Merge pull request #527 from SixLabors/js/faster-gif

Faster Gif Decoding

src/ImageSharp/Common/Helpers/DebugGuard.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Diagnostics;
 
+// TODO: These should just call the guard equivalents
 namespace SixLabors.ImageSharp
 {
     /// <summary>
@@ -114,6 +115,28 @@ namespace SixLabors.ImageSharp
             }
         }
 
+        /// <summary>
+        /// Verifies that the specified value is greater than or equal to a minimum value and less than
+        /// or equal to a maximum value and throws an exception if it is not.
+        /// </summary>
+        /// <param name="value">The target value, which should be validated.</param>
+        /// <param name="min">The minimum value.</param>
+        /// <param name="max">The maximum value.</param>
+        /// <param name="parameterName">The name of the parameter that is to be checked.</param>
+        /// <typeparam name="TValue">The type of the value.</typeparam>
+        /// <exception cref="ArgumentException">
+        /// <paramref name="value"/> is less than the minimum value of greater than the maximum value.
+        /// </exception>
+        [Conditional("DEBUG")]
+        public static void MustBeBetweenOrEqualTo<TValue>(TValue value, TValue min, TValue max, string parameterName)
+           where TValue : IComparable<TValue>
+        {
+            if (value.CompareTo(min) < 0 || value.CompareTo(max) > 0)
+            {
+                throw new ArgumentOutOfRangeException(parameterName, $"Value {value} must be greater than or equal to {min} and less than or equal to {max}.");
+            }
+        }
+
         /// <summary>
         /// Verifies, that the method parameter with specified target value is true
         /// and throws an exception if it is found to be so.

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -5,6 +5,7 @@ using System;
 using System.Buffers;
 using System.IO;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using System.Text;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
@@ -410,13 +411,12 @@ namespace SixLabors.ImageSharp.Formats.Gif
         private void ReadFrameColors<TPixel>(ref Image<TPixel> image, ref ImageFrame<TPixel> previousFrame, Span<byte> indices, Span<byte> colorTable, GifImageDescriptor descriptor)
             where TPixel : struct, IPixel<TPixel>
         {
+            ref byte indicesRef = ref MemoryMarshal.GetReference(indices);
             int imageWidth = this.logicalScreenDescriptor.Width;
             int imageHeight = this.logicalScreenDescriptor.Height;
 
             ImageFrame<TPixel> prevFrame = null;
-
             ImageFrame<TPixel> currentFrame = null;
-
             ImageFrame<TPixel> imageFrame;
 
             if (previousFrame == null)
@@ -479,7 +479,6 @@ namespace SixLabors.ImageSharp.Formats.Gif
                     }
 
                     writeY = interlaceY + descriptor.Top;
-
                     interlaceY += interlaceIncrement;
                 }
                 else
@@ -487,14 +486,13 @@ namespace SixLabors.ImageSharp.Formats.Gif
                     writeY = y;
                 }
 
-                Span<TPixel> rowSpan = imageFrame.GetPixelRowSpan(writeY);
-
+                ref TPixel rowRef = ref MemoryMarshal.GetReference(imageFrame.GetPixelRowSpan(writeY));
                 var rgba = new Rgba32(0, 0, 0, 255);
 
                 // #403 The left + width value can be larger than the image width
-                for (int x = descriptor.Left; x < descriptor.Left + descriptor.Width && x < rowSpan.Length; x++)
+                for (int x = descriptor.Left; x < descriptor.Left + descriptor.Width && x < imageWidth; x++)
                 {
-                    int index = indices[i];
+                    int index = Unsafe.Add(ref indicesRef, i);
 
                     if (this.graphicsControlExtension == null ||
                         this.graphicsControlExtension.TransparencyFlag == false ||
@@ -502,7 +500,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
                     {
                         int indexOffset = index * 3;
 
-                        ref TPixel pixel = ref rowSpan[x];
+                        ref TPixel pixel = ref Unsafe.Add(ref rowRef, x);
                         rgba.Rgb = colorTable.GetRgb24(indexOffset);
 
