Use fixed on filter loops

Settings.StyleCop

@@ -32,6 +32,7 @@
       <Value>Vol</Value>
       <Value>pp</Value>
       <Value>cmyk</Value>
+      <Value>Paeth</Value>
     </CollectionProperty>
   </GlobalSettings>
   <Analyzers>

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

@@ -12,7 +12,7 @@ namespace ImageSharp.Formats
     /// the value of a pixel.
     /// <see href="https://www.w3.org/TR/PNG-Filters.html"/>
     /// </summary>
-    internal static class AverageFilter
+    internal static unsafe class AverageFilter
     {
         /// <summary>
         /// Decodes the scanline
@@ -28,12 +28,17 @@ namespace ImageSharp.Formats
             // Average(x) + floor((Raw(x-bpp)+Prior(x))/2)
             byte[] result = new byte[scanline.Length];
 
-            for (int x = 1; x < scanline.Length; x++)
+            fixed (byte* scan = scanline)
+            fixed (byte* prev = previousScanline)
+            fixed (byte* res = result)
             {
-                byte left = (x - bytesPerPixel < 1) ? (byte)0 : result[x - bytesPerPixel];
-                byte above = previousScanline[x];
+                for (int x = 1; x < scanline.Length; x++)
+                {
+                    byte left = (x - bytesPerPixel < 1) ? (byte)0 : res[x - bytesPerPixel];
+                    byte above = prev[x];
 
-                result[x] = (byte)((scanline[x] + Average(left, above)) % 256);
+                    res[x] = (byte)((scan[x] + Average(left, above)) % 256);
+                }
             }
 
             return result;
@@ -50,19 +55,23 @@ namespace ImageSharp.Formats
         public static byte[] Encode(byte[] scanline, byte[] previousScanline, int bytesPerPixel, int bytesPerScanline)
         {
             // Average(x) = Raw(x) - floor((Raw(x-bpp)+Prior(x))/2)
-            byte[] encodedScanline = new byte[bytesPerScanline + 1];
-
-            encodedScanline[0] = (byte)FilterType.Average;
-
-            for (int x = 0; x < bytesPerScanline; x++)
+            byte[] result = new byte[bytesPerScanline + 1];
+            fixed (byte* scan = scanline)
+            fixed (byte* prev = previousScanline)
+            fixed (byte* res = result)
             {
-                byte left = (x - bytesPerPixel < 0) ? (byte)0 : scanline[x - bytesPerPixel];
-                byte above = previousScanline[x];
+                res[0] = 3;
 
-                encodedScanline[x + 1] = (byte)((scanline[x] - Average(left, above)) % 256);
+                for (int x = 0; x < bytesPerScanline; x++)
+                {
+                    byte left = (x - bytesPerPixel < 0) ? (byte)0 : scan[x - bytesPerPixel];
+                    byte above = prev[x];
+
+                    res[x + 1] = (byte)((scan[x] - Average(left, above)) % 256);
+                }
             }
 
-            return encodedScanline;
+            return result;
         }
 
         /// <summary>
@@ -73,7 +82,7 @@ namespace ImageSharp.Formats
         /// <returns>The <see cref="int"/></returns>
         private static int Average(byte left, byte above)
         {
-            return Convert.ToInt32(Math.Floor((left + above) / 2.0D));
+            return (left + above) >> 1;
         }
     }
-}
+}

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

@@ -34,11 +34,11 @@ namespace ImageSharp.Formats
         public static byte[] Encode(byte[] scanline, int bytesPerScanline)
         {
             // Insert a byte before the data.
-            byte[] encodedScanline = new byte[bytesPerScanline + 1];
-            encodedScanline[0] = (byte)FilterType.None;
-            Buffer.BlockCopy(scanline, 0, encodedScanline, 1, bytesPerScanline);
+            byte[] result = new byte[bytesPerScanline + 1];
+            result[0] = 0;
+            Buffer.BlockCopy(scanline, 0, result, 1, bytesPerScanline);
 
