cv2.resize 中的 None 是什么意思？

Question

I came across the following snippet used to resize an image:

img = cv2.resize(img, None, fx = 1/3, fy = 1/3)

What is the call actually doing? Why is the output size parameter set to None ? When I tried to run this snippet with OpenCV 3.4.3 , it gives an error but works fine with OpenCV 3.4.2 .

I want to emulate this behavior in OpenCV 3.4.3 . What is it doing and how I could do it?

Answer 1

OpenCV function resize has 2 different modes for resizing the image.

1. A fixed output size can be specified in the 2nd parameter.
2. Scaling factor for each dimension can be specified using the fx and fy parameters which are used for calculating the output size.

If both methods are used simultaneously, then fx and fy parameters are ignored. The output size parameter is mandatory for both cases. It means that if we want to use the scaling method, we have to pass None in place of the output size parameter. None indicates that we do not want to use this parameter.

Example Usage:

Fixed output size:

img = cv2.resize( img, (640, 480) )

Dynamic output size:

img = cv2.resize(img, None, fx = 1.0/3.0, fy = 1.0/3.0)