                         pixel.PackFromRgba32(rgba);

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -4,6 +4,8 @@
 using System;
 using System.IO;
 using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using System.Text;
 using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.Memory;
@@ -11,6 +13,9 @@ using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing.Quantization;
 
+// TODO: This is causing more GC collections than I'm happy with.
+// This is likely due to the number of short writes to the stream we are doing.
+// We should investigate reducing them since we know the length of the byte array we require for multiple parts.
 namespace SixLabors.ImageSharp.Formats.Gif
 {
     /// <summary>
@@ -45,11 +50,6 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </summary>
         private int bitDepth;
 
-        /// <summary>
-        /// Whether the current image has multiple frames.
-        /// </summary>
-        private bool hasFrames;
-
         /// <summary>
         /// Initializes a new instance of the <see cref="GifEncoderCore"/> class.
         /// </summary>
@@ -78,8 +78,6 @@ namespace SixLabors.ImageSharp.Formats.Gif
             // Do not use IDisposable pattern here as we want to preserve the stream.
             var writer = new EndianBinaryWriter(Endianness.LittleEndian, stream);
 
-            this.hasFrames = image.Frames.Count > 1;
-
             // Quantize the image returning a palette.
             QuantizedFrame<TPixel> quantized = this.quantizer.CreateFrameQuantizer<TPixel>().QuantizeFrame(image.Frames.RootFrame);
 
@@ -98,9 +96,9 @@ namespace SixLabors.ImageSharp.Formats.Gif
             this.WriteComments(image, writer);
 
             // Write additional frames.
-            if (this.hasFrames)
+            if (image.Frames.Count > 1)
             {
-                this.WriteApplicationExtension(writer, image.MetaData.RepeatCount, image.Frames.Count);
+                this.WriteApplicationExtension(writer, image.MetaData.RepeatCount);
             }
 
             foreach (ImageFrame<TPixel> frame in image.Frames)
@@ -138,11 +136,12 @@ namespace SixLabors.ImageSharp.Formats.Gif
             // Transparent pixels are much more likely to be found at the end of a palette
             int index = -1;
             var trans = default(Rgba32);
+            ref TPixel paletteRef = ref MemoryMarshal.GetReference(quantized.Palette.AsSpan());
             for (int i = quantized.Palette.Length - 1; i >= 0; i--)
             {
-                quantized.Palette[i].ToRgba32(ref trans);
-
-                if (trans.Equals(default(Rgba32)))
+                ref TPixel entry = ref Unsafe.Add(ref paletteRef, i);
+                entry.ToRgba32(ref trans);
+                if (trans.Equals(default))
                 {
                     index = i;
                 }
@@ -155,6 +154,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// Writes the file header signature and version to the stream.
         /// </summary>
         /// <param name="writer">The writer to write to the stream with.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private void WriteHeader(EndianBinaryWriter writer)
         {
             writer.Write(GifConstants.MagicNumber);
@@ -201,11 +201,10 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </summary>
         /// <param name="writer">The writer to write to the stream with.</param>
         /// <param name="repeatCount">The animated image repeat count.</param>
-        /// <param name="frames">The number of image frames.</param>
-        private void WriteApplicationExtension(EndianBinaryWriter writer, ushort repeatCount, int frames)
+        private void WriteApplicationExtension(EndianBinaryWriter writer, ushort repeatCount)
         {
             // Application Extension Header
-            if (repeatCount != 1 && frames > 0)
+            if (repeatCount != 1)
             {
                 this.buffer[0] = GifConstants.ExtensionIntroducer;
                 this.buffer[1] = GifConstants.ApplicationExtensionLabel;
@@ -336,15 +335,14 @@ namespace SixLabors.ImageSharp.Formats.Gif
             var rgb = default(Rgb24);
             using (IManagedByteBuffer colorTable = this.memoryManager.AllocateManagedByteBuffer(colorTableLength))
             {
-                Span<byte> colorTableSpan = colorTable.Span;
+                ref TPixel paletteRef = ref MemoryMarshal.GetReference(image.Palette.AsSpan());
+                ref Rgb24 rgb24Ref = ref Unsafe.As<byte, Rgb24>(ref MemoryMarshal.GetReference(colorTable.Span));
 
