Fix Exif thumbnail offset

Fixes #578

The offset is currently reported relative to the start of the Exif data. However, most users will need the value relative to the start of the outermost data stream. This change attempts to provide that value.

Should work for JPEG and RAW files. Won't work for Exif data embedded within other formats such as PNG, QuickTime, WebP or HEIF. Such support could be added in future.

Source/com/drew/imaging/png/PngMetadataReader.java

@@ -330,7 +330,7 @@ private static void processChunk(@NotNull Metadata metadata, @NotNull PngChunk c
             metadata.addDirectory(directory);
         } else if (chunkType.equals(PngChunkType.eXIf)) {
             try {
-                ExifTiffHandler handler = new ExifTiffHandler(metadata, null);
+                ExifTiffHandler handler = new ExifTiffHandler(metadata, null, 0);
                 new TiffReader().processTiff(new ByteArrayReader(bytes), handler, 0);
             } catch (TiffProcessingException ex) {
                 PngDirectory directory = new PngDirectory(PngChunkType.eXIf);

Source/com/drew/imaging/tiff/TiffMetadataReader.java

@@ -67,7 +67,7 @@ public static Metadata readMetadata(@NotNull InputStream inputStream) throws IOE
     public static Metadata readMetadata(@NotNull RandomAccessReader reader) throws IOException, TiffProcessingException
     {
         Metadata metadata = new Metadata();
-        ExifTiffHandler handler = new ExifTiffHandler(metadata, null);
+        ExifTiffHandler handler = new ExifTiffHandler(metadata, null, 0);
         new TiffReader().processTiff(reader, handler, 0);
         return metadata;
     }

Source/com/drew/metadata/exif/ExifReader.java

@@ -87,7 +87,7 @@ public void extract(@NotNull final RandomAccessReader reader, @NotNull final Met
     /** Reads TIFF formatted Exif data at a specified offset within a {@link RandomAccessReader}. */
     public void extract(@NotNull final RandomAccessReader reader, @NotNull final Metadata metadata, int readerOffset, @Nullable Directory parentDirectory)
     {
-        ExifTiffHandler exifTiffHandler = new ExifTiffHandler(metadata, parentDirectory);
+        ExifTiffHandler exifTiffHandler = new ExifTiffHandler(metadata, parentDirectory, readerOffset);
 
         try {
             // Read the TIFF-formatted Exif data

Source/com/drew/metadata/exif/ExifThumbnailDescriptor.java

@@ -63,7 +63,8 @@ public String getThumbnailLengthDescription()
     @Nullable
     public String getThumbnailOffsetDescription()
     {
-        String value = _directory.getString(TAG_THUMBNAIL_OFFSET);
-        return value == null ? null : value + " bytes";
+        Integer offset = _directory.getAdjustedThumbnailOffset();
+
+        return offset == null ? null : offset.intValue() + " bytes";
     }
 }

Source/com/drew/metadata/exif/ExifThumbnailDirectory.java

@@ -34,7 +34,11 @@
 public class ExifThumbnailDirectory extends ExifDirectoryBase
 {
     /**
-     * The offset to thumbnail image bytes.
+     * The offset to thumbnail image bytes, relative to the start of the IFD.
+     *
+     * To obtain the offset relative to the start of the TIFF data stream, use
+     * <code>getAdjustedThumbnailOffset</code>, which includes the value of
+     * <code>getExifStartOffset</code>.
      */
     public static final int TAG_THUMBNAIL_OFFSET = 0x0201;
     /**
@@ -59,8 +63,11 @@ public class ExifThumbnailDirectory extends ExifDirectoryBase
         _tagNameMap.put(TAG_THUMBNAIL_LENGTH, "Thumbnail Length");
     }
 
-    public ExifThumbnailDirectory()
+    private final int _exifStartOffset;
+
+    public ExifThumbnailDirectory(int exifStartOffset)
     {
+        _exifStartOffset = exifStartOffset;
         this.setDescriptor(new ExifThumbnailDescriptor(this));
     }
 
@@ -77,4 +84,35 @@ protected HashMap<Integer, String> getTagNameMap()
     {
         return _tagNameMap;
     }
+
+    /**
+     * Gets the offset at which the Exif data stream commenced within any containing stream.
+     */
+    public int getExifStartOffset()
+    {
+        return _exifStartOffset;
+    }
+
+    /**
+     * Returns the offset to thumbnail data within the outermost data stream.
+     *
+     * The value for <code>TagThumbnailOffset</code> is relative to the Exif data stream.
+     * Generally, consumers of thumbnail data need this value relative to the outermost stream,
+     * so that the thumbnail data may be extracted from that stream.
+     *
+     * This property adds the value of <code>ExifStartOffset</code> to this tag's value in order
+     * to produce that value.
+     *
+     * Returns <code>null</code> when the tag is not defined in this directory.
+     */
+    public Integer getAdjustedThumbnailOffset()
+    {
+        Integer offset = this.getInteger(TAG_THUMBNAIL_OFFSET);
+
+        if (offset == null) {
+            return null;
+        }
+
+        return offset.intValue() + _exifStartOffset;
+    }
 }

