Merge branch 'foo' into without46

src/ImageSharp/Common/Extensions/ByteExtensions.cs

@@ -24,7 +24,7 @@ namespace ImageSharp
         public static byte[] ToArrayByBitsLength(this byte[] source, int bits)
         {
             Guard.NotNull(source, nameof(source));
-            Guard.MustBeGreaterThan(bits, 0, "bits");
+            Guard.MustBeGreaterThan(bits, 0, nameof(bits));
 
             byte[] result;
 

src/ImageSharp/Common/Extensions/ComparableExtensions.cs

@@ -165,5 +165,18 @@ namespace ImageSharp
         {
             return (byte)value.Clamp(0, 255);
         }
+
+        /// <summary>
+        /// Swaps the references to two objects in memory.
+        /// </summary>
+        /// <param name="first">The first reference.</param>
+        /// <param name="second">The second reference.</param>
+        /// <typeparam name="T">The type of object.</typeparam>
+        public static void Swap<T>(ref T first, ref T second)
+        {
+            T temp = second;
+            second = first;
+            first = temp;
+        }
     }
 }

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

@@ -20,13 +20,9 @@ namespace ImageSharp.Formats
         /// <param name="scanline">The scanline to decode</param>
         /// <param name="previousScanline">The previous scanline.</param>
         /// <param name="bytesPerPixel">The bytes per pixel.</param>
-        /// <returns>
-        /// The <see cref="T:byte[]"/>
-        /// </returns>
-        public static byte[] Decode(byte[] scanline, byte[] previousScanline, int bytesPerPixel)
+        public static void Decode(byte[] scanline, byte[] previousScanline, int bytesPerPixel)
         {
             // Average(x) + floor((Raw(x-bpp)+Prior(x))/2)
-
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
             {
@@ -38,8 +34,6 @@ namespace ImageSharp.Formats
                     scan[x] = (byte)((scan[x] + Average(left, above)) % 256);
                 }
             }
-
-            return scanline;
         }
 
         /// <summary>
@@ -47,13 +41,12 @@ namespace ImageSharp.Formats
         /// </summary>
         /// <param name="scanline">The scanline to encode</param>
         /// <param name="previousScanline">The previous scanline.</param>
+        /// <param name="result">The filtered scanline result.</param>
         /// <param name="bytesPerPixel">The bytes per pixel.</param>
         /// <param name="bytesPerScanline">The number of bytes per scanline</param>
-        /// <returns>The <see cref="T:byte[]"/></returns>
-        public static byte[] Encode(byte[] scanline, byte[] previousScanline, int bytesPerPixel, int bytesPerScanline)
+        public static void Encode(byte[] scanline, byte[] previousScanline, byte[] result, int bytesPerPixel, int bytesPerScanline)
         {
             // Average(x) = Raw(x) - floor((Raw(x-bpp)+Prior(x))/2)
-            byte[] result = new byte[bytesPerScanline + 1];
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
             fixed (byte* res = result)
@@ -68,8 +61,6 @@ namespace ImageSharp.Formats
                     res[x + 1] = (byte)((scan[x] - Average(left, above)) % 256);
                 }
             }
-
-            return result;
         }
 
         /// <summary>

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

@@ -29,14 +29,13 @@ namespace ImageSharp.Formats
         /// Encodes the scanline
         /// </summary>
         /// <param name="scanline">The scanline to encode</param>
+        /// <param name="result">The filtered scanline result.</param>
         /// <param name="bytesPerScanline">The number of bytes per scanline</param>
-        /// <returns>The <see cref="T:byte[]"/></returns>
-        public static byte[] Encode(byte[] scanline, int bytesPerScanline)
+        public static void Encode(byte[] scanline, byte[] result, int bytesPerScanline)
         {
             // Insert a byte before the data.
-            byte[] result = new byte[bytesPerScanline + 1];
+            result[0] = 0;
             Buffer.BlockCopy(scanline, 0, result, 1, bytesPerScanline);
-            return result;
         }
     }
 }

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

@@ -21,11 +21,9 @@ namespace ImageSharp.Formats
         /// <param name="scanline">The scanline to decode</param>
         /// <param name="previousScanline">The previous scanline.</param>
         /// <param name="bytesPerPixel">The bytes per pixel.</param>
-        /// <returns>The <see cref="T:byte[]"/></returns>
-        public static byte[] Decode(byte[] scanline, byte[] previousScanline, int bytesPerPixel)
+        public static void Decode(byte[] scanline, byte[] previousScanline, int bytesPerPixel)
         {
             // Paeth(x) + PaethPredictor(Raw(x-bpp), Prior(x), Prior(x-bpp))
-
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
             {
@@ -38,8 +36,6 @@ namespace ImageSharp.Formats
                     scan[x] = (byte)((scan[x] + PaethPredicator(left, above, upperLeft)) % 256);
                 }
             }
-
-            return scanline;
         }
 
         /// <summary>
@@ -47,13 +43,12 @@ namespace ImageSharp.Formats
         /// </summary>
         /// <param name="scanline">The scanline to encode</param>
         /// <param name="previousScanline">The previous scanline.</param>
+        /// <param name="result">The filtered scanline result.</param>
         /// <param name="bytesPerPixel">The bytes per pixel.</param>
         /// <param name="bytesPerScanline">The number of bytes per scanline</param>
-        /// <returns>The <see cref="T:byte[]"/></returns>
-        public static byte[] Encode(byte[] scanline, byte[] previousScanline, int bytesPerPixel, int bytesPerScanline)
+        public static void Encode(byte[] scanline, byte[] previousScanline, byte[] result, int bytesPerPixel, int bytesPerScanline)
         {
             // Paeth(x) = Raw(x) - PaethPredictor(Raw(x-bpp), Prior(x), Prior(x - bpp))
-            byte[] result = new byte[bytesPerScanline + 1];
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
             fixed (byte* res = result)
@@ -69,8 +64,6 @@ namespace ImageSharp.Formats
                     res[x + 1] = (byte)((scan[x] - PaethPredicator(left, above, upperLeft)) % 256);
                 }
             }
-
-            return result;
         }
 