                 for (int i = 0; i < pixelCount; i++)
                 {
-                    int offset = i * 3;
-                    image.Palette[i].ToRgb24(ref rgb);
-                    colorTableSpan[offset] = rgb.R;
-                    colorTableSpan[offset + 1] = rgb.G;
-                    colorTableSpan[offset + 2] = rgb.B;
+                    ref TPixel entry = ref Unsafe.Add(ref paletteRef, i);
+                    entry.ToRgb24(ref rgb);
+                    Unsafe.Add(ref rgb24Ref, i) = rgb;
                 }
 
                 writer.Write(colorTable.Array, 0, colorTableLength);

src/ImageSharp/Formats/Gif/LzwDecoder.cs

@@ -2,9 +2,9 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-using System.Buffers;
 using System.IO;
-
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Memory;
 
 namespace SixLabors.ImageSharp.Formats.Gif
@@ -115,14 +115,14 @@ namespace SixLabors.ImageSharp.Formats.Gif
             int data = 0;
             int first = 0;
 
-            Span<int> prefixSpan = this.prefix.Span;
-            Span<int> suffixSpan = this.suffix.Span;
-            Span<int> pixelStackSpan = this.pixelStack.Span;
+            ref int prefixRef = ref MemoryMarshal.GetReference(this.prefix.Span);
+            ref int suffixRef = ref MemoryMarshal.GetReference(this.suffix.Span);
+            ref int pixelStackRef = ref MemoryMarshal.GetReference(this.pixelStack.Span);
+            ref byte pixelsRef = ref MemoryMarshal.GetReference(pixels);
 
             for (code = 0; code < clearCode; code++)
             {
-                prefixSpan[code] = 0;
-                suffixSpan[code] = (byte)code;
+                Unsafe.Add(ref suffixRef, code) = (byte)code;
             }
 
             byte[] buffer = new byte[255];
@@ -176,7 +176,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
                     if (oldCode == NullCode)
                     {
-                        pixelStackSpan[top++] = suffixSpan[code];
+                        Unsafe.Add(ref pixelStackRef, top++) = Unsafe.Add(ref suffixRef, code);
                         oldCode = code;
                         first = code;
                         continue;
@@ -185,27 +185,27 @@ namespace SixLabors.ImageSharp.Formats.Gif
                     int inCode = code;
                     if (code == availableCode)
                     {
-                        pixelStackSpan[top++] = (byte)first;
+                        Unsafe.Add(ref pixelStackRef, top++) = (byte)first;
 
                         code = oldCode;
                     }
 
                     while (code > clearCode)
                     {
-                        pixelStackSpan[top++] = suffixSpan[code];
-                        code = prefixSpan[code];
+                        Unsafe.Add(ref pixelStackRef, top++) = Unsafe.Add(ref suffixRef, code);
+                        code = Unsafe.Add(ref prefixRef, code);
                     }
 
-                    first = suffixSpan[code];
-
-                    pixelStackSpan[top++] = suffixSpan[code];
+                    int suffixCode = Unsafe.Add(ref suffixRef, code);
+                    first = suffixCode;
+                    Unsafe.Add(ref pixelStackRef, top++) = suffixCode;
 
                     // Fix for Gifs that have "deferred clear code" as per here :
                     // https://bugzilla.mozilla.org/show_bug.cgi?id=55918
                     if (availableCode < MaxStackSize)
                     {
-                        prefixSpan[availableCode] = oldCode;
-                        suffixSpan[availableCode] = first;
+                        Unsafe.Add(ref prefixRef, availableCode) = oldCode;
+                        Unsafe.Add(ref suffixRef, availableCode) = first;
                         availableCode++;
                         if (availableCode == codeMask + 1 && availableCode < MaxStackSize)
                         {
@@ -221,7 +221,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 top--;
 