Source/com/drew/metadata/exif/ExifTiffHandler.java

@@ -57,9 +57,12 @@
  */
 public class ExifTiffHandler extends DirectoryTiffHandler
 {
-    public ExifTiffHandler(@NotNull Metadata metadata, @Nullable Directory parentDirectory)
+    private final int _exifStartOffset;
+
+    public ExifTiffHandler(@NotNull Metadata metadata, @Nullable Directory parentDirectory, int exifStartOffset)
     {
         super(metadata, parentDirectory);
+        _exifStartOffset = exifStartOffset;
     }
 
     public void setTiffMarker(int marker) throws TiffProcessingException
@@ -149,7 +152,7 @@ public boolean hasFollowerIfd()
             if (_currentDirectory.containsTag(ExifDirectoryBase.TAG_PAGE_NUMBER))
                 pushDirectory(ExifImageDirectory.class);
             else
-                pushDirectory(ExifThumbnailDirectory.class);
+                pushDirectory(new ExifThumbnailDirectory(_exifStartOffset));
             return true;
         }
 

Source/com/drew/metadata/photoshop/PhotoshopTiffHandler.java

@@ -33,7 +33,7 @@ public class PhotoshopTiffHandler extends ExifTiffHandler
 
     public PhotoshopTiffHandler(Metadata metadata, Directory parentDirectory)
     {
-        super(metadata, parentDirectory);
+        super(metadata, parentDirectory, 0);
     }
 
     public boolean customProcessTag(final int tagOffset,

Source/com/drew/metadata/tiff/DirectoryTiffHandler.java

@@ -67,21 +67,26 @@ protected void pushDirectory(@NotNull Class<? extends Directory> directoryClass)
             throw new RuntimeException(e);
         }
 
+        pushDirectory(newDirectory);
+    }
+
+    protected void pushDirectory(@NotNull Directory directory)
+    {
         // If this is the first directory, don't add to the stack
         if (_currentDirectory == null) {
             // Apply any pending root parent to this new directory
             if (_rootParentDirectory != null) {
-                newDirectory.setParent(_rootParentDirectory);
+                directory.setParent(_rootParentDirectory);
                 _rootParentDirectory = null;
             }
         }
         else {
             // The current directory is pushed onto the stack, and set as the new directory's parent
             _directoryStack.push(_currentDirectory);
-            newDirectory.setParent(_currentDirectory);
+            directory.setParent(_currentDirectory);
         }
 
-        _currentDirectory = newDirectory;
+        _currentDirectory = directory;
         _metadata.addDirectory(_currentDirectory);
     }
 

Tests/com/drew/metadata/MetadataTest.java

@@ -96,7 +96,7 @@ public void testOrderOfDifferentTypes()
     {
         Metadata metadata = new Metadata();
         Directory directory1 = new ExifSubIFDDirectory();
-        Directory directory2 = new ExifThumbnailDirectory();
+        Directory directory2 = new ExifThumbnailDirectory(0);
         Directory directory3 = new ExifIFD0Directory();
 
         metadata.addDirectory(directory1);

Tests/com/drew/metadata/exif/ExifDirectoryTest.java

@@ -46,7 +46,7 @@ public void testGetDirectoryName() throws Exception
     {
         Directory subIFDDirectory = new ExifSubIFDDirectory();
         Directory ifd0Directory = new ExifIFD0Directory();
-        Directory thumbDirectory = new ExifThumbnailDirectory();
+        Directory thumbDirectory = new ExifThumbnailDirectory(0);
         Directory gpsDirectory = new GpsDirectory();
 
         assertFalse(subIFDDirectory.hasErrors());

Tests/com/drew/metadata/exif/ExifThumbnailDescriptorTest.java

@@ -36,7 +36,7 @@ public class ExifThumbnailDescriptorTest
     @Test
     public void testGetYCbCrSubsamplingDescription() throws Exception
     {
-        ExifThumbnailDirectory directory = new ExifThumbnailDirectory();
+        ExifThumbnailDirectory directory = new ExifThumbnailDirectory(0);
         directory.setIntArray(TAG_YCBCR_SUBSAMPLING, new int[]{2, 1});
 
         ExifThumbnailDescriptor descriptor = new ExifThumbnailDescriptor(directory);