exif ifd writing fixes

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

@@ -46,7 +46,6 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Exif
         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);
@@ -56,13 +55,13 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Exif
                 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>();
             }
@@ -70,9 +69,10 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Exif
             // 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 + 4;
+            // first IFD offset
+            length += 4;
 
-            var result = new byte[length];
+            byte[] result = new byte[length];
 
             int i = 0;
 
@@ -80,15 +80,13 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Exif
             ExifConstants.LittleEndianByteOrderMarker.CopyTo(result.AsSpan(start: i));
             i += ExifConstants.LittleEndianByteOrderMarker.Length;
 
-            uint ifdOffset = ((uint)i - startIndex) + 4U;
+            uint ifdOffset = (uint)i - startIndex + 4U;
-            uint thumbnailOffset = ifdOffset + ifdLength + exifLength + gpsLength;
 
             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 +101,6 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Exif
                 i = this.WriteData(startIndex, this.gpsValues, result, i);
             }
 
-            WriteUInt32(0, result, i);
-
             return result;
         }
 
@@ -263,7 +259,7 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Exif
             {
                 uint valueLength = GetLength(value);
 
-                length += 2 + 2 + 4 + 4;
+                length += 12;
 
                 if (valueLength > 4)
                 {
@@ -271,6 +267,9 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Exif
                 }
             }
 
+            // next IFD offset
+            length += 4;
+
             return length;
         }
 
@@ -361,6 +360,9 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Exif
                 newOffset += 4;
             }
 
+            // next IFD offset
+            newOffset = WriteUInt32(0, destination, newOffset);
+
             return newOffset;
         }