         /// <summary>

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

@@ -17,11 +17,9 @@ namespace ImageSharp.Formats
         /// </summary>
         /// <param name="scanline">The scanline to decode</param>
         /// <param name="bytesPerPixel">The bytes per pixel.</param>
-        /// <returns>The <see cref="T:byte[]"/></returns>
-        public static byte[] Decode(byte[] scanline, int bytesPerPixel)
+        public static void Decode(byte[] scanline, int bytesPerPixel)
         {
             // Sub(x) + Raw(x-bpp)
-
             fixed (byte* scan = scanline)
             {
                 for (int x = 1; x < scanline.Length; x++)
@@ -30,21 +28,18 @@ namespace ImageSharp.Formats
                     scan[x] = (byte)((scan[x] + priorRawByte) % 256);
                 }
             }
-
-            return scanline;
         }
 
         /// <summary>
         /// Encodes the scanline
         /// </summary>
         /// <param name="scanline">The scanline to encode</param>
+        /// <param name="result">The filtered scanline result.</param>
         /// <param name="bytesPerPixel">The bytes per pixel.</param>
         /// <param name="bytesPerScanline">The number of bytes per scanline</param>
-        /// <returns>The <see cref="T:byte[]"/></returns>
-        public static byte[] Encode(byte[] scanline, int bytesPerPixel, int bytesPerScanline)
+        public static void Encode(byte[] scanline, byte[] result, int bytesPerPixel, int bytesPerScanline)
         {
             // Sub(x) = Raw(x) - Raw(x-bpp)
-            byte[] result = new byte[bytesPerScanline + 1];
             fixed (byte* scan = scanline)
             fixed (byte* res = result)
             {
@@ -57,8 +52,6 @@ namespace ImageSharp.Formats
                     res[x + 1] = (byte)((scan[x] - priorRawByte) % 256);
                 }
             }
-
-            return result;
         }
     }
 }

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

@@ -17,11 +17,9 @@ namespace ImageSharp.Formats
         /// </summary>
         /// <param name="scanline">The scanline to decode</param>
         /// <param name="previousScanline">The previous scanline.</param>
-        /// <returns>The <see cref="T:byte[]"/></returns>
-        public static byte[] Decode(byte[] scanline, byte[] previousScanline)
+        public static void Decode(byte[] scanline, byte[] previousScanline)
         {
             // Up(x) + Prior(x)
-
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
             {
@@ -32,8 +30,6 @@ namespace ImageSharp.Formats
                     scan[x] = (byte)((scan[x] + above) % 256);
                 }
             }
-
-            return scanline;
         }
 
         /// <summary>
@@ -41,12 +37,11 @@ namespace ImageSharp.Formats
         /// </summary>
         /// <param name="scanline">The scanline to encode</param>
         /// <param name="previousScanline">The previous scanline.</param>
+        /// <param name="result">The filtered scanline result.</param>
         /// <param name="bytesPerScanline">The number of bytes per scanline</param>
-        /// <returns>The <see cref="T:byte[]"/></returns>
-        public static byte[] Encode(byte[] scanline, byte[] previousScanline, int bytesPerScanline)
+        public static void Encode(byte[] scanline, byte[] previousScanline, byte[] result, int bytesPerScanline)
         {
             // Up(x) = Raw(x) - Prior(x)
-            byte[] result = new byte[bytesPerScanline + 1];
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
             fixed (byte* res = result)
@@ -60,8 +55,6 @@ namespace ImageSharp.Formats
                     res[x + 1] = (byte)((scan[x] - above) % 256);
                 }
             }
-
-            return result;
         }
     }
 }

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -12,6 +12,8 @@ namespace ImageSharp.Formats
     using System.Linq;
     using System.Text;
 
+    using static ComparableExtensions;
+
     /// <summary>
     /// Performs the png decoding operation.
     /// </summary>
@@ -112,7 +114,8 @@ namespace ImageSharp.Formats
         /// Thrown if the image is larger than the maximum allowable size.
         /// </exception>
         public void Decode<TColor, TPacked>(Image<TColor, TPacked> image, Stream stream)
-            where TColor : struct, IPackedPixel<TPacked> where TPacked : struct
+            where TColor : struct, IPackedPixel<TPacked>
+            where TPacked : struct
         {
             Image<TColor, TPacked> currentImage = image;
             this.currentStream = stream;
@@ -130,39 +133,49 @@ namespace ImageSharp.Formats
                         throw new ImageFormatException("Image does not end with end chunk.");
                     }
 
