Fix #2133

Author: JimBobSquarePants

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -530,6 +530,12 @@ private JpegColorSpace DeduceJpegColorSpace(byte componentCount)
                     return JpegColorSpace.RGB;
                 }
 
+                // If these values are 1-3 for a 3-channel image, then the image is assumed to be YCbCr.
+                if (this.Components[2].Id == 3 && this.Components[1].Id == 2 && this.Components[0].Id == 1)
+                {
+                    return JpegColorSpace.YCbCr;
+                }
+
                 // 3-channel non-subsampled images are assumed to be RGB.
                 if (this.Components[2].VerticalSamplingFactor == 1 && this.Components[1].VerticalSamplingFactor == 1 && this.Components[0].VerticalSamplingFactor == 1 &&
                     this.Components[2].HorizontalSamplingFactor == 1 && this.Components[1].HorizontalSamplingFactor == 1 && this.Components[0].HorizontalSamplingFactor == 1)

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

@@ -216,6 +216,19 @@ public void Issue2057_DecodeWorks<TPixel>(TestImageProvider<TPixel> provider)
             }
         }
 
+        // https://github.com/SixLabors/ImageSharp/issues/2133
+        [Theory]
+        [WithFile(TestImages.Jpeg.Issues.Issue2133DeduceColorSpace, PixelTypes.Rgba32)]
+        public void Issue2133_DeduceColorSpace<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(JpegDecoder))
+            {
+                image.DebugSave(provider);
+                image.CompareToOriginal(provider);
+            }
+        }
+
         // DEBUG ONLY!
         // The PDF.js output should be saved by "tests\ImageSharp.Tests\Formats\Jpg\pdfjs\jpeg-converter.htm"
         // into "\tests\Images\ActualOutput\JpegDecoderTests\"

tests/ImageSharp.Tests/TestImages.cs

@@ -265,6 +265,7 @@ public static class Issues
                 public const string Issue2057App1Parsing = "Jpg/issues/Issue2057-App1Parsing.jpg";
                 public const string ExifNullArrayTag = "Jpg/issues/issue-2056-exif-null-array.jpg";
                 public const string ValidExifArgumentNullExceptionOnEncode = "Jpg/issues/Issue2087-exif-null-reference-on-encode.jpg";
+                public const string Issue2133DeduceColorSpace = "Jpg/issues/Issue2133.jpg";
 
                 public static class Fuzz
                 {