                 // Clear missing pixels
-                pixels[xyz++] = (byte)pixelStackSpan[top];
+                Unsafe.Add(ref pixelsRef, xyz++) = (byte)Unsafe.Add(ref pixelStackRef, top);
             }
         }
 
@@ -238,8 +238,9 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </summary>
         /// <param name="buffer">The buffer to store the block in.</param>
         /// <returns>
-        /// The <see cref="T:byte[]"/>.
+        /// The <see cref="int"/>.
         /// </returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private int ReadBlock(byte[] buffer)
         {
             int bufferSize = this.stream.ReadByte();

src/ImageSharp/Formats/Gif/LzwEncoder.cs

@@ -2,9 +2,9 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-using System.Buffers;
 using System.IO;
-
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Memory;
 
 namespace SixLabors.ImageSharp.Formats.Gif
@@ -233,6 +233,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </summary>
         /// <param name="bitCount">The number of bits</param>
         /// <returns>See <see cref="int"/></returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private static int GetMaxcode(int bitCount)
         {
             return (1 << bitCount) - 1;
@@ -243,10 +244,12 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// flush the packet to disk.
         /// </summary>
         /// <param name="c">The character to add.</param>
+        /// <param name="accumulatorsRef">The reference to the storage for packat accumulators</param>
         /// <param name="stream">The stream to write to.</param>
-        private void AddCharacter(byte c, Stream stream)
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void AddCharacter(byte c, ref byte accumulatorsRef, Stream stream)
         {
-            this.accumulators[this.accumulatorCount++] = c;
+            Unsafe.Add(ref accumulatorsRef, this.accumulatorCount++) = c;
             if (this.accumulatorCount >= 254)
             {
                 this.FlushPacket(stream);
@@ -257,6 +260,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         ///  Table clear for block compress
         /// </summary>
         /// <param name="stream">The output stream.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private void ClearBlock(Stream stream)
         {
             this.ResetCodeTable();
@@ -269,15 +273,10 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// <summary>
         /// Reset the code table.
         /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private void ResetCodeTable()
         {
             this.hashTable.Span.Fill(-1);
-
-            // Original code:
-            // for (int i = 0; i < size; ++i)
-            // {
-            //     this.hashTable[i] = -1;
-            // }
         }
 
         /// <summary>
@@ -309,6 +308,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
             ent = this.NextPixel();
 
+            // TODO: PERF: It looks likt hshift could be calculated once statically.
             hshift = 0;
             for (fcode = this.hsize; fcode < 65536; fcode *= 2)
             {
@@ -323,22 +323,22 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
             this.Output(this.clearCode, stream);
 
-            Span<int> hashTableSpan = this.hashTable.Span;
-            Span<int> codeTableSpan = this.codeTable.Span;
+            ref int hashTableRef = ref MemoryMarshal.GetReference(this.hashTable.Span);
+            ref int codeTableRef = ref MemoryMarshal.GetReference(this.codeTable.Span);
 
             while ((c = this.NextPixel()) != Eof)
             {
                 fcode = (c << this.maxbits) + ent;
                 int i = (c << hshift) ^ ent /* = 0 */;
 
-                if (hashTableSpan[i] == fcode)
+                if (Unsafe.Add(ref hashTableRef, i) == fcode)
                 {
-                    ent = codeTableSpan[i];
+                    ent = Unsafe.Add(ref codeTableRef, i);
                     continue;
                 }
 
                 // Non-empty slot
-                if (hashTableSpan[i] >= 0)
+                if (Unsafe.Add(ref hashTableRef, i) >= 0)
                 {
                     int disp = hsizeReg - i;
                     if (i == 0)
@@ -353,15 +353,15 @@ namespace SixLabors.ImageSharp.Formats.Gif
                             i += hsizeReg;
                         }
 