-                    switch (currentChunk.Type)
+                    try
                     {
-                        case PngChunkTypes.Header:
-                            this.ReadHeaderChunk(currentChunk.Data);
-                            this.ValidateHeader();
-                            break;
-                        case PngChunkTypes.Physical:
-                            this.ReadPhysicalChunk(currentImage, currentChunk.Data);
-                            break;
-                        case PngChunkTypes.Data:
-                            dataStream.Write(currentChunk.Data, 0, currentChunk.Data.Length);
-                            break;
-                        case PngChunkTypes.Palette:
-                            this.palette = currentChunk.Data;
-                            image.Quality = this.palette.Length / 3;
-                            break;
-                        case PngChunkTypes.PaletteAlpha:
-                            this.paletteAlpha = currentChunk.Data;
-                            break;
-                        case PngChunkTypes.Text:
-                            this.ReadTextChunk(currentImage, currentChunk.Data);
-                            break;
-                        case PngChunkTypes.End:
-                            isEndChunkReached = true;
-                            break;
+                        switch (currentChunk.Type)
+                        {
+                            case PngChunkTypes.Header:
+                                this.ReadHeaderChunk(currentChunk.Data);
+                                this.ValidateHeader();
+                                break;
+                            case PngChunkTypes.Physical:
+                                this.ReadPhysicalChunk(currentImage, currentChunk.Data);
+                                break;
+                            case PngChunkTypes.Data:
+                                dataStream.Write(currentChunk.Data, 0, currentChunk.Length);
+                                break;
+                            case PngChunkTypes.Palette:
+                                byte[] pal = new byte[currentChunk.Length];
+                                Buffer.BlockCopy(currentChunk.Data, 0, pal, 0, currentChunk.Length);
+                                this.palette = pal;
+                                image.Quality = pal.Length / 3;
+                                break;
+                            case PngChunkTypes.PaletteAlpha:
+                                byte[] alpha = new byte[currentChunk.Length];
+                                Buffer.BlockCopy(currentChunk.Data, 0, alpha, 0, currentChunk.Length);
+                                this.paletteAlpha = alpha;
+                                break;
+                            case PngChunkTypes.Text:
+                                this.ReadTextChunk(currentImage, currentChunk.Data, currentChunk.Length);
+                                break;
+                            case PngChunkTypes.End:
+                                isEndChunkReached = true;
+                                break;
+                        }
+                    }
+                    finally
+                    {
+                        // Data is rented in ReadChunkData()
+                        ArrayPool<byte>.Shared.Return(currentChunk.Data);
                     }
                 }
 
                 if (this.header.Width > image.MaxWidth || this.header.Height > image.MaxHeight)
                 {
-                    throw new ArgumentOutOfRangeException(
-                              $"The input png '{this.header.Width}x{this.header.Height}' is bigger than the "
-                              + $"max allowed size '{image.MaxWidth}x{image.MaxHeight}'");
+                    throw new ArgumentOutOfRangeException($"The input png '{this.header.Width}x{this.header.Height}' is bigger than the max allowed size '{image.MaxWidth}x{image.MaxHeight}'");
                 }
 
                 image.InitPixels(this.header.Width, this.header.Height);
@@ -182,7 +195,8 @@ namespace ImageSharp.Formats
         /// <param name="image">The image to read to.</param>
         /// <param name="data">The data containing physical data.</param>
         private void ReadPhysicalChunk<TColor, TPacked>(Image<TColor, TPacked> image, byte[] data)
-            where TColor : struct, IPackedPixel<TPacked> where TPacked : struct
+            where TColor : struct, IPackedPixel<TPacked>
+            where TPacked : struct
         {
             data.ReverseBytes(0, 4);
             data.ReverseBytes(4, 4);
@@ -243,7 +257,8 @@ namespace ImageSharp.Formats
         /// <param name="dataStream">The <see cref="MemoryStream"/> containing data.</param>
         /// <param name="pixels"> The pixel data.</param>
         private void ReadScanlines<TColor, TPacked>(MemoryStream dataStream, PixelAccessor<TColor, TPacked> pixels)
-            where TColor : struct, IPackedPixel<TPacked> where TPacked : struct
+            where TColor : struct, IPackedPixel<TPacked>
+            where TPacked : struct
         {
             this.bytesPerPixel = this.CalculateBytesPerPixel();
             this.bytesPerScanline = this.CalculateScanlineLength() + 1;
@@ -268,7 +283,8 @@ namespace ImageSharp.Formats
         /// <param name="compressedStream">The compressed pixel data stream.</param>
         /// <param name="pixels">The image pixel accessor.</param>
         private void DecodePixelData<TColor, TPacked>(Stream compressedStream, PixelAccessor<TColor, TPacked> pixels)
-            where TColor : struct, IPackedPixel<TPacked> where TPacked : struct
+            where TColor : struct, IPackedPixel<TPacked>
+            where TPacked : struct
         {
             byte[] previousScanline = ArrayPool<byte>.Shared.Rent(this.bytesPerScanline);
             byte[] scanline = ArrayPool<byte>.Shared.Rent(this.bytesPerScanline);
@@ -319,9 +335,7 @@ namespace ImageSharp.Formats
 
                     this.ProcessDefilteredScanline(scanline, y, pixels);
 
-                    byte[] temp = previousScanline;
-                    previousScanline = scanline;
-                    scanline = temp;
+                    Swap(ref scanline, ref previousScanline);
                 }
             }
             finally
@@ -396,6 +410,10 @@ namespace ImageSharp.Formats
                                 byte b = this.palette[pixelOffset + 2];
                                 color.PackFromBytes(r, g, b, a);
                             }
+                            else
+                            {
+                                color.PackFromBytes(0, 0, 0, 0);
+                            }
 
                             pixels[x, row] = color;
                         }
@@ -460,13 +478,14 @@ namespace ImageSharp.Formats
         /// <typeparam name="TPacked">The packed format. <example>uint, long, float.</example></typeparam>
         /// <param name="image">The image to decode to.</param>
         /// <param name="data">The <see cref="T:byte[]"/> containing  data.</param>
-        private void ReadTextChunk<TColor, TPacked>(Image<TColor, TPacked> image, byte[] data)
+        /// <param name="length">The maximum length to read.</param>
+        private void ReadTextChunk<TColor, TPacked>(Image<TColor, TPacked> image, byte[] data, int length)
             where TColor : struct, IPackedPixel<TPacked>
             where TPacked : struct
         {
             int zeroIndex = 0;
 
