How to crosscompile OpenCV for Rasberry Pi?

Question

Qt

Hello. I recently started getting into Qt programming on the Raspberry Pi. But I ran into some trouble. I will explain step by step:
1. I built Qt according to the instructions: https://wiki.qt.io/RaspberryPi2EGLFS. And everything went great. All assembled on Ubuntu 16.04
Then I built WiringPi to work with GPIO. Added to the project and everything works fine.
But then it became necessary to work with the camera. I have a Logitech HD Pro C920 USB camera for video recording.
I first tried QtCamera, although the experience with it was not very good. As it was buggy from the first days, it still sells to this day: the recording is only in ogg format, the resolution is also a problem: a maximum of 840 * 640 and on the host machine on a raspberry. I tried different containers and codecs. But nothing helped. Driver v4l also installed.
After that I built OpenCV 3.3.1 for Ubuntu. And made test programs. And everything worked like clockwork. Recorded in AVI 720p, which suited me just fine. Then I started looking for instructions on opencv cross-compilation for Malinka. But did not find anything worthwhile. What I found didn't help. There is of course this stackoverflow answer: https://stackoverflow.com/questions/33276917/cross...
But I can't collect anything from it. Parameters for the assembly I can not set correctly. Can anyone help?
I also found it, but I also couldn’t build it: ( https://docs.opencv.org/2.4/doc/tutorials/introduc...
Help. I don’t know how to solve this anymore...