Adding big junks of custom data to jpg image file

An interesting question. There are file formats that support multiple images per file (multipage TIFF comes to mind) but JPEG doesn't support this natively.

One feature of the JPEG file format is the concept of APP segments. These are regions of the JPEG file that can contain arbitrary information (as a sequence of bytes). Exif is actually stored in one of these segments, and is identified by a preamble.

Take a look at this page: http://www.sno.phy.queensu.ca/~phil/exiftool/#JPEG

You'll see many segments there that start with APP such as APP0 (which can store JFIF data), APP1 (which can contain Exif) and so forth.

There's nothing stopping you storing data in one of these segments. Conformant JPEG readers will ignore this unrecognised data, but you could write software to store/retrieve data from within there. It's even possible to embed another JPEG file within such a segment! There's no precedent I know for doing this however.

Another option would be to include the second image as the thumbnail of the first. Normally thumbnails are very small, but you could store a second image as the thumbnail of the first. Some software might replace or remove this though.

In general I think using two files and a naming convention would be the simplest and least confusing, but you do have options.

There are two ways you can do this.

1) Use and Application Marker (APP0–APPF)—the preferred method

2) Use a Comment Marker (COM)

If you use an APPn marker:

1) Do not make it the first APPn in the file. Every known JPEG file format expects some kind of format specific APPn marker right after the SOI marker. Make sure that your marker is not there.

2) Place a unique application identifier (null terminated string) at the start of the data (something done by convention).

All kinds of applications store additional data this way.

One issue is that the length field is only 16-bits (Big Endian format). If you have a lot of data, you will have to split it across multiple markers.

If you use a COM marker, make sure it comes after the first APPn marker in the file. However, I would discourage using a COM marker for something like this as it might choke applications that try to display the contents.