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

.github/workflows/build-and-test.yml

@@ -69,12 +69,6 @@ jobs:
             sdk-preview: true
             runtime: -x64
             codecov: false
-          - os: macos-13 # macos-latest runs on arm64 runners where libgdiplus is unavailable
-            framework: net10.0
-            sdk: 10.0.x
-            sdk-preview: true
-            runtime: -x64
-            codecov: false
           - os: macos-26
             framework: net10.0
             sdk: 10.0.x
@@ -99,11 +93,6 @@ jobs:
             sdk: 8.0.x
             runtime: -x64
             codecov: false
-          - os: macos-13 # macos-latest runs on arm64 runners where libgdiplus is unavailable
-            framework: net8.0
-            sdk: 8.0.x
-            runtime: -x64
-            codecov: false
           - os: macos-26
             framework: net8.0
             sdk: 8.0.x

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/Rgba16161616TiffColor{TPixel}.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
+
 #nullable disable
 
 using System.Buffers;
@@ -48,31 +49,53 @@ public override void Decode(ReadOnlySpan<byte> data, Buffer2D<TPixel> pixels, in
 
         using IMemoryOwner<Vector4> vectors = hasAssociatedAlpha ? this.memoryAllocator.Allocate<Vector4>(width) : null;
         Span<Vector4> vectorsSpan = hasAssociatedAlpha ? vectors.GetSpan() : [];
-        for (int y = top; y < top + height; y++)
-        {
-            Span<TPixel> pixelRow = pixels.DangerousGetRowSpan(y).Slice(left, width);
 
-            if (this.isBigEndian)
+        if (this.isBigEndian)
+        {
+            if (hasAssociatedAlpha)
             {
-                for (int x = 0; x < pixelRow.Length; x++)
+                for (int y = top; y < top + height; y++)
                 {
-                    ushort r = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset, 2));
-                    offset += 2;
-                    ushort g = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset, 2));
-                    offset += 2;
-                    ushort b = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset, 2));
-                    offset += 2;
-                    ushort a = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset, 2));
-                    offset += 2;
-
-                    pixelRow[x] = hasAssociatedAlpha
-                        ? TiffUtilities.ColorFromRgba64Premultiplied<TPixel>(r, g, b, a)
-                        : TPixel.FromRgba64(new Rgba64(r, g, b, a));
+                    Span<TPixel> pixelRow = pixels.DangerousGetRowSpan(y).Slice(left, width);
+
+                    for (int x = 0; x < pixelRow.Length; x++)
+                    {
+                        ushort r = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset, 2));
+                        ushort g = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset + 2, 2));
+                        ushort b = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset + 4, 2));
+                        ushort a = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset + 6, 2));
+                        offset += 8;
+
+                        pixelRow[x] = TiffUtilities.ColorFromRgba64Premultiplied<TPixel>(r, g, b, a);
+                    }
                 }
             }
             else
             {
+                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++)
+                    {
+                        ushort r = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset, 2));
+                        ushort g = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset + 2, 2));
+                        ushort b = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset + 4, 2));
+                        ushort a = TiffUtilities.ConvertToUShortBigEndian(data.Slice(offset + 6, 2));
+                        offset += 8;
+
+                        pixelRow[x] = TPixel.FromRgba64(new Rgba64(r, g, b, a));
+                    }
+                }
+            }
+        }
+        else
+        {
+            for (int y = top; y < top + height; y++)
+            {
+                Span<TPixel> pixelRow = pixels.DangerousGetRowSpan(y).Slice(left, width);
                 int byteCount = pixelRow.Length * 8;
+
                 PixelOperations<TPixel>.Instance.FromRgba64Bytes(
                     this.configuration,
                     data.Slice(offset, byteCount),

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

@@ -45,7 +45,12 @@ public static TPixel ColorFromRgba64Premultiplied<TPixel>(ushort r, ushort g, us
             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 void TiffDecoder_CanDecode_YccK<TPixel>(TestImageProvider<TPixel> provide
         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 Tiff
         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";