JPEG thumbnail formats

How many ways are there to put an extra “thumbnail” or “preview” image inside a JPEG (.jpg) image file? This isn’t just trivia. One problem with thumbnails is the possibility that they might be neither updated nor deleted when the main image is edited. If you’re not careful, your edited JPEG file could contain a……

GIF plain text extensions

Plain text extensions is an obsolete feature of the GIF (version 89a) image format. Essentially, it allows an animated GIF to have ASCII text frames, as well as the usual graphical frames. As far as I know, it was pretty much never used at all. The only GIF file with plain text extensions that I’m……

X-Face format

X-Face is an odd little graphics format, designed around 1990 as a standard way to include your picture/icon/avatar in the email messages you send. It’s not exactly a file format; it’s a special string of text that can be included in email headers. A number of email applications supported it, and maybe some still do.……

Survey of MacPaint brush patterns

MacPaint is a paint program for early Macintosh computers. You can see it, and try it out, at the Internet Archive: <https://archive.org/details/mac_Paint_2&gt;. Its native image file format is very limited, and very obsolete. But a large number of MacPaint files were created, back in the day, and some of them can still be found. For……

Corrupting ZIP files

The ZIP (PKZIP) compressed file format is organized in a backwards fashion. To unzip a ZIP file, you start by looking for a special byte sequence near the end of it. This is a marker for a data structure (the end of central directory) that points to other data structures that appear earlier in the……

The cpio trailer problem

cpio is an archive file format, similar in concept to ZIP or tar. It combines multiple files (which I’ll call the “member files”) together in a single .cpio archive file. It is an old format, designed to be useful with magnetic tape drives. Here’s a demonstration, using a Unix shell, and the GNU cpio utility.……

An insignificant GIF specification error

I study a lot of file formats, and sometimes I come across a quirk in a format, or its documentation, that isn’t worth making a fuss about, but which might still be interesting to two or three people in the world. I suppose this one doesn’t even meet that criteria, though it is in a……