Cover narrow CCITT Group 4 re-encoding

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

@@ -590,6 +590,20 @@ public class TiffEncoderTests : TiffEncoderBaseTester
     public void TiffEncoder_EncodeBiColor_WithCcittGroup4FaxCompression_BlackIsZero_Works<TPixel>(TestImageProvider<TPixel> provider)
         where TPixel : unmanaged, IPixel<TPixel> => TestTiffEncoderCore(provider, TiffBitsPerPixel.Bit1, TiffPhotometricInterpretation.BlackIsZero, TiffCompression.CcittGroup4Fax);
 
+    [Fact]
+    public void TiffEncoder_ReencodeNarrowCcittGroup4Fax_Works()
+    {
+        byte[] data = BuildCcittGroup4Tiff([0x80, 0x08, 0x00, 0x80]);
+
+        using Image image = Image.Load(data);
+        using MemoryStream output = new();
+        image.Save(output, new TiffEncoder());
+
+        output.Position = 0;
+        using Image decoded = Image.Load(output);
+        Assert.Equal(new Size(1, 1), decoded.Size);
+    }
+
     [Theory]
     [WithFile(Calliphora_BiColorUncompressed, PixelTypes.Rgba32)]
     public void TiffEncoder_EncodeBiColor_WithModifiedHuffmanCompression_WhiteIsZero_Works<TPixel>(TestImageProvider<TPixel> provider)
@@ -647,4 +661,61 @@ public class TiffEncoderTests : TiffEncoderBaseTester
         TiffEncoder encoder = new() { PhotometricInterpretation = photometricInterpretation };
         image.DebugSave(provider, encoder);
     }
+
+    private static byte[] BuildCcittGroup4Tiff(byte[] strip)
+    {
+        (ushort Tag, ushort Type, uint Count, uint Value)[] entries =
+        [
+            (256, 3, 1, 1),
+            (257, 3, 1, 1),
+            (258, 3, 1, 1),
+            (259, 3, 1, 4),
+            (262, 3, 1, 0),
+            (273, 4, 1, 0),
+            (277, 3, 1, 1),
+            (278, 4, 1, 1),
+            (279, 4, 1, (uint)strip.Length),
+        ];
+
+        using MemoryStream stream = new();
+        using BinaryWriter writer = new(stream);
+        writer.Write((byte)'I');
+        writer.Write((byte)'I');
+        writer.Write((ushort)42);
+        writer.Write(8U);
+        writer.Write((ushort)entries.Length);
+
+        long stripOffsetPosition = 0;
+        foreach ((ushort tag, ushort type, uint count, uint value) in entries)
+        {
+            writer.Write(tag);
+            writer.Write(type);
+            writer.Write(count);
+            if (tag == 273)
+            {
+                stripOffsetPosition = stream.Position;
+                writer.Write(0U);
+            }
+            else if (type == 3)
+            {
+                writer.Write((ushort)value);
+                writer.Write((ushort)0);
+            }
+            else
+            {
+                writer.Write(value);
+            }
+        }
+
+        writer.Write(0U);
+        uint stripOffset = (uint)stream.Position;
+        writer.Write(strip);
+
+        long endPosition = stream.Position;
+        stream.Position = stripOffsetPosition;
+        writer.Write(stripOffset);
+        stream.Position = endPosition;
+
+        return stream.ToArray();
+    }
 }