-            for (int i = 0; i < data.Length; i++)
+            for (int i = 0; i < length; i++)
             {
                 if (data[i] == 0)
                 {
@@ -476,7 +495,7 @@ namespace ImageSharp.Formats
             }
 
             string name = Encoding.Unicode.GetString(data, 0, zeroIndex);
-            string value = Encoding.Unicode.GetString(data, zeroIndex + 1, data.Length - zeroIndex - 1);
+            string value = Encoding.Unicode.GetString(data, zeroIndex + 1, length - zeroIndex - 1);
 
             image.Properties.Add(new ImageProperty(name, value));
         }
@@ -577,7 +596,7 @@ namespace ImageSharp.Formats
 
             Crc32 crc = new Crc32();
             crc.Update(this.chunkTypeBuffer);
-            crc.Update(chunk.Data);
+            crc.Update(chunk.Data, 0, chunk.Length);
 
             if (crc.Value != chunk.Crc)
             {
@@ -591,8 +610,8 @@ namespace ImageSharp.Formats
         /// <param name="chunk">The chunk.</param>
         private void ReadChunkData(PngChunk chunk)
         {
-            // TODO: It might be possible to rent this but that could also lead to issues assigning the data to various properties
-            chunk.Data = new byte[chunk.Length];
+            // We rent the buffer here to return it afterwards in Decode()
+            chunk.Data = ArrayPool<byte>.Shared.Rent(chunk.Length);
             this.currentStream.Read(chunk.Data, 0, chunk.Length);
         }
 
@@ -645,4 +664,4 @@ namespace ImageSharp.Formats
             chunk.Length = BitConverter.ToInt32(this.chunkLengthBuffer, 0);
         }
     }
-}
+}

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -13,6 +13,8 @@ namespace ImageSharp.Formats
 
     using Quantizers;
 
+    using static ComparableExtensions;
+
     /// <summary>
     /// Performs the png encoding operation.
     /// </summary>
@@ -58,6 +60,26 @@ namespace ImageSharp.Formats
         /// </summary>
         private int bytesPerPixel;
 
+        /// <summary>
+        /// The buffer for the sub filter
+        /// </summary>
+        private byte[] sub;
+
+        /// <summary>
+        /// The buffer for the up filter
+        /// </summary>
+        private byte[] up;
+
+        /// <summary>
+        /// The buffer for the average filter
+        /// </summary>
+        private byte[] average;
+
+        /// <summary>
+        /// The buffer for the paeth filter
+        /// </summary>
+        private byte[] paeth;
+
         /// <summary>
         /// Gets or sets the quality of output for images.
         /// </summary>
@@ -312,9 +334,10 @@ namespace ImageSharp.Formats
         /// <param name="row">The row.</param>
         /// <param name="previousScanline">The previous scanline.</param>
         /// <param name="rawScanline">The raw scanline.</param>
+        /// <param name="result">The filtered scanline result.</param>
         /// <param name="bytesPerScanline">The number of bytes per scanline.</param>
         /// <returns>The <see cref="T:byte[]"/></returns>
-        private byte[] EncodePixelRow<TColor, TPacked>(PixelAccessor<TColor, TPacked> pixels, int row, byte[] previousScanline, byte[] rawScanline, int bytesPerScanline)
+        private byte[] EncodePixelRow<TColor, TPacked>(PixelAccessor<TColor, TPacked> pixels, int row, byte[] previousScanline, byte[] rawScanline, byte[] result, int bytesPerScanline)
             where TColor : struct, IPackedPixel<TPacked>
             where TPacked : struct
         {
@@ -332,7 +355,7 @@ namespace ImageSharp.Formats
                     break;
             }
 
-            return this.GetOptimalFilteredScanline(rawScanline, previousScanline, bytesPerScanline);
+            return this.GetOptimalFilteredScanline(rawScanline, previousScanline, result, bytesPerScanline);
         }
 
         /// <summary>
@@ -341,45 +364,51 @@ namespace ImageSharp.Formats
         /// </summary>
         /// <param name="rawScanline">The raw scanline</param>
         /// <param name="previousScanline">The previous scanline</param>
+        /// <param name="result">The filtered scanline result.</param>
         /// <param name="bytesPerScanline">The number of bytes per scanline</param>
         /// <returns>The <see cref="T:byte[]"/></returns>
-        private byte[] GetOptimalFilteredScanline(byte[] rawScanline, byte[] previousScanline, int bytesPerScanline)
+        private byte[] GetOptimalFilteredScanline(byte[] rawScanline, byte[] previousScanline, byte[] result, int bytesPerScanline)
         {
             // Palette images don't compress well with adaptive filtering.
             if (this.PngColorType == PngColorType.Palette)
             {
-                return NoneFilter.Encode(rawScanline, bytesPerScanline);
+                NoneFilter.Encode(rawScanline, result, bytesPerScanline);
+                return result;
             }
 
-            // TODO: it would be nice to avoid the memory allocation here.
-            Tuple<byte[], int>[] candidates = new Tuple<byte[], int>[4];
+            SubFilter.Encode(rawScanline, this.sub, this.bytesPerPixel, bytesPerScanline);
+            int currentTotalVariation = this.CalculateTotalVariation(this.sub, bytesPerScanline);
+            int lowestTotalVariation = currentTotalVariation;
 
-            byte[] sub = SubFilter.Encode(rawScanline, this.bytesPerPixel, bytesPerScanline);
-            candidates[0] = new Tuple<byte[], int>(sub, this.CalculateTotalVariation(sub, bytesPerScanline));
+            result = this.sub;
 