-                        if (hashTableSpan[i] == fcode)
+                        if (Unsafe.Add(ref hashTableRef, i) == fcode)
                         {
-                            ent = codeTableSpan[i];
+                            ent = Unsafe.Add(ref codeTableRef, i);
                             break;
                         }
                     }
-                    while (hashTableSpan[i] >= 0);
+                    while (Unsafe.Add(ref hashTableRef, i) >= 0);
 
-                    if (hashTableSpan[i] == fcode)
+                    if (Unsafe.Add(ref hashTableRef, i) == fcode)
                     {
                         continue;
                     }
@@ -371,8 +371,8 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 ent = c;
                 if (this.freeEntry < this.maxmaxcode)
                 {
-                    codeTableSpan[i] = this.freeEntry++; // code -> hashtable
-                    hashTableSpan[i] = fcode;
+                    Unsafe.Add(ref codeTableRef, i) = this.freeEntry++; // code -> hashtable
+                    Unsafe.Add(ref hashTableRef, i) = fcode;
                 }
                 else
                 {
@@ -390,14 +390,12 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// Flush the packet to disk, and reset the accumulator.
         /// </summary>
         /// <param name="outStream">The output stream.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private void FlushPacket(Stream outStream)
         {
-            if (this.accumulatorCount > 0)
-            {
-                outStream.WriteByte((byte)this.accumulatorCount);
-                outStream.Write(this.accumulators, 0, this.accumulatorCount);
-                this.accumulatorCount = 0;
-            }
+            outStream.WriteByte((byte)this.accumulatorCount);
+            outStream.Write(this.accumulators, 0, this.accumulatorCount);
+            this.accumulatorCount = 0;
         }
 
         /// <summary>
@@ -424,6 +422,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// <param name="outs">The stream to write to.</param>
         private void Output(int code, Stream outs)
         {
+            ref byte accumulatorsRef = ref MemoryMarshal.GetReference(this.accumulators.AsSpan());
             this.currentAccumulator &= Masks[this.currentBits];
 
             if (this.currentBits > 0)
@@ -439,7 +438,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
             while (this.currentBits >= 8)
             {
-                this.AddCharacter((byte)(this.currentAccumulator & 0xff), outs);
+                this.AddCharacter((byte)(this.currentAccumulator & 0xFF), ref accumulatorsRef, outs);
                 this.currentAccumulator >>= 8;
                 this.currentBits -= 8;
             }
@@ -467,12 +466,15 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 // At EOF, write the rest of the buffer.
                 while (this.currentBits > 0)
                 {
-                    this.AddCharacter((byte)(this.currentAccumulator & 0xff), outs);
+                    this.AddCharacter((byte)(this.currentAccumulator & 0xFF), ref accumulatorsRef, outs);
                     this.currentAccumulator >>= 8;
                     this.currentBits -= 8;
                 }
 
-                this.FlushPacket(outs);
+                if (this.accumulatorCount > 0)
+                {
+                    this.FlushPacket(outs);
+                }
             }
         }
 

src/ImageSharp/Formats/Gif/PackedField.cs

@@ -2,6 +2,8 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
 
 namespace SixLabors.ImageSharp.Formats.Gif
 {
@@ -21,6 +23,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </summary>
         public byte Byte
         {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get
             {
                 int returnValue = 0;
@@ -53,7 +56,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// <returns>The <see cref="PackedField"/></returns>
         public static PackedField FromInt(byte value)
         {
-            PackedField packed = default(PackedField);
+            PackedField packed = default;
             packed.SetBits(0, 8, value);
             return packed;
         }
@@ -70,12 +73,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </param>
         public void SetBit(int index, bool valueToSet)
         {
-            if (index < 0 || index > 7)
-            {
-                string message = $"Index must be between 0 and 7. Supplied index: {index}";
-                throw new ArgumentOutOfRangeException(nameof(index), message);
-            }
-
+            DebugGuard.MustBeBetweenOrEqualTo(index, 0, 7, nameof(index));
             Bits[index] = valueToSet;
         }
 
