How to drop some frames while playing video?

Hi, all. I am here again :p

I am looking for function which manipulate the frame display on qt media player.
Right now my qt player is working fine.

but I would like to change the framerate dynamically.
I have no idea how to make it but just came up with this idea, maybe the during playing I can make it drop some frames, or dont display some frame.

in this case , the framerate should be changed.

but I have searched decumentation, havent found any.

Did someone know is there any functions for that ?

thanks a lot. weekends comming. last question for this week!