flickering problem while filming a projected screen with webcam

Hello,

For one of my projects I should be able to film and real-time process a projected screen by using webcam. I am facing a serious flickering problem while doing it, how can I solve this issue?

Thank you,

Answer 1

[Walter Roberson]

This is a very common problem.

The projector is displaying some number of frames per second, call it FRout, possibly at regular timing intervals but not necessarily so (some projection systems have slightly different timing when a new display is being loaded.)

The webcam is recording at some number of frames per second, call it FRin, possibly at regular timing intervals, but not necessarily so (some webcams have slightly different timing when the controls are adjusted, or if the data bus is busy.)

Now:

1. FRout, the output frame rate, is not necessarily the same as FRin, the input frame rate. Note that even if both rates are above the normal boundary of human motion perception of 12 frames per second, that there will be a beat frequency at the sum and difference of the two frequencies that will show up as a flicker
2. Even if both frame rates are exactly the same, it is unlikely that you have FRout and FRin hardware synchronized. You could happen to be taking a picture just at the time that the projector is redrawing.

The typical solution to synchronization is to use an output system that can generate a hardware timing signal, typically called Genlock, and to use a camera that accepts that kind of hardware timing signal as a trigger for the camera. Typical webcams do not support genlock or any other hardware trigger signal, but higher end devices might possibly, and some CCTV type cameras support genlock.