JPG Structure

Posted on by Thor 1 Comment

If you hadn’t been over to see the posters made by Ange Albertini, head over now. Below is his poster on the JPG image file format. This is the basic JFIF file format, which stands for JPEG File Interchange Format. There are also raw JPEG streams and Exif, Exchangeable Image File Format.

The basic format is pretty straight forward. There is a start of image marker FFD8 some format information, then the raster compressed data, then an end of image marker FFD9. Identification of a JPEG file should be pretty straight forward. Knowing the start and end marker values and then the type of JPEG based on the Application data, can be very specific. That is until some software engineers start playing fast and loose with the format specifications.

A while back I received a JPG file which didn’t identify using the latest PRONOM signature. It’s happened before, some new phones came out and started using a newer version of the exif specification so I submitted an update to PRONOM for JPG’s using exif 2.3 and greater. But also may need to submit another signature soon for the newly released Exif 3.0 specification! But this JPG I received wasn’t a new version, it should have been identified with the current PRONOM signature. It started with FFD8 and when I went to look at the end of the file for the end of image marker FFD9, it wasn’t where I expected it to be.

This JPG file had an additional 9632 bytes after the FFD9 end of image marker. But why? The image rendered just fine in multiple JPG viewers. The only warning from Exiftool was for “Unrecognized MakerNotes”, which is not too uncommon. So I went to the JPG Exif specification.

EOI, Recording this marker is mandatory. It shall be recorded in this position.

But reading a little further we see…..

Moreover, Exif/DCF readers should be implemented to operate without interruption even if certain kinds of data have been recorded after EOI of the primary image defined in the Exif standard. Specifically, unknown data after EOI of the primary image should be skipped. (see section 4.7.1)

So the extra data is allowed by specification. Any readers should ignore or skip any data after the EOI (End of Image). Well that makes identification more difficult. All the PRONOM signatures are based on having the EOI marker at the “End”. Some have allowance for padding, but not enough for the worst offenders……

The image referenced above was created on a Huawei MHA-L29 cameraphone. But since finding this image, I have also found many Samsung phones do the same thing. Here is one from a Samsung SM-G975U1. Much less padding but enough to throw off identification.

Apple iPhones are also not exempt from this “feature” either. When using the MacOS ImageCapture tool with the HEIC format, a bug can add an excessive amount of empty data at the end of the converted JPG file.

So, when it comes to identification, if your JPG files don’t seem to identify correctly, look closer at the end of the file, it may have some “extra” data.

Corel ArtShow

Posted on by Thor Leave a comment

File extensions are the easiest way to quickly identify a file format, but they can be misleading. This is the reason in Digital Preservation format identification tools like DROID are important to look closer at the file structure to more accurately identify formats. The other complication is some extensions are used for more than one format. Extensions like .DOC or .ISO can be used with many formats.

The PRONOM registry which DROID uses will list extensions associated with each format signature, but for some, they only have an extension and no signature. It’s nice to have an official ID to go with a format but with no signature it only matches based on extension.

This caused a problem awhile back for me while working with some files with the extension CDX. Which according to PRONOM, there are 5 completely different formats which use the extension, and probably others.

My CDX was related to some indexing software called Cindex. At the time the only format with a signature was for the WARC summary file CDX. The other was for a CorelDraw Compressed format with no signature. Confusing right? When I would run format identification on my Cindex files, they would default to the CorelDraw Compressed format, identified by extension. It was easy enough to create a signature for the Cindex format as I had enough samples to know the patterns needed for correct identification. But I was curious about the CorelDraw format. Should be easy to find, right?

Wrong. Finding a sample of this format was very elusive. All I had to go by was the name given to the format by PRONOM and the extension. I scoured every Corel CD and image I could get my hands on. For months I looked and could never find a single CDX file. Each CorelDraw software I was able to run did not have any ability to save in the CDX format. I scoured clipart discs, other Corel software, like Designer, PrintHouse, Photo-Paint, nada, nothing. I started to wonder if the format even existed. That’s when I noticed in the filters included with CorelDraw a reference to the ability to import a CDX but not write to one.

[CDX]
Signature=CORELFILTER - A
FilterEntry=1
Description=CorelDRAW Compressed (CDX)
FilterFullName=CorelDRAW Import Filter
Version=Version 6.00
Company=Corel Corporation
Copyright=Copyright © 1988-1995 Corel Corporation
Extensions=*.CDX
CorelID=0x704
FilterCapability1=0x9000
FilterCapability2=0x0
NoOfCompressions=0

This led to me finding a reference on the old Corel FTP site for knowledge base number 4550.

It mentioned something called ArtShow, where version 5 supported the file format CDX. ArtShow was a gallery of winning designs released on a CD-ROM and book each year. The first one being ArtShow 91, then ArtShow 3, 4, 5, 6, and finally 7 was the last. Each one released used a different proprietary compressed format for storing all the designs, these formats exist nowhere else. The question remains, why didn’t they use other popular Corel formats like CDR, CMX, or CCX which were used on many other clip art titles.

It took some time but I was finally able to find copies of a few of the Artshow CD-ROM discs, especially numbers 5 & 6. Which had the CDX format and the second generation CPX formats.

Each format had a easy to recognize header making a PRONOM signature easy to create. PRONOM already had the PUID for the two formats CDX & CPX, so sending in the signature added to the registry and hopefully will help distinguish between all the CDX formats!