Extra data within image (PPM/PAM/PNM)

Question

Is it possible to store extra data in pixels of a binary PNM file in such a way that it can still be read as an image (hopefully by any decoder, but specifically by ffmpeg)?

I have a simulation that saves its data as PPM currently and I'd like a way to record more than three values per pixel in the file, and yet still be able to use it as an image (obviously only the first three/four values will actually affect the image).

In particle I think the TUPLTYPE of PAM should allow me to do this, but I don't know how make something that's also a readable image from that.

Answer 1

There are two tricks which together can get up to 5 additional bytes per pixel in PAM file.

First trick:

You can try store additional byte of information in alpha channel and then choose to ignore that information in decoder. Enabling alpha channel in PAM is done by adding _APLHA to TUPLTYPE argument, so instead TUPLTYPE RGB you have TUPLTYPE RGB_ALPHA.

Second trick:

You can set MAXVAL in PAM (or equivalent field in PPM and others) to 65535 instead of 255, which means that every pixel will be described by three 16-bit values instead of three 8-bit ones. Now, for these 16-bit values the 8 least significant bits can be used to store information as they do not affect visual properties of image when shown on typical computer screen.

First + second trick:

This gives you additional 3 x 8 = 24 bits for RGB planes and 16 bits in alpha channel. Which means: 5 bytes.

Answer 2

I've not used PNM file format, but I've done this trick with a .bmp file.

Hijack the least significant bit of the image data and stuff it with whatever binary data you want. Nobody will see the difference between a pixel value of a 0 or 1 (00000000 or 00000001), or the the difference between a 254 or 255 (1111110 or 11111111). For every 8 bytes of image data a byte of extra data can be embedded (6 bytes if you use a limited character set). The file viewing software won't know any difference. Any software which could open the file before the encoding, would be able to read it after.

If you want the data to be more covert/hidden, the bits can be stuffed into the image data with a shuffle routine, where the first bit might be location 50, the second in 123, the third in 32... and after locations 0-255 (first 256 bytes if image data) are used (first 32 bytes of extra data), start the shuffle again.