Add JpegColorType to JpegMetadata and use it to configure the JpegEncoder

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -1038,6 +1038,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                 this.Frame.ComponentOrder[i] = (byte)componentIndex;
             }
 
+            this.Metadata.GetJpegMetadata().ColorType = (selectorsCount == 1) ? JpegColorType.Luminance : JpegColorType.YCbCr;
+
             stream.Read(this.temp, 0, 3);
 
             int spectralStart = this.temp[0];

src/ImageSharp/Formats/Jpeg/JpegEncoder.cs

@@ -40,7 +40,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         where TPixel : unmanaged, IPixel<TPixel>
         {
             var encoder = new JpegEncoderCore(this);
-            this.InitializeColorType<TPixel>();
+            this.InitializeColorType<TPixel>(image);
             encoder.Encode(image, stream);
         }
 
@@ -56,16 +56,24 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             where TPixel : unmanaged, IPixel<TPixel>
         {
             var encoder = new JpegEncoderCore(this);
-            this.InitializeColorType<TPixel>();
+            this.InitializeColorType<TPixel>(image);
             return encoder.EncodeAsync(image, stream, cancellationToken);
         }
 
         /// <summary>
-        /// If ColorType was not set, set it based on the given <typeparamref name="TPixel"/>.
+        /// If ColorType was not set, set it based on the given image.
         /// </summary>
-        private void InitializeColorType<TPixel>()
+        private void InitializeColorType<TPixel>(Image<TPixel> image)
             where TPixel : unmanaged, IPixel<TPixel>
         {
+            // First inspect the image metadata.
+            if (this.ColorType == null)
+            {
+                JpegMetadata metadata = image.Metadata.GetJpegMetadata();
+                this.ColorType = metadata.ColorType;
+            }
+
+            // Secondly, inspect the pixel type.
             if (this.ColorType == null)
             {
                 bool isGrayscale =

src/ImageSharp/Formats/Jpeg/JpegMetadata.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
 namespace SixLabors.ImageSharp.Formats.Jpeg
@@ -19,14 +19,23 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// Initializes a new instance of the <see cref="JpegMetadata"/> class.
         /// </summary>
         /// <param name="other">The metadata to create an instance from.</param>
-        private JpegMetadata(JpegMetadata other) => this.Quality = other.Quality;
+        private JpegMetadata(JpegMetadata other)
+        {
+            this.Quality = other.Quality;
+            this.ColorType = other.ColorType;
+        }
 
         /// <summary>
         /// Gets or sets the encoded quality.
         /// </summary>
         public int Quality { get; set; } = 75;
 
+        /// <summary>
+        /// Gets or sets the encoded quality.
+        /// </summary>
+        public JpegColorType? ColorType { get; set; }
+
         /// <inheritdoc/>
         public IDeepCloneable DeepClone() => new JpegMetadata(this);
     }
-}
+}

tests/ImageSharp.Tests/Formats/Jpg/JpegMetadataTests.cs

@@ -12,12 +12,14 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
         [Fact]
         public void CloneIsDeep()
         {
-            var meta = new JpegMetadata { Quality = 50 };
+            var meta = new JpegMetadata { Quality = 50, ColorType = JpegColorType.Luminance };
             var clone = (JpegMetadata)meta.DeepClone();
 
             clone.Quality = 99;
+            clone.ColorType = JpegColorType.YCbCr;
 
             Assert.False(meta.Quality.Equals(clone.Quality));
+            Assert.False(meta.ColorType.Equals(clone.ColorType));
         }
     }
 }