Fix decoding tiff image with BigEndian + 64 bit / pixel + associated alpha

src/ImageSharp/Formats/Tiff/Utils/TiffUtilities.cs

@@ -45,7 +45,12 @@ internal static class TiffUtilities
             return TPixel.FromRgba64(default);
         }
 
-        return TPixel.FromRgba64(new Rgba64((ushort)(r / a), (ushort)(g / a), (ushort)(b / a), a));
+        float scale = 65535f / a;
+        ushort ur = (ushort)Math.Min(r * scale, 65535);
+        ushort ug = (ushort)Math.Min(g * scale, 65535);
+        ushort ub = (ushort)Math.Min(b * scale, 65535);
+
+        return TPixel.FromRgba64(new Rgba64(ur, ug, ub, a));
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]

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

@@ -365,6 +365,19 @@ public class TiffDecoderTests : TiffDecoderBaseTester
         image.CompareToReferenceOutput(ImageComparer.TolerantPercentage(0.0001F), provider);
     }
 
+    [Theory]
+    [WithFile(Issues3031, PixelTypes.Rgba64)]
+    [WithFile(Rgba16BitAssociatedAlphaBigEndian, PixelTypes.Rgba64)]
+    [WithFile(Rgba16BitAssociatedAlphaLittleEndian, PixelTypes.Rgba64)]
+    public void TiffDecoder_CanDecode_64Bit_WithAssociatedAlpha<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage(TiffDecoder.Instance);
+        image.DebugSave(provider);
+
+        image.CompareToReferenceOutput(ImageComparer.Exact, provider);
+    }
+
     [Theory]
     [WithFile(Issues2454_A, PixelTypes.Rgba32)]
     [WithFile(Issues2454_B, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/Formats/Tiff/Utils/TiffUtilitiesTest.cs

@@ -0,0 +1,51 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Tiff.Utils;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Tests.Formats.Tiff.Utils;
+
+[Trait("Format", "Tiff")]
+public class TiffUtilitiesTest
+{
+    [Theory]
+    [InlineData(0, 0, 0, 0)]
+    [InlineData(42, 84, 128, 0)]
+    [InlineData(65535, 65535, 65535, 0)]
+    public void ColorFromRgba64Premultiplied_WithZeroAlpha_ReturnsDefaultPixel(ushort r, ushort g, ushort b, ushort a)
+    {
+        Rgba64 actual = TiffUtilities.ColorFromRgba64Premultiplied<Rgba64>(r, g, b, a);
+
+        Assert.Equal(default, actual);
+    }
+
+    [Theory]
+    [InlineData(65535, 65535, 65535, 65535, 65535, 65535, 65535, 65535)]
+    [InlineData(32767, 0, 0, 65535, 32767, 0, 0, 65535)]
+    [InlineData(0, 32767, 0, 65535, 0, 32767, 0, 65535)]
+    [InlineData(0, 0, 32767, 65535, 0, 0, 32767, 65535)]
+    public void ColorFromRgba64Premultiplied_WithNoAlpha_ReturnExpectedValues(ushort r, ushort g, ushort b, ushort a, ushort expectedR, ushort expectedG, ushort expectedB, ushort expectedA)
+    {
+        Rgba64 actual = TiffUtilities.ColorFromRgba64Premultiplied<Rgba64>(r, g, b, a);
+
+        Assert.Equal(new Rgba64(expectedR, expectedG, expectedB, expectedA), actual);
+    }
+
+    [Theory]
+    [InlineData(32766, 0, 0, 32766, 65535, 0, 0, 32766)] // Red, 50% Alpha
+    [InlineData(0, 32766, 0, 32766, 0, 65535, 0, 32766)] // Green, 50% Alpha
+    [InlineData(0, 0, 32766, 32766, 0, 0, 65535, 32766)] // Blue, 50% Alpha
+    [InlineData(8191, 0, 0, 16383, 32765, 0, 0, 16383)] // Red, 25% Alpha
+    [InlineData(0, 8191, 0, 16383, 0, 32765, 0, 16383)] // Green, 25% Alpha
+    [InlineData(0, 0, 8191, 16383, 0, 0, 32765, 16383)] // Blue, 25% Alpha
+    [InlineData(8191, 0, 0, 0, 0, 0, 0, 0)] // Red, 0% Alpha
+    [InlineData(0, 8191, 0, 0, 0, 0, 0, 0)] // Green, 0% Alpha
+    [InlineData(0, 0, 8191, 0, 0, 0, 0, 0)] // Blue, 0% Alpha
+    public void ColorFromRgba64Premultiplied_WithAlpha_ReturnExpectedValues(ushort r, ushort g, ushort b, ushort a, ushort expectedR, ushort expectedG, ushort expectedB, ushort expectedA)
+    {
+        Rgba64 actual = TiffUtilities.ColorFromRgba64Premultiplied<Rgba64>(r, g, b, a);
+
+        Assert.Equal(new Rgba64(expectedR, expectedG, expectedB, expectedA), actual);
+    }
+}

tests/ImageSharp.Tests/TestImages.cs

@@ -1142,6 +1142,7 @@ public static class TestImages
         public const string Issues2435 = "Tiff/Issues/Issue2435.tiff";
         public const string Issues2454_A = "Tiff/Issues/Issue2454_A.tif";
         public const string Issues2454_B = "Tiff/Issues/Issue2454_B.tif";
+        public const string Issues3031 = "Tiff/Issues/Issue3031.tiff";
         public const string Issues2587 = "Tiff/Issues/Issue2587.tiff";
         public const string Issues2679 = "Tiff/Issues/Issue2679.tiff";
         public const string JpegCompressedGray0000539558 = "Tiff/Issues/JpegCompressedGray-0000539558.tiff";

tests/Images/External/ReferenceOutput/TiffDecoderTests/TiffDecoder_CanDecode_64Bit_WithAssociatedAlpha_Rgba64_Issue3031.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c33a2f63836975bdc7631f26634b3fb0ae98bfaff730300877339cb568141cde
+size 124

tests/Images/External/ReferenceOutput/TiffDecoderTests/TiffDecoder_CanDecode_64Bit_WithAssociatedAlpha_Rgba64_RgbaAssociatedAlpha16bit_lsb.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:e50dfa103459d21642df5e1ca760081fbdfe3b7244624d9d87d8a20f45b51bbe
+size 117953

tests/Images/External/ReferenceOutput/TiffDecoderTests/TiffDecoder_CanDecode_64Bit_WithAssociatedAlpha_Rgba64_RgbaAssociatedAlpha16bit_msb.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:3333503012aa29e23f0d0e52993ad3499514251208fb3318e2bb1560d54650fa
+size 117956

tests/Images/External/ReferenceOutput/TiffDecoderTests/TiffDecoder_CanDecode_BigEndian_AssociatedAlpha_Rgba16161616_Rgba64_Issue3031.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c33a2f63836975bdc7631f26634b3fb0ae98bfaff730300877339cb568141cde
+size 124

tests/Images/Input/Tiff/Issues/Issue3031.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:6e4d2db56a1b7fdea09ed65eab1d10a02952821f6662ca77caa44b8c51b30310
+size 416