Would it be possible for a mobile app to detect a flashing light with its camera, i.e., visual morse code

Is the technology there for the camera of a sm开发者_开发技巧artphone to detect a light flashing and to detect it as morse code, at a maximum of 100m?

There's already at least one app in the iPhone App store that does this for some unknown distance. And the camera can detect luminance at a much greater distance, given enough contrast of the exposure between on and off light levels, a slow enough dot rate to not alias against the frame rate (remember about Nyquist sampling), and maybe a tripod to keep the light centered on some small set of pixels. So the answer is probably yes.

I think it's possible in ideal conditions. Clear air and no other "light noise", like in a dark night in the mountain or so. The problem is that users would try to use it in the city, discos etc... where it would obviously fail.

If you can record a video of the light and easily visually decode it upon watching, then there's a fair chance you may be able to do so programmatically with enough work.

The first challenge would be finding the light in the background, especially if its small and/or there's any movement of the camera or source. You might actually be able to leverage some kinds of video compression technology to help filter out the movement.

The second question is if the phone has enough horsepower and your algorithm enough efficiency to decode it in real time. For a slow enough signaling rate, the answer would be yes.

Finally there might be things you could do to make it easier. For example, if you could get the source to flash at exactly half the camera frame rate when it is on instead of being steady on, it might be easier to identify since it would be in every other frame. You can't synchronize that exactly (unless both devices make good use of GPS time), but might get close enough to be of help.

Yes, the technology is definitely there. I written an Android application for my "Advanced Internet Technology" class, which does exactly what you describe.

The application has still problems with bright noise (when other light sources leave or enter the camera view while recording). The approach that I'm using just uses the overall brightness changes to extract the Morse signal.

There are some more or less complicated algorithms in place to correct the auto exposure problem (the image darkens shortly after the light is "turned on") and to detect the thresholds for the Morse signal strength and speed.

Overall performance of the application is good. I tested it during the night in the mountains and as long as the sending signal is strong enough, there is no problem. In the library (with different light-sources around), it was less accurate. I had to be careful not to have additional light-sources at the "edge" of the camera screen. The application required the length of a "short" Morse signal to be 300ms at least.

The better approach would be to "search" the screen for the actual light-source. For my project it turned out to be too much work, but you should get good detection in noisy environment with this.