I'm no expert, but I like to consider myself an adept when it comes to video and subtitles. Let me clear things up.
Hard Subs
"Hard" subs mean that they cannot be turned off. They are made part of the video images. These are the most common type for fansubs, since this means
AVI can be used. If you're unsure if you have hard subs, the best way to tell is how they look. You could easily pause the video file and check for compression artifacts around the lettering. You could also see if multiple tracks of subtitles appear in different colors, if they move, are slanted/tilted, use different fonts (Or fonts other than Arial), appear in non-Latin (languages not using alphabets similar to English's) or appear in several different places.
Soft Subs
These are subtitles that can be turned on or off. They are separate from the video images. These are a little less common. Generally, if you have soft subs, you know, but just in case, a soft subtitle will usually appear in Arial, will stay near the bottom, centered, will stay the same color, will not move with the picture, no compression artifacts around it, and usually only appear in Latin. Generally, these look a lot like the subtitles one would see in a DVD. Often times, the soft subs will appear sharper than the video they are in, sometimes even sharper when the video is enlarged.
Soft Subs in Video Files
Generally, Soft Subs are stored in one of two methods:
Embedded
Separate file
Separate files are the most common, since they can be easily used with AVI files. To use these files, most of the time all you have to do is make sure the subtitle file has the same name (up to the extension) as the video file. (Ex: FF7AC.avi and FF7AC.ssa would work)
Embedded subtitles are NOT supported by AVI. The AVI format is over ten years old and was never intended for such a use. When the Wikipedia refers to subtitles working "only through hacks" the general gist is that the subtitle data is usually lumped at the end of the file, and is 90% of the time not detected, or readable (from within the file at least) by most video players. The disadvantages to these hacks are that, if the embedded subtitle in AVI works, it's not interleaved (the video is chopped up into chunks that contain all the information needed for a particular part of the file: A chunk of audio, and video, and in non-AVI format, subtitles) so the hard disk often spends a lot of the time seeking, which can slow down read and playback time, as well as adding unnessessary wear and tear to the hard disk. Generally, the more accepted examples are to take the "hacked" subtitles, put them in their own file, and play it that way, or convert the AVI format to OGM of MKV (discussed later, can be done very quickly with no loss of quality. This can even fix some "broken" AVIs which have problems with seeking) with embedded subtitles.
The most common containers (formats) for video with embedded soft subs are either
Matroska (.MKV) or
Ogg Media (.OGM). These formats were intended for videos with multiple video tracks, audio tracks, subtitle tracks, attachments, chapters, and other such toys (think something like DVD in a file, but without menus).
Subtitle Formats
Soft Subs come in three common formats:
SubRip (.SRT),
SubStation Alpha (.SSA or .ASS) and MicroDVD (.SUB). While these are not the only three, nor the best three, they are the most common.
Subtitles and the GP2X
I do not own a GP2X, all I know is what I've read, and my own knowledge.
The GP2X uses a modified version of the renown media application
MPlayer. The full sized version (IE the version used on PCs) has support for a wide variety of video
formats and
codecs, audio
formats and
codecs, and
Subtitle formats.
However, as it is understood so far, MPlayer on the GP2X only understands
SAMI (.SMI) subtitles. There are definate advantages to this format, but the drawback is that it's not that well distributed. However, there is hope.
For Windows users, you can download
SubStudio SAMI. Linux and Mac OS X users can take advantage of MPlayer (which I recomend you use for your media playback anyway) and use the -dumpsami option to convert their soft subs.
Thus far, it's unknown if OGM or MKV files will play on a GP2X (I'd like someone to test this. I can provide a MKV and/or OGM file for them to test with), so it's unknown if embedded subtitles in either of those formats would work. Including the .SMI file with a name identical to the AVI should work fine.
To clear up one thing: Using Vidomi would be a terrible misalocation of resouces for anyone developing on the GP2X. Vidomi does not use the correct specification of AVI, therefore there is the possiblilty of the additional ODML/chunk data confusing certain players. I, for one, am against the bastardization and modification of older formats when new ones offer more than enough. My personal favorite of the two new formats is Matroska, but OGM is perfectly fine in my opinion. Plus, there is the distinct possibility that the GP2X may already play MKV and/or OGM files.