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Extracting IPTC header information from JPEG images

, 27 Aug 2001
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A sample class that manipulates information (e.g. caption, author, copyrights,...) that is stored in a JPEG file

This is our sample file

The sample application

File info from Adobe Photoshop

Introduction

Until I had to write an application that extracts image information from a JPEG image, I didn't know that a JPEG file can contain various information beside the pure image information (size, colors and image data). But it can contain quite a whole lot of textual and other sort of information like copyrights, captions, keywords and other stuff.

Adobe Photoshop can be used to manipulate this information from the File->File information menu. (This is probably the right place to mention that all Photoshop information here is translated from a German copy of Photoshop and thus might not exactly match the real names of the English version).

Ok. So you can edit the data with Photoshop. But what if you would like to use this data in your own application, let's say to store the images in a database along with the extracted information? That's the task of this class. You pass a file name, and then the member variables are filled. You can even modify them and write them back to the JPEG file. However this currently only works with files that already contain IPTC information.

Decoding the file format

As I tried to develop my application I searched for a similar application to reduce my work. I didn't find one. I even had a hard time to gather information on the JPEG file format specification (without buying some books) let alone the specification on the Photoshop headers. After firing up the Hex Editor I tried to find out how the things are stored. So all the structure of the Photoshop specific headers is more or less based on some findings from some sample files. If you have found a complete specification feel free to drop me a line or post it in the comments.

The basic structure is as follows. More information on that can be found here and at The Graphics File Formats Page.

JPEG imageContentsNameDescription
0xFF 0xD8SOIStart of image
Segments (see below)
0xFF 0xD9EOIEnd of image

JPEG segmentsDescription
Segment marker (2 bytes)
Segment size (2 bytes) excl. marker
Segment data

Some JPEG segment markers
ContentsNameDescription
0xFF 0xE0APP0Application marker (in every JPEG file)
0xFF 0xDBDQTQuantization Table
0xFF 0xC0SOF0Start of frame
0xFF 0xC4DHTDefine Huffman Table
0xFF 0xDASOSStart of scan
0xFF 0xEDAPP14This is the marker where Photoshop stores its information

The Photoshop segment

The APP14 segment is the one we are after. Here starts the non-documented area.

APP14 segmentContentsDescription
0xFF 0xEDAPP14 marker
Segment size (2 bytes) excl. marker
Photoshop 3.0\x00Photoshop identification string
8BIM segments (see below)

A JPEG file from Photoshop has various 8BIM (I don't know the real name) headers. The one with the type 0x04 0x04 contains the textual information. The image URL is stored in a different header. That's why it is currently not supported by the demo class. Other headers contain a thumbnail image and other information.

Photoshop 6 introduced a slight variation in this header segment. Basically the 4 byte padding has been replaced by a header description text of variable length. The updated sample can now handle these files as well.

8BIM segmentDescription
8BIM Segment marker (4 bytes)
Segment type (2 bytes)
Zero byte padding (4 bytes)
Segment size (2 bytes excl. marker, type, padding and size)
Segment data

The 8BIM header with the text is divided by even more headers, prefixed by 0x1C 0x02. These blocks then finally contain the information. Multiple blocks with the same type (e.g. Keywords) form a list.

0x1C 0x02 segmentDescription
0x1C 0x02 Segment marker (2 bytes)
Segment type (1 byte)
Segment size (2 bytes excl. marker, type and size)
Segment data

The sample application

With a description of the format of the information it is easy to write an application that scans through the file and extract the interesting bytes. This is exactly what the provided sample class does.

Have fun with it!

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

Christian Tratz
Web Developer
Germany Germany
If you are not living on the edge you are wasting space Wink | ;)

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Comments and Discussions

 
Generalit's really helpful!!! Pin
lucid_bao3-Jun-12 2:59
memberlucid_bao3-Jun-12 2:59 
GeneralVery good. Header parser needs small fix. Pin
Roni Fital7-Sep-10 21:21
memberRoni Fital7-Sep-10 21:21 
Generalgood job Pin
Member 455517230-Jun-10 11:38
memberMember 455517230-Jun-10 11:38 
GeneralVB.NET/C# Pin
pvatanpour4-May-09 4:46
memberpvatanpour4-May-09 4:46 
General8BIM format table is wrong Pin
Vidbuff28-Dec-06 3:19
memberVidbuff28-Dec-06 3:19 
Generalnothing happen Pin
Ibrahim Hamoud11-Dec-06 11:47
memberIbrahim Hamoud11-Dec-06 11:47 
Generalthanks Pin
scarabée15-Nov-06 22:40
memberscarabée15-Nov-06 22:40 
GeneralJPEG header format Pin
8820910-Jul-06 0:00
member8820910-Jul-06 0:00 
GeneralSave crush Pin
Arnon A19-Jan-06 7:22
memberArnon A19-Jan-06 7:22 
GeneralRe: Save crush Pin
Christian Tratz2-May-06 22:05
memberChristian Tratz2-May-06 22:05 
GeneralNew header don't work.... Pin
gothico23-Jan-05 12:48
membergothico23-Jan-05 12:48 
GeneralRe: New header don't work.... Pin
Chris Blue1-Apr-10 18:05
memberChris Blue1-Apr-10 18:05 
GeneralIt's APP13, not APP14 Pin
Phil Harvey13-Jan-05 3:07
sussPhil Harvey13-Jan-05 3:07 
GeneralProblem Pin
André-perpetuo3-Sep-04 6:12
memberAndré-perpetuo3-Sep-04 6:12 
GeneralCode Errors Pin
CodeChampion22-Mar-04 21:55
sussCodeChampion22-Mar-04 21:55 
GeneralRe: Code Errors Pin
saveorg17-Aug-04 5:42
membersaveorg17-Aug-04 5:42 
GeneralSlightly wrong - here's the real 8BIM fmt Pin
imagerodeo21-Feb-04 20:41
memberimagerodeo21-Feb-04 20:41 
GeneralRe: Slightly wrong - here's the real 8BIM fmt Pin
CodeChampion22-Mar-04 22:01
memberCodeChampion22-Mar-04 22:01 
GeneralRead Header of all JPEG Pin
alperen10-Jan-04 18:02
memberalperen10-Jan-04 18:02 
GeneralRe: Read Header of all JPEG Pin
Christian Tratz11-Jan-04 7:16
memberChristian Tratz11-Jan-04 7:16 
GeneralRe: Read Header of all JPEG Pin
alperen11-Jan-04 8:44
memberalperen11-Jan-04 8:44 
GeneralNeed 8BIM Help Pin
jdoklovic24-Oct-03 7:00
memberjdoklovic24-Oct-03 7:00 
GeneralApplication Independent IPTC data extraction Pin
Priyanka Ahuja18-Sep-03 4:31
memberPriyanka Ahuja18-Sep-03 4:31 
GeneralRe: Application Independent IPTC data extraction Pin
Christian Tratz18-Sep-03 5:09
memberChristian Tratz18-Sep-03 5:09 
GeneralRe: Application Independent IPTC data extraction Pin
Priyanka Ahuja18-Sep-03 22:21
memberPriyanka Ahuja18-Sep-03 22:21 

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