-            byte[] up = UpFilter.Encode(rawScanline, previousScanline, bytesPerScanline);
-            candidates[1] = new Tuple<byte[], int>(up, this.CalculateTotalVariation(up, bytesPerScanline));
+            UpFilter.Encode(rawScanline, previousScanline, this.up, bytesPerScanline);
+            currentTotalVariation = this.CalculateTotalVariation(this.up, bytesPerScanline);
 
-            byte[] average = AverageFilter.Encode(rawScanline, previousScanline, this.bytesPerPixel, bytesPerScanline);
-            candidates[2] = new Tuple<byte[], int>(average, this.CalculateTotalVariation(average, bytesPerScanline));
+            if (currentTotalVariation < lowestTotalVariation)
+            {
+                lowestTotalVariation = currentTotalVariation;
+                result = this.up;
+            }
 
-            byte[] paeth = PaethFilter.Encode(rawScanline, previousScanline, this.bytesPerPixel, bytesPerScanline);
-            candidates[3] = new Tuple<byte[], int>(paeth, this.CalculateTotalVariation(paeth, bytesPerScanline));
+            AverageFilter.Encode(rawScanline, previousScanline, this.average, this.bytesPerPixel, bytesPerScanline);
+            currentTotalVariation = this.CalculateTotalVariation(this.average, bytesPerScanline);
 
-            int lowestTotalVariation = int.MaxValue;
-            int lowestTotalVariationIndex = 0;
+            if (currentTotalVariation < lowestTotalVariation)
+            {
+                lowestTotalVariation = currentTotalVariation;
+                result = this.average;
+            }
 
-            for (int i = 0; i < candidates.Length; i++)
+            PaethFilter.Encode(rawScanline, previousScanline, this.paeth, this.bytesPerPixel, bytesPerScanline);
+            currentTotalVariation = this.CalculateTotalVariation(this.paeth, bytesPerScanline);
+
+            if (currentTotalVariation < lowestTotalVariation)
             {
-                if (candidates[i].Item2 < lowestTotalVariation)
-                {
-                    lowestTotalVariationIndex = i;
-                    lowestTotalVariation = candidates[i].Item2;
-                }
+                result = this.paeth;
             }
 
-            // ReSharper disable once RedundantAssignment
-            return candidates[lowestTotalVariationIndex].Item1;
+            return result;
         }
 
         /// <summary>
@@ -592,6 +621,15 @@ namespace ImageSharp.Formats
             byte[] previousScanline = ArrayPool<byte>.Shared.Rent(bytesPerScanline);
             byte[] rawScanline = ArrayPool<byte>.Shared.Rent(bytesPerScanline);
             int resultLength = bytesPerScanline + 1;
+            byte[] result = ArrayPool<byte>.Shared.Rent(resultLength);
+
+            if (this.PngColorType != PngColorType.Palette)
+            {
+                this.sub = ArrayPool<byte>.Shared.Rent(resultLength);
+                this.up = ArrayPool<byte>.Shared.Rent(resultLength);
+                this.average = ArrayPool<byte>.Shared.Rent(resultLength);
+                this.paeth = ArrayPool<byte>.Shared.Rent(resultLength);
+            }
 
             byte[] buffer;
             int bufferLength;
@@ -603,12 +641,9 @@ namespace ImageSharp.Formats
                 {
                     for (int y = 0; y < this.height; y++)
                     {
-                        deflateStream.Write(this.EncodePixelRow(pixels, y, previousScanline, rawScanline, bytesPerScanline), 0, resultLength);
+                        deflateStream.Write(this.EncodePixelRow(pixels, y, previousScanline, rawScanline, result, bytesPerScanline), 0, resultLength);
 
-                        // Do a bit of shuffling;
-                        byte[] tmp = rawScanline;
-                        rawScanline = previousScanline;
-                        previousScanline = tmp;
+                        Swap(ref rawScanline, ref previousScanline);
                     }
                 }
 
@@ -620,6 +655,15 @@ namespace ImageSharp.Formats
                 memoryStream?.Dispose();
                 ArrayPool<byte>.Shared.Return(previousScanline);
                 ArrayPool<byte>.Shared.Return(rawScanline);
+                ArrayPool<byte>.Shared.Return(result);
+
+                if (this.PngColorType != PngColorType.Palette)
+                {
+                    ArrayPool<byte>.Shared.Return(this.sub);
+                    ArrayPool<byte>.Shared.Return(this.up);
+                    ArrayPool<byte>.Shared.Return(this.average);
+                    ArrayPool<byte>.Shared.Return(this.paeth);
+                }
             }
 
             // Store the chunks in repeated 64k blocks.

src/ImageSharp/IO/EndianBitConverter.cs

@@ -20,9 +20,11 @@ namespace ImageSharp.IO
     [SuppressMessage("StyleCop.CSharp.OrderingRules", "SA1201:ElementsMustAppearInTheCorrectOrder", Justification = "Reviewed. Suppression is OK here. Better readability.")]
     [SuppressMessage("StyleCop.CSharp.OrderingRules", "SA1202:ElementsMustBeOrderedByAccess", Justification = "Reviewed. Suppression is OK here. Better readability.")]
     [SuppressMessage("StyleCop.CSharp.OrderingRules", "SA1204:StaticElementsMustAppearBeforeInstanceElements", Justification = "Reviewed. Suppression is OK here. Better readability.")]
+    [SuppressMessage("StyleCop.CSharp.ReadabilityRules", "SA1124:DoNotUseRegions", Justification = "Reviewed. Suppression is OK here. Better readability.")]
     internal abstract class EndianBitConverter
     {
         #region Endianness of this converter
+
         /// <summary>
         /// Indicates the byte order ("endianness") in which data is converted using this class.
         /// </summary>
@@ -41,6 +43,7 @@ namespace ImageSharp.IO
         #endregion
 
