Ensure pixel is assigned and add encoded comparison and

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -342,7 +342,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 {
                     ref TPixel entry = ref Unsafe.Add(ref paletteRef, i);
                     entry.ToRgb24(ref rgb);
-                    Unsafe.Add(ref rgb24Ref, i);
+                    Unsafe.Add(ref rgb24Ref, i) = rgb;
                 }
 
                 writer.Write(colorTable.Array, 0, colorTableLength);

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

@@ -2,11 +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
 
@@ -15,6 +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)]
@@ -23,7 +24,22 @@ 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");
             }
         }
 

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,

tests/Images/External

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