[Opencv-python] cv2.createTrackbar(...) creates a slider

1. Background

For detailed documentation, please refer to official information.
This blog is as simple and clear as possible. If there are any errors, please correct me.

The python and cv2 versions of this blog are as follows:

python:3.6.8 (tags/v3.6.8:3c6b436a57, Dec 24 2018, 00:16:47) [MSC v.1916 64 bit (AMD64)]
opencv-python:4.5.1.48

opencv provides an api to create a slider on the image form:

cv2.createTrackbar(trackbarName, windowName, value, count, onChange) ->