The Metadata in JPEG files¶

This article shares my investigations about the metadata structure in JPEG files. It also introduces briefly the algorithm used in exiv2 to read and write Exif, IPTC, XMP data and image comments on the JPEG files.

1. Background¶

JPEG ISO standard is a commonly used method of lossy compression for digital photography. The name "JPEG" stands for Joint Photographic Experts Group, the name of the committee.

JPEG refers only to a class of compression algorithms, not to a specific file format. In order to produce files with embedded JPEG streams, a number of file format standards have been adapted or devised. Some of them are JPEG /JFIF, JPEG /SPIFF (Still Picture Interchange File Format), JPEG /CIFF, JPEG/Exif (Exchangeable image file format).

Among them, the most common types are JPEG/Exif and JPEG/JFIF.

• JPEG/Exif is the most common image format used by digital cameras and other photographic image capture devices.
• JPEG/JFIF is the most common format for storing and transmitting photographic images on the World Wide Web.

2. The metadata structure in JPEG¶

A JPEG file contains several segments; each segment contains different kinds of data, delimited by two-byte codes called markers. The markers are hexadecimal; they begin with 0xFF and end with a code (1 byte) indicating the kind of marker.

Some markers consist of just those two bytes; others are followed by two bytes indicating the length of marker-specific payload data that follows. The length includes the two bytes for the length, but not the two bytes for the marker.

Short name	Bytes	Payload	Name and Comments
SOI	0xFF, 0xD8	None	Start Of Image
SOF0	0xFF, 0xC0	Variable size	Start Of Frame (Baseline DCT) Indicates that this is a baseline DCT-based JPEG, and specifies the width, height, number of components, and component subsampling
SOF2	0xFF, 0xC2	Variable size	Start Of Frame (Progressive DCT) Indicates that this is a progressive DCT-based JPEG, and specifies the width, height, number of components, and component subsampling
DHT	0xFF, 0xC4	Variable size	Define Huffman Table(s)
DQT	0xFF, 0xDB	Variable size	Define Quantization Table(s)
DRI	0xFF, 0xDD	2 bytes	Define Restart Interval Specifies the interval between RSTn markers, in macroblocks. This marker is followed by two bytes indicating the fixed size so it can be treated like any other variable size segment.
SOS	0xFF, 0xDA	Variable size	Start Of Scan Begins a top-to-bottom scan of the image. In baseline DCT JPEG images, there is generally a single scan. Progressive DCT JPEG images usually contain multiple scans. This marker specifies which slice of data it will contain, and is immediately followed by entropy-coded data.
RSTn	0xFF, 0xDn n(n=0..7)	None	Restart Inserted every r macroblocks, where r is the restart interval set by a DRI marker. Not used if there was no DRI marker. The low 3 bits of the marker code cycle in value from 0 to 7.
APPn	0xFF, 0xEn	Variable size	Application-specific For example, an Exif JPEG file uses an APP1 marker to store metadata, laid out in a structure based closely on TIFF.
COM	0xFF, 0xFE	Variable size	Comment
EOI	0xFF, 0xD9	None	End Of Image

Fig.1. The common JPEG markers. From Wikipedia, https://en.wikipedia.org/wiki/JPEG

The metadata in JPEG file is stored in APPn (0xFF, 0xEn) segment and the comment is stored in COM segment (0xFF, 0xFE). Several vendors might use the same APPn marker type to include their information, so these markers often begin with a vendor name (e.g., "Exif" or "Adobe") or some other identifying string.

Exiv2 provides fast and easy read write access to the Exif, IPTC and XMP. Hence, this article only focuses on the position of Exif, IPTC and XMP data in JPEG files.

