prosthetic knowledge

Screenshot-proof images via temporal dithering 

Proof-of-concept code to protect images online - by persistant.info:

Snapchat’s (and now Facebook Poke’s) main claim to fame is that it lets you send “self-destructing” image messages. Setting aside the debate about the uses of this beyond sexting, the key vulnerability in both apps is the built-in ability to take screenshots. Both take a reactive approach, where you’re notified if the recipient took a screenshot, but can’t really do anything about it.

I was thinking about ways of mitigating this issue, and figured that perhaps turning the image into an animation where individual frames are not (or at least less) recognizable would be the right path. This is a variant of temporal dithering, except we’re intentionally pretending like each frame has a limited amount of precision, and only when averaged together is the original image re-created.

I’ve created a proof of concept (source) of this. It loads the image into a <canvas> and generates a “positive” and “negative” frame out of it. The positive frame has a random offset added to each pixel’s RGB components, while the negative one has it subtracted. When displayed in quick sequence (requestAnimationFrame is used to do this every time the screen refreshes) the two offsets should cancel out, and the resulting image should re-appear.

The GIF above doesn’t really demonstrate the idea well, you can get a better idea of how it works in this online demo here, and more info can be found here.