-            return encodedScanline;
+            return result;
         }
     }
 }

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

@@ -13,7 +13,7 @@ namespace ImageSharp.Formats
     /// This technique is due to Alan W. Paeth.
     /// <see href="https://www.w3.org/TR/PNG-Filters.html"/>
     /// </summary>
-    internal static class PaethFilter
+    internal static unsafe class PaethFilter
     {
         /// <summary>
         /// Decodes the scanline
@@ -27,13 +27,18 @@ namespace ImageSharp.Formats
             // Paeth(x) + PaethPredictor(Raw(x-bpp), Prior(x), Prior(x-bpp))
             byte[] result = new byte[scanline.Length];
 
-            for (int x = 1; x < scanline.Length; x++)
+            fixed (byte* scan = scanline)
+            fixed (byte* prev = previousScanline)
+            fixed (byte* res = result)
             {
-                byte left = (x - bytesPerPixel < 1) ? (byte)0 : result[x - bytesPerPixel];
-                byte above = previousScanline[x];
-                byte upperLeft = (x - bytesPerPixel < 1) ? (byte)0 : previousScanline[x - bytesPerPixel];
+                for (int x = 1; x < scanline.Length; x++)
+                {
+                    byte left = (x - bytesPerPixel < 1) ? (byte)0 : res[x - bytesPerPixel];
+                    byte above = prev[x];
+                    byte upperLeft = (x - bytesPerPixel < 1) ? (byte)0 : prev[x - bytesPerPixel];
 
-                result[x] = (byte)((scanline[x] + PaethPredicator(left, above, upperLeft)) % 256);
+                    res[x] = (byte)((scan[x] + PaethPredicator(left, above, upperLeft)) % 256);
+                }
             }
 
             return result;
@@ -50,19 +55,25 @@ namespace ImageSharp.Formats
         public static byte[] Encode(byte[] scanline, byte[] previousScanline, int bytesPerPixel, int bytesPerScanline)
         {
             // Paeth(x) = Raw(x) - PaethPredictor(Raw(x-bpp), Prior(x), Prior(x - bpp))
-            byte[] encodedScanline = new byte[bytesPerScanline + 1];
-            encodedScanline[0] = (byte)FilterType.Paeth;
+            byte[] result = new byte[bytesPerScanline + 1];
 
-            for (int x = 0; x < bytesPerScanline; x++)
+            fixed (byte* scan = scanline)
+            fixed (byte* prev = previousScanline)
+            fixed (byte* res = result)
             {
-                byte left = (x - bytesPerPixel < 0) ? (byte)0 : scanline[x - bytesPerPixel];
-                byte above = previousScanline[x];
-                byte upperLeft = (x - bytesPerPixel < 0) ? (byte)0 : previousScanline[x - bytesPerPixel];
+                res[0] = 4;
 
-                encodedScanline[x + 1] = (byte)((scanline[x] - PaethPredicator(left, above, upperLeft)) % 256);
+                for (int x = 0; x < bytesPerScanline; x++)
+                {
+                    byte left = (x - bytesPerPixel < 0) ? (byte)0 : scan[x - bytesPerPixel];
+                    byte above = prev[x];
+                    byte upperLeft = (x - bytesPerPixel < 0) ? (byte)0 : prev[x - bytesPerPixel];
+
+                    res[x + 1] = (byte)((scan[x] - PaethPredicator(left, above, upperLeft)) % 256);
+                }
             }
 
-            return encodedScanline;
+            return result;
         }
 
         /// <summary>

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

@@ -10,7 +10,7 @@ namespace ImageSharp.Formats
     /// of the prior pixel.
     /// <see href="https://www.w3.org/TR/PNG-Filters.html"/>
     /// </summary>
-    internal static class SubFilter
+    internal static unsafe class SubFilter
     {
         /// <summary>
         /// Decodes the scanline
@@ -23,11 +23,15 @@ namespace ImageSharp.Formats
             // Sub(x) + Raw(x-bpp)
             byte[] result = new byte[scanline.Length];
 