2.1 Exif¶

Exif JPEG file uses an APP1 segment to store the information (and multiples APP2 segments for flashPix data). Exif APP1 segment stores a great amount of information on photographic parameters for digital cameras and it is the preferred way to store thumbnail images nowadays. It can also host an additional section with GPS data. All details about Exif are available at http://www.exif.org/Exif2-2.PDF

In theory, Exif APP1 is recorded immediately after the SOI marker (the marker indicating the beginning of the file). However, this leads to the incompatibility between the Exif and JFIF standards because both of them specify that their particular application segment (APP0 for JFIF, APP1 for Exif) must be the first in the image file. In practice, most JPEG files contain a JFIF marker segment (APP0) that precedes the Exif APP1. This allows older readers to correctly handle the format JFIF segment, while newer readers also decode the following Exif segment, being less strict about requiring it to appear first. This way will not affect the image decoding for most decoders, but poorly designed JFIF or Exif parsers may not recognize the file properly.

Exif APP1 segment consists of the APP1 marker (0xFFE1), Exif identifier string (“Exif\0\0”), and the attribute information itself. The identifier string "Exif\0\0” is used to avoid a conflict with other applications using APP1 (e.g XMP).

Fig.2. Basic Structure of JPEG Files. From Exif.org, http://www.exif.org/Exif2-2.PDF

Exif does not use APPn segments other than APP1, APP2 and COM segments. However, some unknown APPn may still exist on the file structure and Exif readers should be designed to skip over them.

2.2 XMP¶

In a typical edited JPEG file, XMP (eXtensible Metadata Platform) information is typically included alongside Exif and IPTC (Information Interchange Model data). XMP uses an APP1 segment in order to store metadata information; the storage format is RDF (Resource Description Framework) implemented as an application of XML.

XMP APP1 segment consists of the APP1 marker (0xFFE1), XMP identifier string (“http://ns.adobe.com/xap/1.0/\x00”), and Unicode XMP packet (the encoding is usually UTF-8, but it can also be UTF-16 or UTF-32). The packet cannot be split in multiple segments, so there is a maximum size of approximately 64KB (2^16-1 bytes).

The structure of the packet content can be found at http://www.w3.org/TR/REC-rdf-syntax/.
The reference document for XMP 3.2 can be downloaded from Adobe Systems Incorporated http://xml.coverpages.org/xmp.html

2.3 IPTC¶

Adobe Photoshop uses the APP13 segment for storing non-graphic information, such as layers, paths, IPTC data and more. The content of an APP13 segment is formed by APP1 marker (0xFFE1), an identifier string (usually "Photoshop 3.0\000", but also 'Adobe_Photoshop2.5:', used by earlier versions) followed by a sequence of resource data blocks. In general, a resource block contains only a few bytes, but there is the important IPTC block can be quite large. The IPTC block may not fit into one APP13 segment, so it can be split into multiple APP13 segments.

The reference document for the Photoshop file format is available at http://www.adobe.com/devnet-apps/photoshop/fileformatashtml/

3. Exiv2 JPEG read/write metadata algorithms¶

This section introduces briefly about the algorithm used in Exiv2. For more details, please download the source code of Exiv2 and read the code in src/jpgimage.cpp file.

3.1 Read Algorithm¶

From the above investigation, the read algorithm is quite simple. We just need to go through the markers, find and read the content of Exif APP1, XMP APP1, IPTC APP13 segments. Those segments all locate before the SOS segment (which is immediately followed by entropy-coded data) and often locate right after the SOI segment (not guaranteed). Hence, it’s not necessary to read the whole JPEG file to check whether the metadata exists or not; the process can be stopped whenever the marker SOS is found.

Fig.3. Flowchart of readMetadata

Notes:

• The standard JPEG files should have at most one Exif APP1 segment and one XMP APP1 segment. Hence, if there’s more than one Exif APP1 segment or one XMP APP1 segment, exiv2 only reads the first segment.
• JPEG files can have multiple comments, but for now exiv2 only reads the first one (most of the jpeg files only have one anyway).

3.2 Write Algorithm¶

The general idea for the write algorithm is to find Exif APP1, XMP APP1, IPTC APP13 segments, remove them from the JPEG file and replace them with the new metadata. To simplify this a bit, we agree the following rules (this is standard in most jpegs anyway).

