EXIF IDF pointer

src/ImageSharp/Metadata/Profiles/Exif/ExifWriter.cs

@@ -46,49 +46,42 @@ public ExifWriter(IList<IExifValue> values, ExifParts allowedParts)
         public byte[] GetData()
         {
             const uint startIndex = 0;
-            uint length;
 
             IExifValue exifOffset = GetOffsetValue(this.ifdValues, this.exifValues, ExifTag.SubIFDOffset);
             IExifValue gpsOffset = GetOffsetValue(this.ifdValues, this.gpsValues, ExifTag.GPSIFDOffset);
 
-            if (this.ifdValues.Count == 0 && this.exifValues.Count == 0 && this.gpsValues.Count == 0)
-            {
-                return Array.Empty<byte>();
-            }
-
-            uint ifdLength = this.GetLength(this.ifdValues) + 4U;
+            uint ifdLength = this.GetLength(this.ifdValues);
             uint exifLength = this.GetLength(this.exifValues);
             uint gpsLength = this.GetLength(this.gpsValues);
 
-            length = ifdLength + exifLength + gpsLength;
+            uint length = ifdLength + exifLength + gpsLength;
 
-            if (length == 4U)
+            if (length == 0)
             {
                 return Array.Empty<byte>();
             }
 
             // two bytes for the byte Order marker 'II' or 'MM', followed by the number 42 (0x2A) and a 0, making 4 bytes total
             length += (uint)ExifConstants.LittleEndianByteOrderMarker.Length;
 
-            length += 4 + 2;
+            // first IFD offset
+            length += 4;
 
-            var result = new byte[length];
+            byte[] result = new byte[length];
 
             int i = 0;
 
             // The byte order marker for little-endian, followed by the number 42 and a 0
             ExifConstants.LittleEndianByteOrderMarker.CopyTo(result.AsSpan(start: i));
             i += ExifConstants.LittleEndianByteOrderMarker.Length;
 
-            uint ifdOffset = ((uint)i - startIndex) + 4U;
-            uint thumbnailOffset = ifdOffset + ifdLength + exifLength + gpsLength;
+            uint ifdOffset = (uint)i - startIndex + 4U;
 
             exifOffset?.TrySetValue(ifdOffset + ifdLength);
             gpsOffset?.TrySetValue(ifdOffset + ifdLength + exifLength);
 
             i = WriteUInt32(ifdOffset, result, i);
             i = this.WriteHeaders(this.ifdValues, result, i);
-            i = WriteUInt32(thumbnailOffset, result, i);
             i = this.WriteData(startIndex, this.ifdValues, result, i);
 
             if (exifLength > 0)
@@ -103,8 +96,6 @@ public byte[] GetData()
                 i = this.WriteData(startIndex, this.gpsValues, result, i);
             }
 
-            WriteUInt16(0, result, i);
-
             return result;
         }
 
@@ -263,14 +254,17 @@ private uint GetLength(IList<IExifValue> values)
             {
                 uint valueLength = GetLength(value);
 
-                length += 2 + 2 + 4 + 4;
+                length += 12;
 
                 if (valueLength > 4)
                 {
                     length += valueLength;
                 }
             }
 
+            // next IFD offset
+            length += 4;
+
             return length;
         }
 
@@ -361,6 +355,9 @@ private int WriteHeaders(List<IExifValue> values, Span<byte> destination, int of
                 newOffset += 4;
             }
 
+            // next IFD offset
+            newOffset = WriteUInt32(0, destination, newOffset);
+
             return newOffset;
         }
 

tests/ImageSharp.Tests/Metadata/Profiles/Exif/ExifProfileTests.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Buffers.Binary;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -86,6 +87,18 @@ public void ConstructorEmpty()
             new ExifProfile(Array.Empty<byte>());
         }
 
+        [Fact]
+        public void EmptyWriter()
+        {
+            var profile = new ExifProfile() { Parts = ExifParts.GpsTags };
+            profile.SetValue(ExifTag.Copyright, "Copyright text");
+
+            byte[] bytes = profile.ToByteArray();
+
+            Assert.NotNull(bytes);
+            Assert.Empty(bytes);
+        }
+
         [Fact]
         public void ConstructorCopy()
         {
@@ -420,7 +433,7 @@ public void TestArrayValueWithUnspecifiedSize()
             Assert.Equal(2, profile.Values.Count(v => (ExifTagValue)(ushort)v.Tag == ExifTagValue.DateTime));
 
             byte[] bytes = profile.ToByteArray();
-            Assert.Equal(525, bytes.Length);
+            Assert.Equal(531, bytes.Length);
 
             var profile2 = new ExifProfile(bytes);
             Assert.Equal(25, profile2.Values.Count);
@@ -487,6 +500,22 @@ private static ExifProfile CreateExifProfile()
             return profile;
         }
 
+        [Fact]
+        public void IfdStructure()
+        {
+            var exif = new ExifProfile();
+            exif.SetValue(ExifTag.XPAuthor, Encoding.GetEncoding("UCS-2").GetBytes("Dan Petitt"));
+
+            Span<byte> actualBytes = exif.ToByteArray();
+
+            // Assert
+            int ifdOffset = ExifConstants.LittleEndianByteOrderMarker.Length;
+            Assert.Equal(8U, BinaryPrimitives.ReadUInt32LittleEndian(actualBytes.Slice(ifdOffset, 4)));
+
+            int nextIfdPointerOffset = ExifConstants.LittleEndianByteOrderMarker.Length + 4 + 2 + 12;
+            Assert.Equal(0U, BinaryPrimitives.ReadUInt32LittleEndian(actualBytes.Slice(nextIfdPointerOffset, 4)));
+        }
+
         internal static ExifProfile GetExifProfile()
         {
             using Image<Rgba32> image = TestFile.Create(TestImages.Jpeg.Baseline.Floorplan).CreateRgba32Image();