-            for (int x = 1; x < scanline.Length; x++)
+            fixed (byte* scan = scanline)
+            fixed (byte* res = result)
             {
-                byte priorRawByte = (x - bytesPerPixel < 1) ? (byte)0 : result[x - bytesPerPixel];
+                for (int x = 1; x < scanline.Length; x++)
+                {
+                    byte priorRawByte = (x - bytesPerPixel < 1) ? (byte)0 : res[x - bytesPerPixel];
 
-                result[x] = (byte)((scanline[x] + priorRawByte) % 256);
+                    res[x] = (byte)((scan[x] + priorRawByte) % 256);
+                }
             }
 
             return result;
@@ -43,17 +47,22 @@ namespace ImageSharp.Formats
         public static byte[] Encode(byte[] scanline, int bytesPerPixel, int bytesPerScanline)
         {
             // Sub(x) = Raw(x) - Raw(x-bpp)
-            byte[] encodedScanline = new byte[bytesPerScanline + 1];
-            encodedScanline[0] = (byte)FilterType.Sub;
+            byte[] result = new byte[bytesPerScanline + 1];
 
-            for (int x = 0; x < bytesPerScanline; x++)
+            fixed (byte* scan = scanline)
+            fixed (byte* res = result)
             {
-                byte priorRawByte = (x - bytesPerPixel < 0) ? (byte)0 : scanline[x - bytesPerPixel];
+                res[0] = 1;
 
-                encodedScanline[x + 1] = (byte)((scanline[x] - priorRawByte) % 256);
+                for (int x = 0; x < bytesPerScanline; x++)
+                {
+                    byte priorRawByte = (x - bytesPerPixel < 0) ? (byte)0 : scan[x - bytesPerPixel];
+
+                    res[x + 1] = (byte)((scan[x] - priorRawByte) % 256);
+                }
             }
 
-            return encodedScanline;
+            return result;
         }
     }
 }

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

@@ -10,7 +10,7 @@ namespace ImageSharp.Formats
     /// rather than just to its left, is used as the predictor.
     /// <see href="https://www.w3.org/TR/PNG-Filters.html"/>
     /// </summary>
-    internal static class UpFilter
+    internal static unsafe class UpFilter
     {
         /// <summary>
         /// Decodes the scanline
@@ -23,11 +23,16 @@ namespace ImageSharp.Formats
             // Up(x) + Prior(x)
             byte[] result = new byte[scanline.Length];
 
-            for (int x = 1; x < scanline.Length; x++)
+            fixed (byte* scan = scanline)
+            fixed (byte* prev = previousScanline)
+            fixed (byte* res = result)
             {
-                byte above = previousScanline[x];
+                for (int x = 1; x < scanline.Length; x++)
+                {
+                    byte above = prev[x];
 
-                result[x] = (byte)((scanline[x] + above) % 256);
+                    res[x] = (byte)((scan[x] + above) % 256);
+                }
             }
 
             return result;
@@ -43,17 +48,23 @@ namespace ImageSharp.Formats
         public static byte[] Encode(byte[] scanline, int bytesPerScanline, byte[] previousScanline)
         {
             // Up(x) = Raw(x) - Prior(x)
-            byte[] encodedScanline = new byte[bytesPerScanline + 1];
-            encodedScanline[0] = (byte)FilterType.Up;
+            byte[] result = new byte[bytesPerScanline + 1];
 
-            for (int x = 0; x < bytesPerScanline; x++)
+            fixed (byte* scan = scanline)
+            fixed (byte* prev = previousScanline)
+            fixed (byte* res = result)
             {
-                byte above = previousScanline[x];
+                res[0] = 2;
 
-                encodedScanline[x + 1] = (byte)((scanline[x] - above) % 256);
+                for (int x = 0; x < bytesPerScanline; x++)
+                {
+                    byte above = prev[x];
+
+                    res[x + 1] = (byte)((scan[x] - above) % 256);
+                }
             }
 
-            return encodedScanline;
+            return result;
         }
     }
 }