I want to make a tool for myself (and maybe others if it comes out good) for low level inspection/modification of JFIF files. Sort of like TweakPNG, but for JFIFs. So far I haven't found any tool that does this.
In order to do that I need to read all the relevant specifications. Unfortunately the official ones are pretty costly, so I'm looking for alternate sources. Is there anything you can recommend which covers as much ground as possible?
To answer my own question - the specification comes in several parts:
can be downloaded from the W3 page and the corrigendum (which contains only patent information) from the ITU page. Direct links: Part 1, Corrigendum.
I cannot find, but apparently it only includes test cases for application compliance to part 1. Useful, but not necessary.
and its amendment detail some extensions and are again available from ITU. Direct links: Part 3, Amendment.
is available from ITU as well, and details... I'm not sure yet, what. Either some more extensions and a SPIFF file format (never heard of it before); or methods for registering new extensions. Possibly both. Anyway, direct link: Part 4.
...oh, wait, there are only 4 parts! But there is a JFIF specification. That one's a bit hairy. Apparently it's at version 1.02, and was developed in good old 1992, but that's pretty unofficial. ISO is still working on formalizing it, but in the meantime ECMA has already done so by simply reformatting it to their typical specification style. Anyways, the two things should be the same, as I understand. You can find them on the W3 page and ECMA page. Direct links: Original, ECMA style (edition June 2009).
(I like imaginary parts!) would be the Exif standard. According to Wikipedia it's never been standardized, but everyone adheres to the unoffical standard by JEIDA/CIPA. Wikipedia also had this direct link to the latest version of the spec (2010!): EXIF 2.3 (2023): Exif 3.0.
Well, that's it folks! Happy JPEGing!