[Opencv-python] cv2.cvtColor(...) convert color space

1. Background

The blog python and cv2 versions are as follows:

python:3.7.9 (default, Aug 31 2020, 17:10:11) [MSC v.1916 64 bit (AMD64)]
opencv-python:4.5.1.48

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.

Cv2 conversion color space interface:

cv2.cvtColor(src, code[, dst[, dstCn