Add PhotometricInterpretation to the tiff metadata

src/ImageSharp/Formats/Tiff/ITiffEncoderOptions.cs

@@ -23,7 +23,7 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
         DeflateCompressionLevel CompressionLevel { get; }
 
         /// <summary>
-        /// Gets the encoding mode to use. RGB, RGB with color palette or gray.
+        /// Gets the encoding mode to use. Possible options are RGB, RGB with a color palette, gray or BiColor.
         /// If no mode is specified in the options, RGB will be used.
         /// </summary>
         TiffEncodingMode Mode { get; }

src/ImageSharp/Formats/Tiff/TiffDecoderMetadataCreator.cs

@@ -2,10 +2,9 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-using System.Buffers.Binary;
 using System.Collections.Generic;
 using System.Linq;
-using System.Runtime.InteropServices;
+
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.Metadata.Profiles.Exif;
 using SixLabors.ImageSharp.Metadata.Profiles.Icc;
@@ -43,6 +42,7 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
             tiffMetadata.ByteOrder = byteOrder;
             tiffMetadata.BitsPerPixel = GetBitsPerPixel(rootFrameMetadata);
             tiffMetadata.Compression = rootFrameMetadata.Compression;
+            tiffMetadata.PhotometricInterpretation = rootFrameMetadata.PhotometricInterpretation;
 
             if (!ignoreMetadata)
             {

src/ImageSharp/Formats/Tiff/TiffEncoderCore.cs

@@ -70,10 +70,10 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
         public TiffEncoderCore(ITiffEncoderOptions options, MemoryAllocator memoryAllocator)
         {
             this.memoryAllocator = memoryAllocator;
-            this.CompressionType = options.Compression;
             this.Mode = options.Mode;
             this.quantizer = options.Quantizer ?? KnownQuantizers.Octree;
             this.UseHorizontalPredictor = options.UseHorizontalPredictor;
+            this.CompressionType = options.Compression;
             this.compressionLevel = options.CompressionLevel;
             this.maxStripBytes = options.MaxStripBytes;
         }
@@ -271,8 +271,7 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
                         this.Mode = TiffEncodingMode.BiColor;
                         break;
                     case TiffBitsPerPixel.Pixel8:
-                        // todo: can gray or palette
-                        this.Mode = TiffEncodingMode.Gray;
+                        this.Mode = tiffMetadata.PhotometricInterpretation != TiffPhotometricInterpretation.PaletteColor ? TiffEncodingMode.Gray : TiffEncodingMode.Rgb;
                         break;
                     default:
                         this.Mode = TiffEncodingMode.Rgb;

src/ImageSharp/Formats/Tiff/TiffMetadata.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
+using System;
 using SixLabors.ImageSharp.Formats.Experimental.Tiff.Constants;
 
 namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
@@ -24,9 +25,11 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
         private TiffMetadata(TiffMetadata other)
         {
             this.ByteOrder = other.ByteOrder;
-            this.XmpProfile = other.XmpProfile;
             this.BitsPerPixel = other.BitsPerPixel;
             this.Compression = other.Compression;
+            this.PhotometricInterpretation = other.PhotometricInterpretation;
+            this.XmpProfile = other.XmpProfile != null ? new byte[other.XmpProfile.Length] : null;
+            other.XmpProfile?.AsSpan().CopyTo(this.XmpProfile.AsSpan());
         }
 
         /// <summary>
@@ -44,6 +47,11 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
         /// </summary>
         public TiffCompression Compression { get; internal set; } = TiffCompression.None;
 
+        /// <summary>
+        /// Gets the photometric interpretation which indicates how the pixels are to be interpreted, e.g. if the image is bicolor, RGB, color paletted etc.
+        /// </summary>
+        public TiffPhotometricInterpretation PhotometricInterpretation { get; internal set; }
+
         /// <summary>
         /// Gets or sets the XMP profile.
         /// For internal use only. ImageSharp not support XMP profile.

