Hi Rainer
Pasjpeg is basically just a Pascal translation of sources originally written in ANSI C by IJG, Independent JPEG Group. You can find this C code with documentation at http://www.ijg.org/. At these URL there are also some informations about JPEG file coding (actually JFIF, JPEG is a standard for compressing image data and JFIF is a file format that uses JPEG compression; JFIF stands for "JPEG File Interchange Format"). But you will not find there a complete specification of JPEG format and, to be honest, I don't know where you can find it on the internet. Definitely, coding JPEG [de]compression by yourself is really an advanced topic and I would not advice you this.
Besides, Pasjpeg can really do all that you want. Pasjpeg contains some extra units not found in IJG code - mainly Jeg.pas and Pasjpeg.pas.
The first one implements a class that loads a jpeg image into a TGraphic class descendant from Graphics unit. Graphics unit comes from Delphi's VCL and it's not available with FPC - but I know Lazarus tries to implement library compatible with VCL (but Lazarus does it for FPC and for many platforms FPC supports), so maybe you could integrate Jpeg with Lazarus (I don't know, maybe they even already did that in Lazarus ?). So that's the first option you can try.
The second option is to take Jpeg unit and tweak it so that it is capable of reading and storing jpeg images in your preferable format, like simple two-dimensional array of RGB. For that you will definitely need IJG documentation to understand how Jpeg unit works - it's really too big subject to describe it in this letter.
Third option is probably the easiest one - you can use Pasjpeg unit to convert jpeg files into BMP files. Than you can learn how to read BMP files, it's really simple, and after doing that you will be able to read both BMP and JPEG images in quite the same way, you will only have to "filter" JPEG images through Pasjpeg. Not very elegant way to read JPEG images, but simple and working.
There is also a forth option, probably the hardets one : read IJG documentation and write your own code to handle pasjpeg library without Pasjpeg and Jpeg units (I hope you don't get confused when I sometimes use "pasjpeg" to name the whole "pasjpeg library" and sometimes to name the specific unit named "Pasjpeg.pas" in that library). Actually, if you learn how to do that you will not need pasjpeg library - under Linux there is usually a libjpeg.so shared library that contains whole IJG code, so you will not have to use pasjpeg library - just link dynamically to libjpeg.so (or statically to libjpeg.a).
Final words : what do I advise : take the easiest way - learn how to read BMP images and than use Pasjpeg unit. Also, there is a question - do you really need the JPEG format ? If you want just ANY image format than thinking about JPEG is a big mistake - there are many image formats MUCH easier to read. E.g. PPM (the simplest possible image format, you can handle it with a few lines of code), PNG (nice format with lots of capabilities and lossless compression (you DO know that JPEG compression is lossy, do you?), very easily readable with libpng) and so on. BMP format is somewhere between, it's a bit more complicated and a bit more functional than PPM - if you don't care about JPEGs anymore I would advice you PPM (or more general PNM) or PNG formats.
And remember there are many programs that can convert JPEG images into other formats (like PPM and PNG).
Oh, one more thing - Pasjpeg unit uses TStream classes and you said you dont like Classes. But, really, they are so simple - I guarantee that you will be glad when you learn how to use them and when you see what TStream and it's descendants can do for you.
Hope this helps,
