hi. i did something like that about 20-30 years ago, except it was in Commodore BASIC 2.0 on my trusty old C64 (i kid you not -- i was newly exposed to AutoCAD on IBM PCs and i wanted to do something similar)
(and no i no longer have my old source code but i sure wish i did -- and mine drew crosshairs using ONLY ASCII characters driven by cursor keys or maybe a joystick, and it was super slow, too! lol)

so i was wondering if you ever figured your problem out and found out how to do what you asked? yes i know this post is 10 years old lol
i want to do something like this and learn QT5 as well. I have learned some C++ now as it is (i am a newb).