tests/ImageSharp.Tests/Formats/Tiff/TiffEncoderTests.cs

@@ -73,6 +73,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
         [WithFile(Calliphora_BiColorUncompressed, PixelTypes.Rgba32, TiffBitsPerPixel.Pixel1)]
         [WithFile(GrayscaleUncompressed, PixelTypes.Rgba32, TiffBitsPerPixel.Pixel8)]
         [WithFile(RgbUncompressed, PixelTypes.Rgba32, TiffBitsPerPixel.Pixel24)]
+        [WithFile(Calliphora_PaletteUncompressed, PixelTypes.Rgba32, TiffBitsPerPixel.Pixel24)]
         public void TiffEncoder_PreserveBitsPerPixel<TPixel>(TestImageProvider<TPixel> provider, TiffBitsPerPixel expectedBitsPerPixel)
             where TPixel : unmanaged, IPixel<TPixel>
         {

tests/ImageSharp.Tests/Formats/Tiff/TiffMetadataTests.cs

@@ -36,12 +36,14 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
         [Fact]
         public void CloneIsDeep()
         {
+            byte[] xmpData = { 1, 1, 1 };
             var meta = new TiffMetadata
             {
                 Compression = TiffCompression.Deflate,
                 BitsPerPixel = TiffBitsPerPixel.Pixel8,
                 ByteOrder = ByteOrder.BigEndian,
-                XmpProfile = new byte[3]
+                XmpProfile = xmpData,
+                PhotometricInterpretation = TiffPhotometricInterpretation.Rgb
             };
 
             var clone = (TiffMetadata)meta.DeepClone();
@@ -49,12 +51,14 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
             clone.Compression = TiffCompression.None;
             clone.BitsPerPixel = TiffBitsPerPixel.Pixel24;
             clone.ByteOrder = ByteOrder.LittleEndian;
-            clone.XmpProfile = new byte[1];
+            clone.PhotometricInterpretation = TiffPhotometricInterpretation.YCbCr;
 
             Assert.False(meta.Compression == clone.Compression);
             Assert.False(meta.BitsPerPixel == clone.BitsPerPixel);
             Assert.False(meta.ByteOrder == clone.ByteOrder);
-            Assert.False(meta.XmpProfile.SequenceEqual(clone.XmpProfile));
+            Assert.False(meta.PhotometricInterpretation == clone.PhotometricInterpretation);
+            Assert.False(meta.XmpProfile.Equals(clone.XmpProfile));
+            Assert.True(meta.XmpProfile.SequenceEqual(clone.XmpProfile));
         }
 
         [Theory]
@@ -95,6 +99,23 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
             Assert.Equal(expectedCompression, tiffMetadata.Compression);
         }
 
+        [Theory]
+        [InlineData(Calliphora_RgbUncompressed, TiffPhotometricInterpretation.Rgb)]
+        [InlineData(Calliphora_BiColorUncompressed, TiffPhotometricInterpretation.BlackIsZero)]
+        [InlineData(Calliphora_PaletteUncompressed, TiffPhotometricInterpretation.PaletteColor)]
+        public void Identify_DetectsCorrectPhotometricInterpretation(string imagePath, TiffPhotometricInterpretation expectedPhotometricInterpretation)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using var stream = new MemoryStream(testFile.Bytes, false);
+
+            IImageInfo imageInfo = Image.Identify(this.configuration, stream);
+
+            Assert.NotNull(imageInfo);
+            TiffMetadata tiffMetadata = imageInfo.Metadata.GetTiffMetadata();
+            Assert.NotNull(tiffMetadata);
+            Assert.Equal(expectedPhotometricInterpretation, tiffMetadata.PhotometricInterpretation);
+        }
+
         [Theory]
         [InlineData(GrayscaleUncompressed, ByteOrder.BigEndian)]
         [InlineData(LittleEndianByteOrder, ByteOrder.LittleEndian)]