@@ -88,18 +86,8 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// <param name="valueToSet">The value to set the bits to.</param>
         public void SetBits(int startIndex, int length, int valueToSet)
         {
-            if (startIndex < 0 || startIndex > 7)
-            {
-                string message = $"Start index must be between 0 and 7. Supplied index: {startIndex}";
-                throw new ArgumentOutOfRangeException(nameof(startIndex), message);
-            }
-
-            if (length < 1 || startIndex + length > 8)
-            {
-                string message = "Length must be greater than zero and the sum of length and start index must be less than 8. "
-                                 + $"Supplied length: {length}. Supplied start index: {startIndex}";
-                throw new ArgumentOutOfRangeException(nameof(length), message);
-            }
+            DebugGuard.MustBeBetweenOrEqualTo(startIndex, 0, 7, nameof(startIndex));
+            DebugCheckLength(startIndex, length);
 
             int bitShift = length - 1;
             for (int i = startIndex; i < startIndex + length; i++)
@@ -121,12 +109,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </returns>
         public bool GetBit(int index)
         {
-            if (index < 0 || index > 7)
-            {
-                string message = $"Index must be between 0 and 7. Supplied index: {index}";
-                throw new ArgumentOutOfRangeException(nameof(index), message);
-            }
-
+            DebugGuard.MustBeBetweenOrEqualTo(index, 0, 7, nameof(index));
             return Bits[index];
         }
 
@@ -140,19 +123,8 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </returns>
         public int GetBits(int startIndex, int length)
         {
-            if (startIndex < 0 || startIndex > 7)
-            {
-                string message = $"Start index must be between 0 and 7. Supplied index: {startIndex}";
-                throw new ArgumentOutOfRangeException(nameof(startIndex), message);
-            }
-
-            if (length < 1 || startIndex + length > 8)
-            {
-                string message = "Length must be greater than zero and the sum of length and start index must be less than 8. "
-                                 + $"Supplied length: {length}. Supplied start index: {startIndex}";
-
-                throw new ArgumentOutOfRangeException(nameof(length), message);
-            }
+            DebugGuard.MustBeBetweenOrEqualTo(startIndex, 1, 8, nameof(startIndex));
+            DebugCheckLength(startIndex, length);
 
             int returnValue = 0;
             int bitShift = length - 1;
@@ -189,5 +161,17 @@ namespace SixLabors.ImageSharp.Formats.Gif
         {
             return this.Byte.GetHashCode();
         }
+
+        [Conditional("DEBUG")]
+        private static void DebugCheckLength(int startIndex, int length)
+        {
+            if (length < 1 || startIndex + length > 8)
+            {
+                string message = "Length must be greater than zero and the sum of length and start index must be less than 8. "
+                                 + $"Supplied length: {length}. Supplied start index: {startIndex}";
+
+                throw new ArgumentOutOfRangeException(nameof(length), message);
+            }
+        }
     }
 }

src/ImageSharp/Processing/Quantization/FrameQuantizers/PaletteFrameQuantizer{TPixel}.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.PixelFormats;
 
@@ -45,18 +46,18 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
             TPixel sourcePixel = source[0, 0];
             TPixel previousPixel = sourcePixel;
             byte pixelValue = this.QuantizePixel(sourcePixel);
-            TPixel[] colorPalette = this.GetPalette();
-            TPixel transformedPixel = colorPalette[pixelValue];
+            ref TPixel colorPaletteRef = ref MemoryMarshal.GetReference(this.GetPalette().AsSpan());
+            TPixel transformedPixel = Unsafe.Add(ref colorPaletteRef, pixelValue);
 
             for (int y = 0; y < height; y++)
             {
-                Span<TPixel> row = source.GetPixelRowSpan(y);
+                ref TPixel rowRef = ref MemoryMarshal.GetReference(source.GetPixelRowSpan(y));
 
