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This code needs to be reorganized if you want it to work well.

ros has restrictions on what you can do inside a subscriber callback. A well written app does not do anything that takes a long time (looping or heavy cpu processing) and it must never sleep. Creating a subscriber object inside a callback is not a good idea because it may be a little slow and may not see a message sent too soon afterward, but it is technically permitted to do so.

All of your code in callback() should be in a find_face() method that is called from the main loop. You are allowed to loop and sleep in the main loop, and in functions that it calls. Your callback() should simply set a flag that controls whether the main loop calls find_face() or not. It will work better if you create the stop_callback() subscriber just once before you enter the main loop.

This code needs to be reorganized if you want it to work well.

ros has restrictions on what you can do inside a subscriber callback. A well written app does not do anything inside the callback that takes a long time (looping or heavy cpu processing) and it must never sleep. sleep. Creating a subscriber object inside a callback is not a good idea because it may be a little slow and may not see a message sent too soon afterward, but it is technically permitted to do so.

All of your code in callback() should be in a find_face() method that is called from the main loop. You are allowed to loop and sleep in the main loop, and in functions that it calls. Your callback() should simply set a flag that controls whether the main loop calls find_face() or not. It will work better if you create the stop_callback() subscriber just once before you enter the main loop.