Add support for decoding tiff images with CieLab color space

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/CieLabTiffColor{TPixel}.cs

@@ -0,0 +1,46 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Numerics;
+using SixLabors.ImageSharp.ColorSpaces;
+using SixLabors.ImageSharp.ColorSpaces.Conversion;
+using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
+{
+    /// <summary>
+    /// Implements decoding pixel data with photometric interpretation of type 'CieLab'.
+    /// </summary>
+    internal class CieLabTiffColor<TPixel> : TiffBaseColorDecoder<TPixel>
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        private readonly ColorSpaceConverter colorSpaceConverter = new();
+
+        private const float Inv255 = 1.0f / 255.0f;
+
+        /// <inheritdoc/>
+        public override void Decode(ReadOnlySpan<byte> data, Buffer2D<TPixel> pixels, int left, int top, int width, int height)
+        {
+            var color = default(TPixel);
+            int offset = 0;
+            for (int y = top; y < top + height; y++)
+            {
+                Span<TPixel> pixelRow = pixels.DangerousGetRowSpan(y).Slice(left, width);
+
+                for (int x = 0; x < pixelRow.Length; x++)
+                {
+                    float l = (data[offset] & 0xFF) * 100f * Inv255;
+                    var lab = new CieLab(l, (sbyte)data[offset + 1], (sbyte)data[offset + 2]);
+                    var rgb = this.colorSpaceConverter.ToRgb(lab);
+
+                    color.FromVector4(new Vector4(rgb.R, rgb.G, rgb.B, 1.0f));
+                    pixelRow[x] = color;
+
+                    offset += 3;
+                }
+            }
+        }
+    }
+}

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/TiffColorDecoderFactory{TPixel}.cs

@@ -385,8 +385,23 @@ namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
                     return new PaletteTiffColor<TPixel>(bitsPerSample, colorMap);
 
                 case TiffColorType.YCbCr:
+                    DebugGuard.IsTrue(
+                        bitsPerSample.Channels == 3
+                        && bitsPerSample.Channel2 == 8
+                        && bitsPerSample.Channel1 == 8
+                        && bitsPerSample.Channel0 == 8,
+                        "bitsPerSample");
                     return new YCbCrTiffColor<TPixel>(memoryAllocator, referenceBlackAndWhite, ycbcrCoefficients, ycbcrSubSampling);
 
+                case TiffColorType.CieLab:
+                    DebugGuard.IsTrue(
+                        bitsPerSample.Channels == 3
+                        && bitsPerSample.Channel2 == 8
+                        && bitsPerSample.Channel1 == 8
+                        && bitsPerSample.Channel0 == 8,
+                        "bitsPerSample");
+                    return new CieLabTiffColor<TPixel>();
+
                 default:
                     throw TiffThrowHelper.InvalidColorType(colorType.ToString());
             }

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/TiffColorType.cs

@@ -276,6 +276,11 @@ namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
         /// <summary>
         /// The pixels are stored in YCbCr format as planar.
         /// </summary>
-        YCbCrPlanar
+        YCbCrPlanar,
+
+        /// <summary>
+        /// The pixels are stored in CieLab format.
+        /// </summary>
+        CieLab,
     }
 }

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/YCbCrTiffColor{TPixel}.cs

@@ -9,6 +9,9 @@ using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
 {
+    /// <summary>
+    /// Implements decoding pixel data with photometric interpretation of type 'YCbCr'.
+    /// </summary>
     internal class YCbCrTiffColor<TPixel> : TiffBaseColorDecoder<TPixel>
         where TPixel : unmanaged, IPixel<TPixel>
     {

src/ImageSharp/Formats/Tiff/TiffDecoderOptionsParser.cs

@@ -381,7 +381,7 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                     options.ColorMap = exifProfile.GetValue(ExifTag.ColorMap)?.Value;
                     if (options.BitsPerSample.Channels != 3)
                     {
-                        TiffThrowHelper.ThrowNotSupported("The number of samples in the TIFF BitsPerSample entry is not supported.");
+                        TiffThrowHelper.ThrowNotSupported("The number of samples in the TIFF BitsPerSample entry is not supported for YCbCr images.");
                     }
 
                     ushort bitsPerChannel = options.BitsPerSample.Channel0;
@@ -395,6 +395,24 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                     break;
                 }
 
+                case TiffPhotometricInterpretation.CieLab:
+                {
+                    options.ColorType = TiffColorType.CieLab;
+
+                    if (options.BitsPerSample.Channels != 3)
+                    {
+                        TiffThrowHelper.ThrowNotSupported("The number of samples in the TIFF BitsPerSample entry is not supported for CieLab images.");
+                    }
+
+                    ushort bitsPerChannel = options.BitsPerSample.Channel0;
+                    if (bitsPerChannel != 8)
+                    {
+                        TiffThrowHelper.ThrowNotSupported("Only 8 bits per channel is supported for CieLab images.");
+                    }
+
+                    break;
+                }
+
                 default:
                 {
                     TiffThrowHelper.ThrowNotSupported($"The specified TIFF photometric interpretation is not supported: {options.PhotometricInterpretation}");