                 // And loop through each column
                 for (int x = 0; x < width; x++)
                 {
                     // Get the pixel.
-                    sourcePixel = row[x];
+                    sourcePixel = Unsafe.Add(ref rowRef, x);
 
                     // Check if this is the same as the last pixel. If so use that value
                     // rather than calculating it again. This is an inexpensive optimization.
@@ -70,7 +71,7 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
 
                         if (this.Dither)
                         {
-                            transformedPixel = colorPalette[pixelValue];
+                            transformedPixel = Unsafe.Add(ref colorPaletteRef, pixelValue);
                         }
                     }
 
@@ -86,6 +87,7 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
         }
 
         /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         protected override TPixel[] GetPalette()
         {
             return this.colors;

tests/ImageSharp.Tests/Formats/Gif/GifEncoderTests.cs

@@ -2,10 +2,11 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.IO;
-using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Gif;
 using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Quantization;
+using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 using Xunit;
 // ReSharper disable InconsistentNaming
 
@@ -14,6 +15,7 @@ namespace SixLabors.ImageSharp.Tests
     public class GifEncoderTests
     {
         private const PixelTypes TestPixelTypes = PixelTypes.Rgba32 | PixelTypes.RgbaVector | PixelTypes.Argb32;
+        private static readonly ImageComparer ValidatorComparer = ImageComparer.TolerantPercentage(0.001F);
 
         [Theory]
         [WithTestPatternImages(100, 100, TestPixelTypes)]
@@ -22,28 +24,43 @@ namespace SixLabors.ImageSharp.Tests
         {
             using (Image<TPixel> image = provider.GetImage())
             {
-                provider.Utility.SaveTestOutputFile(image, "gif", new GifEncoder());
+                var encoder = new GifEncoder()
+                {
+                    // Use the palette quantizer without dithering to ensure results 
+                    // are consistant
+                    Quantizer = new PaletteQuantizer(false)
+                };
+
+                // Always save as we need to compare the encoded output.
+                provider.Utility.SaveTestOutputFile(image, "gif", encoder);
+            }
+
+            // Compare encoded result
+            string path = provider.Utility.GetTestOutputFileName("gif", null, true);
+            using (var encoded = Image.Load(path))
+            {
+                encoded.CompareToReferenceOutput(ValidatorComparer, provider, null, "gif");
             }
         }
 
         [Fact]
         public void Encode_IgnoreMetadataIsFalse_CommentsAreWritten()
         {
-            GifEncoder options = new GifEncoder()
+            var options = new GifEncoder()
             {
                 IgnoreMetadata = false
             };
 
-            TestFile testFile = TestFile.Create(TestImages.Gif.Rings);
+            var testFile = TestFile.Create(TestImages.Gif.Rings);
 
             using (Image<Rgba32> input = testFile.CreateImage())
             {
-                using (MemoryStream memStream = new MemoryStream())
+                using (var memStream = new MemoryStream())
                 {
                     input.Save(memStream, options);
 
                     memStream.Position = 0;
-                    using (Image<Rgba32> output = Image.Load<Rgba32>(memStream))
+                    using (var output = Image.Load<Rgba32>(memStream))
                     {
                         Assert.Equal(1, output.MetaData.Properties.Count);
                         Assert.Equal("Comments", output.MetaData.Properties[0].Name);
@@ -56,21 +73,21 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void Encode_IgnoreMetadataIsTrue_CommentsAreNotWritten()
         {
-            GifEncoder options = new GifEncoder()
+            var options = new GifEncoder()
             {
                 IgnoreMetadata = true
             };
 
