Add support for decoding planar tiff with cielab colorspace

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

@@ -0,0 +1,49 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Buffers;
+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' with the planar configuration.
+    /// </summary>
+    internal class CieLabPlanarTiffColor<TPixel> : TiffBasePlanarColorDecoder<TPixel>
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        private readonly ColorSpaceConverter colorSpaceConverter = new();
+
+        private const float Inv255 = 1.0f / 255.0f;
+
+        /// <inheritdoc/>
+        public override void Decode(IMemoryOwner<byte>[] data, Buffer2D<TPixel> pixels, int left, int top, int width, int height)
+        {
+            Span<byte> l = data[0].GetSpan();
+            Span<byte> a = data[1].GetSpan();
+            Span<byte> b = data[2].GetSpan();
+
+            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++)
+                {
+                    var lab = new CieLab((l[offset] & 0xFF) * 100f * Inv255, (sbyte)a[offset], (sbyte)b[offset]);
+                    var rgb = this.colorSpaceConverter.ToRgb(lab);
+
+                    color.FromVector4(new Vector4(rgb.R, rgb.G, rgb.B, 1.0f));
+                    pixelRow[x] = color;
+
+                    offset++;
+                }
+            }
+        }
+    }
+}

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

@@ -430,6 +430,9 @@ namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
                 case TiffColorType.YCbCrPlanar:
                     return new YCbCrPlanarTiffColor<TPixel>(referenceBlackAndWhite, ycbcrCoefficients, ycbcrSubSampling);
 
+                case TiffColorType.CieLabPlanar:
+                    return new CieLabPlanarTiffColor<TPixel>();
+
                 case TiffColorType.Rgb161616Planar:
                     DebugGuard.IsTrue(colorMap == null, "colorMap");
                     return new Rgb16PlanarTiffColor<TPixel>(byteOrder == ByteOrder.BigEndian);

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

@@ -282,5 +282,10 @@ namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
         /// The pixels are stored in CieLab format.
         /// </summary>
         CieLab,
+
+        /// <summary>
+        /// The pixels are stored in CieLab format as planar.
+        /// </summary>
+        CieLabPlanar,
     }
 }

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/YCbCrPlanarTiffColor{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' with the planar configuration.
+    /// </summary>
     internal class YCbCrPlanarTiffColor<TPixel> : TiffBasePlanarColorDecoder<TPixel>
         where TPixel : unmanaged, IPixel<TPixel>
     {

src/ImageSharp/Formats/Tiff/TiffDecoderOptionsParser.cs

@@ -397,8 +397,6 @@ namespace SixLabors.ImageSharp.Formats.Tiff
 
                 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.");
@@ -410,6 +408,8 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                         TiffThrowHelper.ThrowNotSupported("Only 8 bits per channel is supported for CieLab images.");
                     }
 
+                    options.ColorType = options.PlanarConfiguration == TiffPlanarConfiguration.Chunky ? TiffColorType.CieLab : TiffColorType.CieLabPlanar;
+
                     break;
                 }