[PD] need a workaround for detecting movement in Mac OS

[Matteo Sisti Sette]

Hi,

Bug 3191771 renders the combination pix_video+pix_movement useless in 
Mac OS unless you can lower gem's framerate to ensure it is lower than 
the camera's framerate (which is not always an option), or unless one 
finds some other workaround.

The problem is that whenever a new frame from the camera is not 
available, pix_movement computes the new input frame as a black frame, 
and hence the resulting difference is almost all-white.

Note that the problem is NOT that pix_movement computes the difference 
between two identical frames yelding a black frame as a result. It's 
worse: when the new frame is not available, it seems it considers it as 
an all-black frame, so the output is the difference between a real frame 
and a black frame. That causes white flickering, not black flickering.


Does anybody use pix_movement and pix_video on Mac OS? How do you work 
around this issue?

Ideally I would like to find a simple and easy-to-understand solution, 
because I am teaching a Pd course and I'd like to give the students 
(most of whom use Mac) a workaround they can understand.

Thank you in advance
m.