-            TestFile testFile = TestFile.Create(TestImages.Gif.Rings);
+            var testFile = TestFile.Create(TestImages.Gif.Rings);
 
             using (Image<Rgba32> input = testFile.CreateImage())
             {
-                using (MemoryStream memStream = new MemoryStream())
+                using (var memStream = new MemoryStream())
                 {
                     input.SaveAsGif(memStream, options);
 
                     memStream.Position = 0;
-                    using (Image<Rgba32> output = Image.Load<Rgba32>(memStream))
+                    using (var output = Image.Load<Rgba32>(memStream))
                     {
                         Assert.Equal(0, output.MetaData.Properties.Count);
                     }
@@ -81,17 +98,17 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void Encode_WhenCommentIsTooLong_CommentIsTrimmed()
         {
-            using (Image<Rgba32> input = new Image<Rgba32>(1, 1))
+            using (var input = new Image<Rgba32>(1, 1))
             {
                 string comments = new string('c', 256);
                 input.MetaData.Properties.Add(new ImageProperty("Comments", comments));
 
-                using (MemoryStream memStream = new MemoryStream())
+                using (var memStream = new MemoryStream())
                 {
                     input.Save(memStream, new GifEncoder());
 
                     memStream.Position = 0;
-                    using (Image<Rgba32> output = Image.Load<Rgba32>(memStream))
+                    using (var output = Image.Load<Rgba32>(memStream))
                     {
                         Assert.Equal(1, output.MetaData.Properties.Count);
                         Assert.Equal("Comments", output.MetaData.Properties[0].Name);

tests/ImageSharp.Tests/TestUtilities/TestImageExtensions.cs

@@ -87,6 +87,37 @@ namespace SixLabors.ImageSharp.Tests
             return image;
         }
 
+        /// <summary>
+        /// Saves the image only when not running in the CI server.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format</typeparam>
+        /// <param name="image">The image</param>
+        /// <param name="provider">The image provider</param>
+        /// <param name="encoder">The image encoder</param>
+        /// <param name="testOutputDetails">Details to be concatenated to the test output file, describing the parameters of the test.</param>
+        /// <param name="appendPixelTypeToFileName">A boolean indicating whether to append the pixel type to the  output file name.</param>
+        public static Image<TPixel> DebugSave<TPixel>(
+            this Image<TPixel> image,
+            ITestImageProvider provider,
+            IImageEncoder encoder,
+            object testOutputDetails = null,
+            bool appendPixelTypeToFileName = true)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            if (TestEnvironment.RunsOnCI)
+            {
+                return image;
+            }
+
+            // We are running locally then we want to save it out
+            provider.Utility.SaveTestOutputFile(
+                image,
+                encoder: encoder,
+                testOutputDetails: testOutputDetails,
+                appendPixelTypeToFileName: appendPixelTypeToFileName);
+            return image;
+        }
+
         public static Image<TPixel> DebugSaveMultiFrame<TPixel>(
             this Image<TPixel> image,
             ITestImageProvider provider,
@@ -168,7 +199,7 @@ namespace SixLabors.ImageSharp.Tests
                 provider,
                 testOutputDetails,
                 extension,
-                appendPixelTypeToFileName)) 
+                appendPixelTypeToFileName))
             {
                 comparer.VerifySimilarity(referenceImage, image);
             }
@@ -272,7 +303,7 @@ namespace SixLabors.ImageSharp.Tests
             }
 
             Image<TPixel> firstTemp = temporaryFrameImages[0];
-            
+
             var result = new Image<TPixel>(firstTemp.Width, firstTemp.Height);
 
             foreach (Image<TPixel> fi in temporaryFrameImages)
@@ -345,7 +376,7 @@ namespace SixLabors.ImageSharp.Tests
         {
             return CompareToOriginal(image, provider, ImageComparer.Tolerant());
         }
-        
+
         public static Image<TPixel> CompareToOriginal<TPixel>(
             this Image<TPixel> image,
             ITestImageProvider provider,

tests/Images/External

@@ -1 +1 @@
-Subproject commit 5a66c9c6da02bf27345f90adc05d415c0d0450ea
+Subproject commit 01af5f36912ec7080cae3187a48905d1e54f6ea7