Get top-down view

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Hi guys, I need your help with next task:

1. how to get top-down view from camera view.
2. measuring real world distances in the picture (just on surface).

What i know or have:

1. camera field of view (vertical, horizontal)
2. camera picture resolution (vertical, horizontal)
3. angle of camera view
4. height of camera
5. picture with compensated lens distortion.

I know, everything is about using next methods: warpPerspective(), getPerpectiveTransform, or similarly with affine transform.

But I really don't know how to correctly use it.

Thanks in advance David.