Length; x++)
                 {
-                    byte priorRawByte = (x - bytesPerPixel < 1) ? (byte)0 : res[x - bytesPerPixel];
-
-                    res[x] = (byte)((scan[x] + priorRawByte) % 256);
+                    byte priorRawByte = (x - bytesPerPixel < 1) ? (byte)0 : scan[x - bytesPerPixel];
+                    scan[x] = (byte)((scan[x] + priorRawByte) % 256);
                 }
             }
 
-            return result;
+            return scanline;
         }
 
         /// <summary>

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

@@ -21,21 +21,19 @@ namespace ImageSharp.Formats
         public static byte[] Decode(byte[] scanline, byte[] previousScanline)
         {
             // Up(x) + Prior(x)
-            byte[] result = new byte[scanline.Length];
 
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
-            fixed (byte* res = result)
             {
                 for (int x = 1; x < scanline.Length; x++)
                 {
                     byte above = prev[x];
 
-                    res[x] = (byte)((scan[x] + above) % 256);
+                    scan[x] = (byte)((scan[x] + above) % 256);
                 }
             }
 
-            return result;
+            return scanline;
         }
 
         /// <summary>

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -10,7 +10,7 @@ namespace ImageSharp.Formats
     using System.IO;
     using System.Linq;
     using System.Text;
-
+    
     /// <summary>
     /// Performs the png decoding operation.
     /// </summary>
@@ -276,45 +276,42 @@ namespace ImageSharp.Formats
             // TODO: ArrayPool<byte>.Shared.Rent(this.bytesPerScanline)
             byte[] previousScanline = new byte[this.bytesPerScanline];
             byte[] scanline = new byte[this.bytesPerScanline];
+          
             for (int y = 0; y < this.header.Height; y++)
             {
                 compressedStream.Read(scanline, 0, this.bytesPerScanline);
 
                 FilterType filterType = (FilterType)scanline[0];
-                byte[] defilteredScanline;
 
-                // TODO: It would be good if we can reduce the memory usage here - Each filter is creating a new row.
-                // Every time I try to use the same approach as I have in the encoder though I keep messing up.
-                // Fingers crossed someone with a big brain and a kind heart will come along and finish optimizing this for me.
                 switch (filterType)
                 {
                     case FilterType.None:
 
-                        defilteredScanline = NoneFilter.Decode(scanline);
+                        NoneFilter.Decode(scanline);
 
                         break;
 
                     case FilterType.Sub:
 
-                        defilteredScanline = SubFilter.Decode(scanline, this.bytesPerPixel);
+                        SubFilter.Decode(scanline, this.bytesPerPixel);
 
                         break;
 
                     case FilterType.Up:
 
-                        defilteredScanline = UpFilter.Decode(scanline, previousScanline);
+                         UpFilter.Decode(scanline, previousScanline);
 
                         break;
 
                     case FilterType.Average:
 
-                        defilteredScanline = AverageFilter.Decode(scanline, previousScanline, this.bytesPerPixel);
+                         AverageFilter.Decode(scanline, previousScanline, this.bytesPerPixel);
 
                         break;
 
                     case FilterType.Paeth:
 
-                        defilteredScanline = PaethFilter.Decode(scanline, previousScanline, this.bytesPerPixel);
+                         PaethFilter.Decode(scanline, previousScanline, this.bytesPerPixel);
 
                         break;
 
@@ -322,8 +319,11 @@ namespace ImageSharp.Formats
                         throw new ImageFormatException("Unknown filter type.");
                 }
 
-                previousScanline = defilteredScanline;
-                this.ProcessDefilteredScanline(defilteredScanline, y, pixels);
+                this.ProcessDefilteredScanline(scanline, y, pixels);
+
+                byte[] temp = previousScanline;
+                previousScanline = scanline;
+                scanline = temp;
             }
         }
 

tests/ImageSharp.Tests/FileTestBase.cs

@@ -32,6 +32,12 @@ namespace ImageSharp.Tests
             // 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.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
+            // new TestFile(TestImages.Png.Filter3), // Perf: Enable for local testing only
+            // new TestFile(TestImages.Png.Filter4), // Perf: Enable for local testing only
+            // new TestFile(TestImages.Png.FilterVar), // Perf: Enable for local testing only
             new TestFile(TestImages.Gif.Rings),
             // new TestFile(TestImages.Gif.Giphy) // Perf: Enable for local testing only
         };

tests/ImageSharp.Tests/TestImages.cs

@@ -19,6 +19,15 @@ namespace ImageSharp.Tests
             public static string Blur => folder + "blur.png";
             public static string Indexed => folder + "indexed.png";
             public static string Splash => folder + "splash.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";
+            public static string Filter2 => folder + "filter2.png";
+            public static string Filter3 => folder + "filter3.png";
+            public static string Filter4 => folder + "filter4.png";
+            // filter changing per scanline     
+            public static string FilterVar => folder + "filterVar.png";
         }
 
         public static class Jpeg

tests/ImageSharp.Tests/TestImages/Formats/Png/filter0.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:0fe92b2aa2da04c885d1dbd85c834716f6cdd946364d97dcd597bb79d9e14427
+size 2475

tests/ImageSharp.Tests/TestImages/Formats/Png/filter1.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:ef072ec6815ebf9b33e0553d2e4e4e7ed6911860a2512c67bcd10a9f0f09b9de
+size 1180

tests/ImageSharp.Tests/TestImages/Formats/Png/filter2.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:ca4b937b3c587d5c007f193a2eec14dc96b0d23ff7d6aa9004e3badd1af9fe8f
+size 1729

tests/ImageSharp.Tests/TestImages/Formats/Png/filter3.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:2d101e3ef4f78a69437034671e93fe11faac0cfc4d44210dcca1b944caa886f7
+size 1291

tests/ImageSharp.Tests/TestImages/Formats/Png/filter4.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c365c24153cb69fd3c162f00b296ae23a71a1595645d1aeb0ad23af680d7b4be
+size 985

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

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:8ac0f095d2a943157e820fa121bccde08d5230af1b5830c3041d5f4da3524eba
+size 426