• The order of the Exif, XMP, IPTC segments in the output file (regardless of their positions in the input file)
  SOI | (APP0) | (Exif App1) | (XMP App1) | (IPTC App13) | … | EOI
• The COM segment is located just before SOFn in the output file (regardless of its position in the input file).

Fig.4. Flowchart of writeMetadata

Example:
The following is an example to show the changes in the structure of JPEG files after the writeMetadata. In this example, after adding the Exif.Photo.UserComment, not only the Exif APP1 segment is added, but the IPTC APP13 is also moved to right after Exif APP1.

$ ./exiv2 -pa test.jpg 
Iptc.Application2.RecordVersion              Short       1  2
Iptc.Application2.Keywords                   String      4  2010
Iptc.Application2.Keywords                   String     10  California
Iptc.Application2.Keywords                   String      9  Christmas
Iptc.Application2.Keywords                   String      9  canderson
Iptc.Application2.Keywords                   String      6  nature
Iptc.Application2.Keywords                   String      6  winter
Iptc.Application2.Copyright                  String     12  (C)CANDERSON
$ ./exifprint test.jpg -struc 
STRUTURE OF FILE:
marker | size | signature
0xd8            
0xe0        16  JFIFHH
0xe2      3160  ICC_PROFILE
                           HLinomntrRGB 
0xed     18462  Photoshop 3.08BIM
0xe1     34447  XMP://ns.adobe.com/xmp/extension/
0xdb        67  
0xdb        67  
0xc0        17  
0xc4        31  
0xc4        70  
0xc4        29  
0xc4        68  
0xda        12  
0xd9
-----------------
Caught Exiv2 exception 'test.jpg: No Exif data found in the file'
$ ./exiv2 -M"add Exif.Photo.UserComment Hello" test.jpg   
$ ./exifprint test.jpg -struc                          
STRUTURE OF FILE:
marker | size | signature
0xd8            
0xe0        16  JFIFHH
0xe1        66  ExifIIi
0xed     18462  Photoshop 3.08BIM
0xe2      3160  ICC_PROFILE
                           HLinomntrRGB 
0xe1     34447  XMP://ns.adobe.com/xmp/extension/
0xdb        67  
0xdb        67  
0xc0        17  
0xc4        31  
0xc4        70  
0xc4        29  
0xc4        68  
0xda        12  
0xd9
-----------------
Exif.Image.ExifTag                           0x8769 Long        1  26
Exif.Photo.UserComment                       0x9286 Undefined  13  Hello

Note:

• XMP://ns.adobe.com/xmp/extension/ is not the valid XMP identifier string in Exiv2. The valid string should be http://ns.adobe.com/xap/1.0/\x00. Hence, XMP doesn't appear in the output.
• The test.jpg is attached at the bottom of this wiki. You can download and reproduce the above example easily.

References¶

1. Wikipedia. JPEG. http://en.wikipedia.org/wiki/JPEG.
2. Wikipedia. Exif. http://en.wikipedia.org/wiki/Exif.
3. Standard of Japan Electronics and Information Technology Industries Association. Exchangeable image file format for digital still cameras Exif Version 2.2. (Apr, 2002) http://www.exif.org/Exif2-2.PDF
4. C-Cube Microsystems. JPEG File Interchange Format Version 1.02. (Sep, 1992). http://www.w3.org/Graphics/JPEG/jfif3.pdf
5. Adobe. Adobe Photoshop File Formats Specification. (Jun, 2013). http://www.adobe.com/devnet-apps/photoshop/fileformatashtml/
6. Coverpages.org. Extensible Metadata Platform (XMP). http://xml.coverpages.org/xmp.html
7. W3C. RDF/XML Syntax Specification. (Feb, 2004). http://www.w3.org/TR/REC-rdf-syntax/
8. CPAN. The structure of JPEG files. http://search.cpan.org/dist/Image-MetaData-JPEG/lib/Image/MetaData/JPEG/Structures.pod
9. Fileformat.info. JPEG File Interchange Format File Format Summary. http://www.fileformat.info/format/jpeg/egff.htm
10. Ehow.com. The Structure of a JPEG. http://www.ehow.com/info_12202329_structure-jpeg.html