i) to just once call that singleshot timer, why not just place that call before the loop?
ii) QTimer events are process only by a running event loop; your code is busy running your while loop, thus no event loop is run, thus no timer events are process and your slot is never called
iii) if you want, you can place the "crossCapture" stuff into a separate thread (if it is a long running operation)
iv) either way:
use a signal to trigger crossCapture(); the exit the calling function (**)
use another signal, to "continue" where (**) left off

HTH