Add dominant pixel encoding.

Add functionality as described at
https://manu.ninja/dominant-colors-for-lazy-loading-images

Former-commit-id: 6877d52aec61a5cff850b8fc52b98fe4328db115
Former-commit-id: 231b6016c32e434d9699fd01b90662257c7edb59
Former-commit-id: 504ae4d40e60270d85797b646b659becd4303500

src/ImageProcessorCore/ImageExtensions.cs

@@ -10,6 +10,7 @@ namespace ImageProcessorCore
 
     using Formats;
 
+    using ImageProcessorCore.Quantizers;
     using ImageProcessorCore.Samplers;
 
     /// <summary>
@@ -51,6 +52,53 @@ namespace ImageProcessorCore
         /// <exception cref="ArgumentNullException">Thrown if the stream is null.</exception>
         public static void SaveAsGif(this ImageBase source, Stream stream, int quality = 256) => new GifEncoder() { Quality = quality }.Encode(source, stream);
 
+        /// <summary>
+        /// Returns a 1x1 pixel Base64 encoded gif from the given image containing a single dominant color. 
+        /// </summary>
+        /// <remarks>
+        /// The idea and code is based on the article at <see href="https://manu.ninja/dominant-colors-for-lazy-loading-images"/>
+        /// </remarks>
+        /// <param name="source">The image this method extends.</param>
+        /// <returns>The <see cref="string"/></returns>
+        public static string ToBase64GifPixelString(this Image source)
+        {
+            // Leave the original image intact
+            using (Image temp = new Image(source))
+            {
+                Bgra32 color = new OctreeQuantizer().Quantize(temp.Resize(250, 250), 1).Palette[0];
+
+                byte[] gif = {
+                0x47, 0x49, 0x46, 0x38, 0x39, 0x61, // Header
+                0x01, 0x00, 0x01, 0x00, 0x80, 0x01, 0x00, // Logical Screen Descriptor
+                color.R, color.G, color.B, 0x00, 0x00, 0x00, // Global Color Table
+                0x2C, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, // Image Descriptor
+                0x02, 0x02, 0x44, 0x01, 0x00 // Image Data
+               };
+
+                return "data:image/gif;base64," + Convert.ToBase64String(gif);
+            }
+        }
+
+        /// <summary>
+        /// Returns a 3x3 pixel Base64 encoded gif from the given image. 
+        /// </summary>
+        /// <remarks>
+        /// The idea and code is based on the article at <see href="https://manu.ninja/dominant-colors-for-lazy-loading-images"/>
+        /// </remarks>
+        /// <param name="source">The image this method extends.</param>
+        /// <returns>The <see cref="string"/></returns>
+        public static string ToBase64GifString(this Image source)
+        {
+            // Leave the original image intact
+            using (Image temp = new Image(source))
+            using (MemoryStream stream = new MemoryStream())
+            {
+                temp.Resize(3, 3).SaveAsGif(stream);
+                stream.Flush();
+                return "data:image/gif;base64," + Convert.ToBase64String(stream.ToArray());
+            }
+        }
+
         /// <summary>
         /// Applies the collection of processors to the image.
         /// <remarks>This method does not resize the target image.</remarks>

tests/ImageProcessorCore.Tests/Formats/EncoderDecoderTests.cs

@@ -17,6 +17,32 @@ namespace ImageProcessorCore.Tests
 
     public class EncoderDecoderTests : FileTestBase
     {
+        [Fact]
+        public void ImageCanEncodeToDominantPixel()
+        {
+            if (!Directory.Exists("TestOutput/Dominant"))
+            {
+                Directory.CreateDirectory("TestOutput/Dominant");
+            }
+
+            foreach (string file in Files)
+            {
+                using (FileStream stream = File.OpenRead(file))
+                {
+                    Stopwatch watch = Stopwatch.StartNew();
+                    using (Image image = new Image(stream))
+                    {
+                        string filename = "TestOutput/Dominant/" + Path.GetFileNameWithoutExtension(file) + ".txt";
+                        string filename2 = "TestOutput/Dominant/Pixel-" + Path.GetFileNameWithoutExtension(file) + ".txt";
+                        File.WriteAllText(filename, image.ToBase64GifString());
+                        File.WriteAllText(filename2, image.ToBase64GifPixelString());
+                    }
+
+                    Trace.WriteLine($"{watch.ElapsedMilliseconds}ms");
+                }
+            }
+        }
+
         [Fact]
         public void DecodeThenEncodeImageFromStreamShouldSucceed()
         {