         #region Factory properties
+
         /// <summary>
         /// The little-endian bit converter.
         /// </summary>
@@ -65,6 +68,7 @@ namespace ImageSharp.IO
         #endregion
 
         #region Double/primitive conversions
+
         /// <summary>
         /// Converts the specified double-precision floating point number to a
         /// 64-bit signed integer. Note: the endianness of this converter does not
@@ -115,6 +119,7 @@ namespace ImageSharp.IO
         #endregion
 
         #region To(PrimitiveType) conversions
+
         /// <summary>
         /// Returns a Boolean value converted from one byte at a specified position in a byte array.
         /// </summary>
@@ -279,6 +284,7 @@ namespace ImageSharp.IO
         #endregion
 
         #region ToString conversions
+
         /// <summary>
         /// Returns a String converted from the elements of a byte array.
         /// </summary>
@@ -326,6 +332,7 @@ namespace ImageSharp.IO
         #endregion
 
         #region Decimal conversions
+
         /// <summary>
         /// Returns a decimal value converted from sixteen bytes
         /// at a specified position in a byte array.
@@ -382,6 +389,7 @@ namespace ImageSharp.IO
         #endregion
 
         #region GetBytes conversions
+
         /// <summary>
         /// Returns an array with the given number of bytes formed
         /// from the least significant bytes of the specified value.
@@ -508,6 +516,7 @@ namespace ImageSharp.IO
         #endregion
 
         #region CopyBytes conversions
+
         /// <summary>
         /// Copies the given number of bytes from the least-specific
         /// end of the specified value into the specified byte array, beginning
@@ -669,6 +678,7 @@ namespace ImageSharp.IO
         #endregion
 
         #region Private struct used for Single/Int32 conversions
+
         /// <summary>
         /// Union used solely for the equivalent of DoubleToInt64Bits and vice versa.
         /// </summary>

src/ImageSharp/Numerics/Rectangle.cs

@@ -213,6 +213,16 @@ namespace ImageSharp
             return !left.Equals(right);
         }
 
+        /// <summary>
+        /// Returns the center point of the given <see cref="Rectangle"/>
+        /// </summary>
+        /// <param name="rectangle">The rectangle</param>
+        /// <returns><see cref="Point"/></returns>
+        public static Point Center(Rectangle rectangle)
+        {
+            return new Point(rectangle.Left + (rectangle.Width / 2), rectangle.Top + (rectangle.Height / 2));
+        }
+
         /// <summary>
         /// Determines if the specfied point is contained within the rectangular region defined by
         /// this <see cref="Rectangle"/>.
@@ -229,16 +239,6 @@ namespace ImageSharp
                    && y < this.Bottom;
         }
 
-        /// <summary>
-        /// Returns the center point of the given <see cref="Rectangle"/>
-        /// </summary>
-        /// <param name="rectangle">The rectangle</param>
-        /// <returns><see cref="Point"/></returns>
-        public static Point Center(Rectangle rectangle)
-        {
-            return new Point(rectangle.Left + (rectangle.Width / 2), rectangle.Top + (rectangle.Height / 2));
-        }
-
         /// <inheritdoc/>
         public override int GetHashCode()
         {

src/ImageSharp/Profiles/Exif/ExifReader.cs

@@ -15,8 +15,6 @@ namespace ImageSharp
     /// </summary>
     internal sealed class ExifReader
     {
-        private delegate TDataType ConverterMethod<TDataType>(byte[] data);
-
         private readonly Collection<ExifTag> invalidTags = new Collection<ExifTag>();
         private byte[] exifData;
         private uint currentIndex;
@@ -25,6 +23,13 @@ namespace ImageSharp
         private uint gpsOffset;
         private uint startIndex;
 
+        private delegate TDataType ConverterMethod<TDataType>(byte[] data);
+
+        /// <summary>
+        /// Gets the invalid tags.
+        /// </summary>
+        public IEnumerable<ExifTag> InvalidTags => this.invalidTags;
+
         /// <summary>
         /// Gets the thumbnail length in the byte stream
         /// </summary>
@@ -112,10 +117,40 @@ namespace ImageSharp
             return result;
         }
 
-        /// <summary>
-        /// Gets the invalid tags.
-        /// </summary>
-        public IEnumerable<ExifTag> InvalidTags => this.invalidTags;
+        private static TDataType[] ToArray<TDataType>(ExifDataType dataType, byte[] data, ConverterMethod<TDataType> converter)
+        {
+            int dataTypeSize = (int)ExifValue.GetSize(dataType);
+            int length = data.Length / dataTypeSize;
+
+            TDataType[] result = new TDataType[length];
+            byte[] buffer = new byte[dataTypeSize];
+
+            for (int i = 0; i < length; i++)
+            {
+                Array.Copy(data, i * dataTypeSize, buffer, 0, dataTypeSize);
+
+                result.SetValue(converter(buffer), i);
+            }
+
+            return result;
+        }
+
+        private static byte ToByte(byte[] data)
+        {
+            return data[0];
+        }
+
+        private static string ToString(byte[] data)
+        {
+            string result = Encoding.UTF8.GetString(data, 0, data.Length);
+            int nullCharIndex = result.IndexOf('\0');
+            if (nullCharIndex != -1)
+            {
+                result = result.Substring(0, nullCharIndex);
+            }
+
+            return result;
+        }
 
