Optimize x-bit scanline packing.

src/ImageSharp/Formats/Png/IPngEncoderOptions.cs

@@ -24,7 +24,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <summary>
         /// Gets the filter method.
         /// </summary>
-        PngFilterMethod FilterMethod { get; }
+        PngFilterMethod? FilterMethod { get; }
 
         /// <summary>
         /// Gets the compression level 1-9.

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -396,18 +396,18 @@ namespace SixLabors.ImageSharp.Formats.Png
             }
 
             buffer = this.memoryAllocator.AllocateManagedByteBuffer(bytesPerScanline * 8 / bits, AllocationOptions.Clean);
-            byte[] result = buffer.Array;
+            ref byte sourceRef = ref MemoryMarshal.GetReference(source);
+            ref byte resultRef = ref buffer.Array[0];
             int mask = 0xFF >> (8 - bits);
             int resultOffset = 0;
 
             for (int i = 0; i < bytesPerScanline; i++)
             {
-                byte b = source[i];
+                byte b = Unsafe.Add(ref sourceRef, i);
                 for (int shift = 0; shift < 8; shift += bits)
                 {
                     int colorIndex = (b >> (8 - bits - shift)) & mask;
-                    result[resultOffset] = (byte)colorIndex;
-
+                    Unsafe.Add(ref resultRef, resultOffset) = (byte)colorIndex;
                     resultOffset++;
                 }
             }

src/ImageSharp/Formats/Png/PngEncoder.cs

@@ -27,7 +27,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <summary>
         /// Gets or sets the filter method.
         /// </summary>
-        public PngFilterMethod FilterMethod { get; set; } = PngFilterMethod.Paeth;
+        public PngFilterMethod? FilterMethod { get; set; }
 
         /// <summary>
         /// Gets or sets the compression level 1-9.

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -4,6 +4,8 @@
 using System;
 using System.Buffers.Binary;
 using System.IO;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Formats.Png.Filters;
@@ -162,8 +164,10 @@ namespace SixLabors.ImageSharp.Formats.Png
             this.pngBitDepth = options.BitDepth;
             this.pngColorType = options.ColorType;
 
-            // Palette compresses better with none and spec recommends it.
-            this.pngFilterMethod = options.ColorType.Equals(PngColorType.Palette) ? PngFilterMethod.None : options.FilterMethod;
+            // Specification recommends default filter method None for paletted images and Paeth for others.
+            this.pngFilterMethod = options.FilterMethod ?? (options.ColorType.Equals(PngColorType.Palette)
+                ? PngFilterMethod.None
+                : PngFilterMethod.Paeth);
             this.compressionLevel = options.CompressionLevel;
             this.gamma = options.Gamma;
             this.quantizer = options.Quantizer;
@@ -212,9 +216,11 @@ namespace SixLabors.ImageSharp.Formats.Png
                 // Create quantized frame returning the palette and set the bit depth.
                 quantized = this.quantizer.CreateFrameQuantizer<TPixel>().QuantizeFrame(image.Frames.RootFrame);
                 byte quantizedBits = (byte)ImageMaths.GetBitsNeededForColorDepth(quantized.Palette.Length).Clamp(1, 8);
+                bits = Math.Max(bits, quantizedBits);
 
                 // Png only supports in four pixel depths: 1, 2, 4, and 8 bits when using the PLTE chunk
-                bits = Math.Max(bits, quantizedBits);
+                // We check again for the bit depth as the bit depth of the color palette from a given quantizer might not
+                // be within the acceptable range.
                 if (bits == 3)
                 {
                     bits = 4;
@@ -228,6 +234,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             }
             else
             {
+                // TODO: Get the correct bit depth for grayscale images.
                 this.bitDepth = (byte)(this.use16Bit ? 16 : 8);
             }
 
@@ -416,13 +423,13 @@ namespace SixLabors.ImageSharp.Formats.Png
             {
                 case PngColorType.Palette:
 
-                    int stride = this.rawScanline.Length();
                     if (this.bitDepth < 8)
                     {
                         this.ScaleDownFrom8BitArray(quantized.GetRowSpan(row), this.rawScanline.GetSpan(), this.bitDepth);
                     }
                     else
                     {
+                        int stride = this.rawScanline.Length();
                         quantized.GetPixelSpan().Slice(row * stride, stride).CopyTo(this.rawScanline.GetSpan());
                     }
 
@@ -841,12 +848,16 @@ namespace SixLabors.ImageSharp.Formats.Png
             stream.Write(this.buffer, 0, 4); // write the crc
         }
 
+        /// <summary>
+        /// Packs the given 8 bit array into and array of <paramref name="bits"/> depths.
+        /// </summary>
+        /// <param name="source">The source span in 8 bits.</param>
+        /// <param name="result">The resultant span in <paramref name="bits"/>.</param>
+        /// <param name="bits">The bit depth.</param>
         private void ScaleDownFrom8BitArray(ReadOnlySpan<byte> source, Span<byte> result, int bits)
         {
-            if (bits >= 8)
-            {
-                return;
-            }
+            ref byte sourceRef = ref MemoryMarshal.GetReference(source);
+            ref byte resultRef = ref MemoryMarshal.GetReference(result);
 
             byte mask = (byte)(0xFF >> (8 - bits));
             byte shift0 = (byte)(8 - bits);
@@ -856,13 +867,13 @@ namespace SixLabors.ImageSharp.Formats.Png
 
             for (int i = 0; i < source.Length; i++)
             {
-                int value = source[i] & mask;
+                int value = Unsafe.Add(ref sourceRef, i) & mask;
                 v |= value << shift;
 
                 if (shift == 0)
                 {
                     shift = shift0;
-                    result[resultOffset] = (byte)v;
+                    Unsafe.Add(ref resultRef, resultOffset) = (byte)v;
                     resultOffset++;
                     v = 0;
                 }
@@ -874,7 +885,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
             if (shift != shift0)
             {
-                result[resultOffset] = (byte)v;
+                Unsafe.Add(ref resultRef, resultOffset) = (byte)v;
             }
         }
 

src/ImageSharp/Processing/Processors/Quantization/QuantizedFrame{TPixel}.cs

@@ -3,7 +3,7 @@
 
 using System;
 using System.Buffers;
-
+using System.Runtime.CompilerServices;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Memory;
@@ -57,6 +57,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         /// Gets the pixels of this <see cref="QuantizedFrame{TPixel}"/>.
         /// </summary>
         /// <returns>The <see cref="Span{T}"/></returns>
+        [MethodImpl(InliningOptions.ShortMethod)]
         public Span<byte> GetPixelSpan() => this.pixels.GetSpan();
 
         /// <summary>
@@ -65,6 +66,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         /// </summary>
         /// <param name="rowIndex">The row.</param>
         /// <returns>The <see cref="Span{T}"/></returns>
+        [MethodImpl(InliningOptions.ShortMethod)]
         public Span<byte> GetRowSpan(int rowIndex) => this.GetPixelSpan().Slice(rowIndex * this.Width, this.Width);
 
         /// <inheritdoc/>