Why the CPU usage is higher when using OpenCV on C++ than on Python

Question

I am using Ubuntu 20.04.4, and I compiled OpenCV as release mode. Whenever I read frames, it consumes quite a lot of my CPU. I tested this in other machines as well. However, using a very similar script on python, it uses much less CPU. I found this question that seems to have a similar problem as mine. Although I am using the Release version.

Also, my python seems to be using the same OpenCV version as the one I compiled: 4.5.5.

Here is the C++ test code:

#include "opencv2/opencv.hpp"

int main(){
    cv::VideoCapture vo = cv::VideoCapture(2);
    //Set fourc for better performance.
    vo.set(cv::CAP_PROP_FOURCC, cv::VideoWriter::fourcc('M','J','P','G'));
    vo.set(cv::CAP_PROP_FPS,30);
    //Setting buffersize to one will make vi.read() blocking until next frame is available.
    vo.set(cv::CAP_PROP_BUFFERSIZE,1);
    vo.set(cv::CAP_PROP_FRAME_WIDTH,1920);
    vo.set(cv::CAP_PROP_FRAME_HEIGHT,1080); 
    cv::Mat frame;
    while (vo.isOpened())
    {
        vo.read(frame);
    }
    
}

And the python code:

import cv2

vo = cv2.VideoCapture(2)
vo.set(cv2.CAP_PROP_FPS,30)
vo.set(cv2.CAP_PROP_BUFFERSIZE,1)
vo.set(cv2.CAP_PROP_FRAME_WIDTH,1920)
vo.set(cv2.CAP_PROP_FRAME_HEIGHT,1080)

while(vo.isOpened()):
    ret, frame = vo.read()

The python script consumes around 10% of my CPU while the C++ consumes around 30%. I work in an environment where CPU resource is critical. I'd like to know if there is any way to decrease this usage. Am I missing something?

Answer 1

Thanks for @ChristophRackwitz. Apparently, it was the fourcc configuration that was causing the high CPU usage. Using a resolution of 1920x1080 will cramp FPS to 5 using the default YUYV encoding. This is likely why I got lower CPU usage using python.

If I set the fourcc on python to MJPG the CPU usage spikes.