         /// <summary>
         /// Adds the collection of EXIF values to the reader.
@@ -369,29 +404,6 @@ namespace ImageSharp
             }
         }
 
-        private static TDataType[] ToArray<TDataType>(ExifDataType dataType, byte[] data, ConverterMethod<TDataType> converter)
-        {
-            int dataTypeSize = (int)ExifValue.GetSize(dataType);
-            int length = data.Length / dataTypeSize;
-
-            TDataType[] result = new TDataType[length];
-            byte[] buffer = new byte[dataTypeSize];
-
-            for (int i = 0; i < length; i++)
-            {
-                Array.Copy(data, i * dataTypeSize, buffer, 0, dataTypeSize);
-
-                result.SetValue(converter(buffer), i);
-            }
-
-            return result;
-        }
-
-        private static byte ToByte(byte[] data)
-        {
-            return data[0];
-        }
-
         private double ToDouble(byte[] data)
         {
             if (!this.ValidateArray(data, 8))
@@ -432,18 +444,6 @@ namespace ImageSharp
             return BitConverter.ToSingle(data, 0);
         }
 
-        private static string ToString(byte[] data)
-        {
-            string result = Encoding.UTF8.GetString(data, 0, data.Length);
-            int nullCharIndex = result.IndexOf('\0');
-            if (nullCharIndex != -1)
-            {
-                result = result.Substring(0, nullCharIndex);
-            }
-
-            return result;
-        }
-
         private Rational ToRational(byte[] data)
         {
             if (!this.ValidateArray(data, 8, 4))

src/ImageSharp/Profiles/Exif/ExifValue.cs

@@ -41,6 +41,24 @@ namespace ImageSharp
             }
         }
 
+        internal ExifValue(ExifTag tag, ExifDataType dataType, bool isArray)
+        {
+            this.Tag = tag;
+            this.DataType = dataType;
+            this.IsArray = isArray;
+
+            if (dataType == ExifDataType.Ascii)
+            {
+                this.IsArray = false;
+            }
+        }
+
+        internal ExifValue(ExifTag tag, ExifDataType dataType, object value, bool isArray)
+          : this(tag, dataType, isArray)
+        {
+            this.exifValue = value;
+        }
+
         /// <summary>
         /// Gets the data type of the exif value.
         /// </summary>
@@ -81,6 +99,57 @@ namespace ImageSharp
             }
         }
 
+        internal bool HasValue
+        {
+            get
+            {
+                if (this.exifValue == null)
+                {
+                    return false;
+                }
+
+                if (this.DataType == ExifDataType.Ascii)
+                {
+                    return ((string)this.exifValue).Length > 0;
+                }
+
+                return true;
+            }
+        }
+
+        internal int Length
+        {
+            get
+            {
+                if (this.exifValue == null)
+                {
+                    return 4;
+                }
+
+                int size = (int)(GetSize(this.DataType) * this.NumberOfComponents);
+
+                return size < 4 ? 4 : size;
+            }
+        }
+
+        internal int NumberOfComponents
+        {
+            get
+            {
+                if (this.DataType == ExifDataType.Ascii)
+                {
+                    return Encoding.UTF8.GetBytes((string)this.exifValue).Length;
+                }
+
+                if (this.IsArray)
+                {
+                    return ((Array)this.exifValue).Length;
+                }
+
+                return 1;
+            }
+        }
+
         /// <summary>
         /// Determines whether the specified ExifValue instances are considered equal.
         /// </summary>
@@ -173,75 +242,6 @@ namespace ImageSharp
             return sb.ToString();
         }
 
-        internal bool HasValue
-        {
-            get
-            {
-                if (this.exifValue == null)
-                {
-                    return false;
-                }
-
-                if (this.DataType == ExifDataType.Ascii)
-                {
-                    return ((string)this.exifValue).Length > 0;
-                }
-
-                return true;
-            }
-        }
-
-        internal int Length
-        {
-            get
-            {
-                if (this.exifValue == null)
-                {
-                    return 4;
-                }
-
-                int size = (int)(GetSize(this.DataType) * this.NumberOfComponents);
-
-                return size < 4 ? 4 : size;
-            }
-        }
-
-        internal int NumberOfComponents
-        {
-            get
-            {
-                if (this.DataType == ExifDataType.Ascii)
-                {
-                    return Encoding.UTF8.GetBytes((string)this.exifValue).Length;
-                }
-
-                if (this.IsArray)
-                {
-                    return ((Array)this.exifValue).Length;
-                }
-
-                return 1;
-            }
-        }
-
-        internal ExifValue(ExifTag tag, ExifDataType dataType, bool isArray)
-        {
-            this.Tag = tag;
-            this.DataType = dataType;
-            this.IsArray = isArray;
-
-            if (dataType == ExifDataType.Ascii)
-            {
-                this.IsArray = false;
-            }
-        }
-
-        internal ExifValue(ExifTag tag, ExifDataType dataType, object value, bool isArray)
-          : this(tag, dataType, isArray)
-        {
-            this.exifValue = value;
-        }
-
         internal static ExifValue Create(ExifTag tag, object value)
         {
             Guard.IsFalse(tag == ExifTag.Unknown, nameof(tag), "Invalid Tag");
@@ -574,6 +574,26 @@ namespace ImageSharp
             }
         }
 
