Allow additional and undefined extra samples

Author: JimBobSquarePants

src/ImageSharp/Formats/Tiff/TiffDecoderOptionsParser.cs

@@ -25,20 +25,22 @@ internal static class TiffDecoderOptionsParser
     /// <returns>True, if the image uses tiles. Otherwise the images has strip's.</returns>
     public static bool VerifyAndParse(this TiffDecoderCore options, ExifProfile exifProfile, TiffFrameMetadata frameMetadata)
     {
-        IExifValue extraSamplesExifValue = exifProfile.GetValueInternal(ExifTag.ExtraSamples);
-        if (extraSamplesExifValue is not null)
+        if (exifProfile.TryGetValue(ExifTag.ExtraSamples, out IExifValue<ushort[]> samples))
         {
-            short[] extraSamples = (short[])extraSamplesExifValue.GetValue();
-            if (extraSamples.Length != 1)
+            // We only support a single sample pertaining to alpha data.
+            // Other information is discarded.
+            TiffExtraSampleType sampleType = (TiffExtraSampleType)samples.Value[0];
+            if (sampleType is TiffExtraSampleType.CorelDrawUnassociatedAlphaData)
             {
-                TiffThrowHelper.ThrowNotSupported("ExtraSamples is only supported with one extra sample for alpha data.");
+                // According to libtiff, this CorelDRAW-specific value indicates unassociated alpha.
+                // Patch required for compatibility with malformed CorelDRAW-generated TIFFs.
+                // https://libtiff.gitlab.io/libtiff/releases/v3.9.0beta.html
+                sampleType = TiffExtraSampleType.UnassociatedAlphaData;
             }
 
-            TiffExtraSampleType extraSamplesType = (TiffExtraSampleType)extraSamples[0];
-            options.ExtraSamplesType = extraSamplesType;
-            if (extraSamplesType is not (TiffExtraSampleType.UnassociatedAlphaData or TiffExtraSampleType.AssociatedAlphaData))
+            if (sampleType is (TiffExtraSampleType.UnassociatedAlphaData or TiffExtraSampleType.AssociatedAlphaData))
             {
-                TiffThrowHelper.ThrowNotSupported("Decoding Tiff images with ExtraSamples is not supported with UnspecifiedData.");
+                options.ExtraSamplesType = sampleType;
             }
         }
 

src/ImageSharp/Formats/Tiff/TiffExtraSampleType.cs

@@ -22,5 +22,11 @@ internal enum TiffExtraSampleType
     /// The extra data is unassociated alpha data is transparency information that logically exists independent of an image;
     /// it is commonly called a soft matte.
     /// </summary>
-    UnassociatedAlphaData = 2
+    UnassociatedAlphaData = 2,
+
+    /// <summary>
+    /// A CorelDRAW-specific value observed in damaged files, indicating unassociated alpha.
+    /// Not part of the official TIFF specification; patched in ImageSharp for compatibility.
+    /// </summary>
+    CorelDrawUnassociatedAlphaData = 999
 }

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

@@ -772,4 +772,9 @@ public void TiffDecoder_Decode_Resize<TPixel>(TestImageProvider<TPixel> provider
             testOutputDetails: details,
             appendPixelTypeToFileName: false);
     }
+
+    [Theory]
+    [WithFile(ExtraSamplesUnspecified, PixelTypes.Rgba32)]
+    public void TiffDecoder_CanDecode_ExtraSamplesUnspecified<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -1112,6 +1112,7 @@ public static class Tiff
         public const string Issues2587 = "Tiff/Issues/Issue2587.tiff";
         public const string JpegCompressedGray0000539558 = "Tiff/Issues/JpegCompressedGray-0000539558.tiff";
         public const string Tiled0000023664 = "Tiff/Issues/tiled-0000023664.tiff";
+        public const string ExtraSamplesUnspecified = "Tiff/Issues/ExtraSamplesUnspecified.tif";
 
         public const string SmallRgbDeflate = "Tiff/rgb_small_deflate.tiff";
         public const string SmallRgbLzw = "Tiff/rgb_small_lzw.tiff";