Tiff decoder now respects byte order for 16 bit gray images with white is zero

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

@@ -32,6 +32,11 @@ namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
                     DebugGuard.IsTrue(colorMap == null, "colorMap");
                     return new WhiteIsZero8TiffColor<TPixel>();
 
+                case TiffColorType.WhiteIsZero16:
+                    DebugGuard.IsTrue(bitsPerSample.Channels == 1 && bitsPerSample.Channel0 == 16, "bitsPerSample");
+                    DebugGuard.IsTrue(colorMap == null, "colorMap");
+                    return new WhiteIsZero16TiffColor<TPixel>(byteOrder == ByteOrder.BigEndian);
+
                 case TiffColorType.BlackIsZero:
                     DebugGuard.IsTrue(bitsPerSample.Channels == 1, "bitsPerSample");
                     DebugGuard.IsTrue(colorMap == null, "colorMap");

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

@@ -53,6 +53,11 @@ namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
         /// </summary>
         WhiteIsZero8,
 
+        /// <summary>
+        /// Grayscale: 0 is imaged as white. The maximum value is imaged as black. Optimized implementation for 16-bit images.
+        /// </summary>
+        WhiteIsZero16,
+
         /// <summary>
         /// Palette-color.
         /// </summary>

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/WhiteIsZero16TiffColor{TPixel}.cs

@@ -0,0 +1,48 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using SixLabors.ImageSharp.Formats.Tiff.Utils;
+using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
+{
+    /// <summary>
+    /// Implements the 'WhiteIsZero' photometric interpretation for 16-bit grayscale images.
+    /// </summary>
+    internal class WhiteIsZero16TiffColor<TPixel> : TiffBaseColorDecoder<TPixel>
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        private readonly bool isBigEndian;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="WhiteIsZero16TiffColor{TPixel}" /> class.
+        /// </summary>
+        /// <param name="isBigEndian">if set to <c>true</c> decodes the pixel data as big endian, otherwise as little endian.</param>
+        public WhiteIsZero16TiffColor(bool isBigEndian) => this.isBigEndian = isBigEndian;
+
+        /// <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;
+
+            var l16 = default(L16);
+            for (int y = top; y < top + height; y++)
+            {
+                for (int x = left; x < left + width; x++)
+                {
+                    ushort intensity = (ushort)(ushort.MaxValue - TiffUtils.ConvertToShort(data.Slice(offset, 2), this.isBigEndian));
+                    offset += 2;
+
+                    l16.PackedValue = intensity;
+                    color.FromL16(l16);
+
+                    pixels[x, y] = color;
+                }
+            }
+        }
+    }
+}

src/ImageSharp/Formats/Tiff/TiffDecoderOptionsParser.cs

@@ -111,6 +111,12 @@ namespace SixLabors.ImageSharp.Formats.Tiff
 
                     switch (bitsPerChannel)
                     {
+                        case 16:
+                        {
+                            options.ColorType = TiffColorType.WhiteIsZero16;
+                            break;
+                        }
+
                         case 8:
                         {
                             options.ColorType = TiffColorType.WhiteIsZero8;

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

@@ -142,6 +142,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
         [Theory]
         [WithFile(Flower16BitGrayLittleEndian, PixelTypes.Rgba32)]
         [WithFile(Flower16BitGray, PixelTypes.Rgba32)]
+        [WithFile(Flower16BitGrayMinIsWhiteLittleEndian, PixelTypes.Rgba32)]
         public void TiffDecoder_CanDecode_16Bit<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
 

tests/ImageSharp.Tests/TestImages.cs

@@ -583,6 +583,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Flower14BitGray = "Tiff/flower-minisblack-14.tiff";
             public const string Flower16BitGray = "Tiff/flower-minisblack-16.tiff";
             public const string Flower16BitGrayLittleEndian = "Tiff/flower-minisblack-16_lsb.tiff";
+            public const string Flower16BitGrayMinIsWhiteLittleEndian = "Tiff/flower-miniswhite-16_lsb.tiff";
             public const string Issues1716Rgb161616BitLittleEndian = "Tiff/Issues/Issue1716.tiff";
 
             public const string SmallRgbDeflate = "Tiff/rgb_small_deflate.tiff";

tests/Images/Input/Tiff/flower-miniswhite-16_lsb.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:435c92b453587e1943940111b66afabf70307beb0e1d65e9701fd9bb753eead2
+size 6588