+        private static ExifValue CreateNumber(ExifTag tag, Type type, bool isArray)
+        {
+            if (type == null || type == typeof(ushort))
+            {
+                return new ExifValue(tag, ExifDataType.Short, isArray);
+            }
+
+            if (type == typeof(short))
+            {
+                return new ExifValue(tag, ExifDataType.SignedShort, isArray);
+            }
+
+            if (type == typeof(uint))
+            {
+                return new ExifValue(tag, ExifDataType.Long, isArray);
+            }
+
+            return new ExifValue(tag, ExifDataType.SignedLong, isArray);
+        }
+
         private void CheckValue(object value)
         {
             if (value == null)
@@ -639,26 +659,6 @@ namespace ImageSharp
             }
         }
 
-        private static ExifValue CreateNumber(ExifTag tag, Type type, bool isArray)
-        {
-            if (type == null || type == typeof(ushort))
-            {
-                return new ExifValue(tag, ExifDataType.Short, isArray);
-            }
-
-            if (type == typeof(short))
-            {
-                return new ExifValue(tag, ExifDataType.SignedShort, isArray);
-            }
-
-            if (type == typeof(uint))
-            {
-                return new ExifValue(tag, ExifDataType.Long, isArray);
-            }
-
-            return new ExifValue(tag, ExifDataType.SignedLong, isArray);
-        }
-
         private string ToString(object value)
         {
             string description = ExifTagDescriptionAttribute.GetDescription(this.Tag, value);

src/ImageSharp/Profiles/Exif/ExifWriter.cs

@@ -12,6 +12,8 @@ namespace ImageSharp
 
     internal sealed class ExifWriter
     {
+        private const int StartIndex = 6;
+
         private static readonly ExifTag[] IfdTags = new ExifTag[127]
         {
             ExifTag.SubfileType,
@@ -274,8 +276,6 @@ namespace ImageSharp
             ExifTag.GPSDifferential
         };
 
-        private const int StartIndex = 6;
-
         private ExifParts allowedParts;
         private Collection<ExifValue> values;
         private Collection<int> dataOffsets;
@@ -379,6 +379,13 @@ namespace ImageSharp
             return result;
         }
 
+        private static int Write(byte[] source, byte[] destination, int offset)
+        {
+            Buffer.BlockCopy(source, 0, destination, offset, source.Length);
+
+            return offset + source.Length;
+        }
+
         private int GetIndex(Collection<int> indexes, ExifTag tag)
         {
             foreach (int index in indexes)
@@ -443,13 +450,6 @@ namespace ImageSharp
             return length;
         }
 
-        private static int Write(byte[] source, byte[] destination, int offset)
-        {
-            Buffer.BlockCopy(source, 0, destination, offset, source.Length);
-
-            return offset + source.Length;
-        }
-
         private int WriteArray(ExifValue value, byte[] destination, int offset)
         {
             if (value.DataType == ExifDataType.Ascii)

src/ImageSharp/Samplers/Processors/RotateProcessor.cs

@@ -33,23 +33,6 @@ namespace ImageSharp.Processors
         /// </summary>
         public bool Expand { get; set; } = true;
 
-        /// <inheritdoc/>
-        protected override void OnApply(ImageBase<TColor, TPacked> target, ImageBase<TColor, TPacked> source, Rectangle targetRectangle, Rectangle sourceRectangle)
-        {
-            const float Epsilon = .0001F;
-
-            if (Math.Abs(this.Angle) < Epsilon || Math.Abs(this.Angle - 90) < Epsilon || Math.Abs(this.Angle - 180) < Epsilon || Math.Abs(this.Angle - 270) < Epsilon)
-            {
-                return;
-            }
-
-            this.processMatrix = Point.CreateRotation(new Point(0, 0), -this.Angle);
-            if (this.Expand)
-            {
-                CreateNewTarget(target, sourceRectangle, this.processMatrix);
-            }
-        }
-
         /// <inheritdoc/>
         public override void Apply(ImageBase<TColor, TPacked> target, ImageBase<TColor, TPacked> source, Rectangle targetRectangle, Rectangle sourceRectangle, int startY, int endY)
         {
@@ -83,6 +66,23 @@ namespace ImageSharp.Processors
             }
         }
 
+        /// <inheritdoc/>
+        protected override void OnApply(ImageBase<TColor, TPacked> target, ImageBase<TColor, TPacked> source, Rectangle targetRectangle, Rectangle sourceRectangle)
+        {
+            const float Epsilon = .0001F;
+
+            if (Math.Abs(this.Angle) < Epsilon || Math.Abs(this.Angle - 90) < Epsilon || Math.Abs(this.Angle - 180) < Epsilon || Math.Abs(this.Angle - 270) < Epsilon)
+            {
+                return;
+            }
+
+            this.processMatrix = Point.CreateRotation(new Point(0, 0), -this.Angle);
+            if (this.Expand)
+            {
+                CreateNewTarget(target, sourceRectangle, this.processMatrix);
+            }
+        }
+
         /// <summary>
         /// Rotates the images with an optimized method when the angle is 90, 180 or 270 degrees.
         /// </summary>

tests/ImageSharp.Tests/Formats/Png/PngTests.cs

@@ -8,6 +8,7 @@ using ImageSharp.Formats;
 namespace ImageSharp.Tests
 {
     using System.IO;
+    using System.Threading.Tasks;
 
     using Formats;
 
@@ -31,5 +32,23 @@ namespace ImageSharp.Tests
                 }
             }
         }
+
+        [Fact]
+        public void ImageCanSavePngInParallel()
+        {
+            string path = this.CreateOutputDirectory("Png");
+
+            Parallel.ForEach(
+                Files,
+                file =>
+                    {
+                        Image image = file.CreateImage();
+
+                        using (FileStream output = File.OpenWrite($"{path}/{file.FileNameWithoutExtension}.png"))
+                        {
+                            image.Save(output, new PngFormat());
+                        }
+                    });
+        }
     }
 }