无法在Python环境下通过OpenCV调整分辨率

Question

I am currently try to adjust the resolution of the video by opencv , according to my research, cap.set is the method to do so. thus, I write the code below:

cap = cv2.VideoCapture('IMG_2957.MOV')
for i in range(19):
    print (""+str(i)+" "+str(cap.get(i)))
cap.set(3,160) #Width
cap.set(4,120) #Height
for i in range(19):
    print (""+str(i)+" "+str(cap.get(i)))

very surprise, the value of "CV_CAP_PROP_FRAME_WIDTH" and "CV_CAP_PROP_FRAME_HEIGHT" remain unchanged

Before the cv.set 
3 1920.0
4 1080.0

And after the setting, the frame was not set as what I want

After the cv.set 
3 1920.0
4 1080.0

Is that we are not able to change the resolution by setting its property? Can anyone advice me the usage of .set of cv2.videocapture ?

More information about the version:
Python 3.6.0  [MSC v.1900 64 bit (AMD64)] on win32
>>> import cv2
>>> cv2.__version__
'3.4.0'

Answer 1

First, it is important to understand that VideoCapture is more than just a class to playback a video file. It can also connect to a stream or a camera for example. Then it may have properties that will work only for specific cases and that is why they have a set / get generic function. With a webcam, if your webcam/drivers allow it you may set the resolution for example with the code snippet you supply.

The set function returns a bool value in C++ (and in python) not sure if this means that it is set or not, but you can check it out, probably yours returns false. I am not sure, since I had not checked that and the documentation does not provide an explanation.

In a video, it will read each frame, but it may not change the properties of the video. What can you do then? Depends on what you are planning to do with it.

You may use the resize , for example:

retval, frame = cap.read()
res = cv2.resize(frame,(160, 120), interpolation = cv2.INTER_CUBIC)

Then you may save the video with VideoWritter or do any image manipulation to it.