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.
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.