I want to do some particle image velocimetry on dancing people but I was not able to record movies from the top (a bird's eye view). Instead, I managed to shot them from an upper position and my goal is to transform this series of images into a top/down bird's eye view using warpPerspective from openCV in Python. The issues that I have are the following:

• I do not have reference points on the image;
• I do not know the distortion of the camera (the movie was shot with a Nikon D7000 and a 18-140 mm Nikon varifocal lens).

Here is the image I want to transform

As a comment, the angle of the camera is very strong to extract a top down view of the desired area, your image will be stretched a lot.