Pico-8 is a fantasy console from the creator of Voxatron http://www.lexaloffle.com/.  Its a virtual console, similar in many respects to the game boy color, since it features tile/sprite based graphics, 128×128 screen, 4 channel synth.

One part of the Pico-8 that is particularly cool is that games written for it can be distributed as regular PNG images.  Here are two examples:

Its not immediately obvious how the programs are stored inside the images themselves, however it seems this is a fine example of steganography:

Steganography (US ⁱ/ˌstɛ.ɡʌnˈɔː.ɡrʌ.fi/, UK /ˌstɛɡ.ənˈɒɡ.rə.fi/) is the practice of concealing a file, message, image, or video within another file, message, image, or video. The word steganography combines the Ancient Greek words steganos (στεγανός), meaning “covered, concealed, or protected”, and graphein (γράφειν) meaning “writing”.

A simple scheme for hiding data in pictures is to hijack the least significant bits of the color channels, and use them to transport the data, which will have the lowest visually perceivable impact.  I had a hunch that was what was happening here.

The quickest way I could think to validate my hypothesis was to take two cartridges (the above two) and blend them using difference mode in Artweaver.  I hoped that would cut out some of the cartridge surround, and make the data somewhat more visible.  I also upped the contrast for good measure so those least significant bits move towards the more significant bits and become brighter.

Here is the result:

Structured noise is readily apparent, and is most certainly the hidden program data.  Taking a quick look at the range of colors present in the noise, I would guess that two bits of each color channel are being hijacked, and for RGBA that would allow one full byte to be hidden per pixel.  At an image size of 160×205 that would allow ~32k of data to be stored.  That middle band of data also seems very random, leading me to guess it may be compressed.

I might one day whip up a program to try and extract and decode the actual data from these cartridges.

I love the idea of a fantasy virtual console, and I love the idea of distributing picture “cartridges” with their programs embedded.  Hats off to lexaloffle, I think this is just fantastic stuff.