Reduce png encoder memory usage by 3/4

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

@@ -47,13 +47,12 @@ 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 +67,6 @@ 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

@@ -47,13 +47,12 @@ 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 +68,6 @@ namespace ImageSharp.Formats
                     res[x + 1] = (byte)((scan[x] - PaethPredicator(left, above, upperLeft)) % 256);
                 }
             }
-
-            return result;
         }
 
         /// <summary>

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

@@ -21,7 +21,6 @@ namespace ImageSharp.Formats
         public static byte[] Decode(byte[] scanline, int bytesPerPixel)
         {
             // Sub(x) + Raw(x-bpp)
-
             fixed (byte* scan = scanline)
             {
                 for (int x = 1; x < scanline.Length; x++)
@@ -38,13 +37,12 @@ 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="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 +55,6 @@ namespace ImageSharp.Formats
                     res[x + 1] = (byte)((scan[x] - priorRawByte) % 256);
                 }
             }
-
-            return result;
         }
     }
 }

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

@@ -21,7 +21,6 @@ namespace ImageSharp.Formats
         public static byte[] Decode(byte[] scanline, byte[] previousScanline)
         {
             // Up(x) + Prior(x)
-
             fixed (byte* scan = scanline)
             fixed (byte* prev = previousScanline)
             {
@@ -41,12 +40,12 @@ 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="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 +59,6 @@ namespace ImageSharp.Formats
                     res[x + 1] = (byte)((scan[x] - above) % 256);
                 }
             }
-
-            return result;
         }
     }
 }

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>
@@ -312,9 +314,10 @@ 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 +335,7 @@ namespace ImageSharp.Formats
                     break;
             }
 
-            return this.GetOptimalFilteredScanline(rawScanline, previousScanline, bytesPerScanline);
+            return this.GetOptimalFilteredScanline(rawScanline, previousScanline, result, bytesPerScanline);
         }
 
         /// <summary>
@@ -341,45 +344,66 @@ 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];
+            byte[] sub = ArrayPool<byte>.Shared.Rent(bytesPerScanline + 1);
+            byte[] up = ArrayPool<byte>.Shared.Rent(bytesPerScanline + 1);
+            byte[] average = ArrayPool<byte>.Shared.Rent(bytesPerScanline + 1);
+            byte[] paeth = ArrayPool<byte>.Shared.Rent(bytesPerScanline + 1);
 
-            byte[] sub = SubFilter.Encode(rawScanline, this.bytesPerPixel, bytesPerScanline);
-            candidates[0] = new Tuple<byte[], int>(sub, this.CalculateTotalVariation(sub, bytesPerScanline));
+            try
+            {
+                SubFilter.Encode(rawScanline, sub, this.bytesPerPixel, bytesPerScanline);
+                int currentTotalVariation = this.CalculateTotalVariation(sub, bytesPerScanline);
+                int lowestTotalVariation = currentTotalVariation;
 
-            byte[] up = UpFilter.Encode(rawScanline, previousScanline, bytesPerScanline);
-            candidates[1] = new Tuple<byte[], int>(up, this.CalculateTotalVariation(up, bytesPerScanline));
+                result = sub;
 
-            byte[] average = AverageFilter.Encode(rawScanline, previousScanline, this.bytesPerPixel, bytesPerScanline);
-            candidates[2] = new Tuple<byte[], int>(average, this.CalculateTotalVariation(average, bytesPerScanline));
+                UpFilter.Encode(rawScanline, previousScanline, up, bytesPerScanline);
+                currentTotalVariation = this.CalculateTotalVariation(up, bytesPerScanline);
 
-            byte[] paeth = PaethFilter.Encode(rawScanline, previousScanline, this.bytesPerPixel, bytesPerScanline);
-            candidates[3] = new Tuple<byte[], int>(paeth, this.CalculateTotalVariation(paeth, bytesPerScanline));
+                if (currentTotalVariation < lowestTotalVariation)
+                {
+                    lowestTotalVariation = currentTotalVariation;
+                    result = up;
+                }
 
-            int lowestTotalVariation = int.MaxValue;
-            int lowestTotalVariationIndex = 0;
+                AverageFilter.Encode(rawScanline, previousScanline, average, this.bytesPerPixel, bytesPerScanline);
+                currentTotalVariation = this.CalculateTotalVariation(average, bytesPerScanline);
 
-            for (int i = 0; i < candidates.Length; i++)
-            {
-                if (candidates[i].Item2 < lowestTotalVariation)
+                if (currentTotalVariation < lowestTotalVariation)
                 {
-                    lowestTotalVariationIndex = i;
-                    lowestTotalVariation = candidates[i].Item2;
+                    lowestTotalVariation = currentTotalVariation;
+                    result = average;
                 }
-            }
 
-            // ReSharper disable once RedundantAssignment
-            return candidates[lowestTotalVariationIndex].Item1;
+                PaethFilter.Encode(rawScanline, previousScanline, paeth, this.bytesPerPixel, bytesPerScanline);
+                currentTotalVariation = this.CalculateTotalVariation(paeth, bytesPerScanline);
+
+                if (currentTotalVariation < lowestTotalVariation)
+                {
+                    result = paeth;
+                }
+
+                return result;
+            }
+            finally
+            {
+                ArrayPool<byte>.Shared.Return(sub);
+                ArrayPool<byte>.Shared.Return(up);
+                ArrayPool<byte>.Shared.Return(average);
+                ArrayPool<byte>.Shared.Return(paeth);
+            }
         }
 
         /// <summary>
@@ -592,6 +616,7 @@ 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);
 
             byte[] buffer;
             int bufferLength;
@@ -603,12 +628,9 @@ 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 +642,7 @@ namespace ImageSharp.Formats
                 memoryStream?.Dispose();
                 ArrayPool<byte>.Shared.Return(previousScanline);
                 ArrayPool<byte>.Shared.Return(rawScanline);
+                ArrayPool<byte>.Shared.Return(result);
             }
 
             // Store the chunks in repeated 64k blocks.