Does performance differ between Python or C++ coding of OpenCV?

Question

I aim to start opencv little by little but first I need to decide which API of OpenCV is more useful. I predict that Python implementation is shorter but running time will b

Answer 1

All google results for openCV state the same: that python will only be slightly slower. But not once have I seen any profiling on that. So I decided to do some and discovered:

Python is significantly slower than C++ with opencv, even for trivial programs.

The most simple example I could think of was to display the output of a webcam on-screen and display the number of frames per second. With python, I achieved 50FPS (on an Intel atom). With C++, I got 65FPS, an increase of 25%. In both cases, the CPU usage was using a single core, and to the best of my knowledge, was bound by the performance of the CPU. Additionally this test case about aligns with what I have seen in projects I've ported from one to the other in the past.

Where does this difference come from? In python, all of the openCV functions return new copies of the image matrices. Whenever you capture an image, or if you resize it - in C++ you can re-use existing memory. In python you cannot. I suspect this time spent allocating memory is the major difference, because as others have said: the underlying code of openCV is C++.

Before you throw python out the window: python is much faster to develop in, and if long as you aren't running into hardware-constraints, or if development speed it more important than performance, then use python. In many applications I've done with openCV, I've started in python and later converted only the computer vision components to C++ (eg using python's ctype module and compiling the CV code into a shared library).

Python Code:

import cv2
import time

FPS_SMOOTHING = 0.9

cap = cv2.VideoCapture(2)
fps = 0.0
prev = time.time()
while True:
    now = time.time()
    fps = (fps*FPS_SMOOTHING + (1/(now - prev))*(1.0 - FPS_SMOOTHING))
    prev = now

    print("fps: {:.1f}".format(fps))

    got, frame = cap.read()
    if got:
        cv2.imshow("asdf", frame)
    if (cv2.waitKey(2) == 27):
        break

C++ Code:

#include 
#include 

using namespace std;
using namespace cv;

#define FPS_SMOOTHING 0.9

int main(int argc, char** argv){
    VideoCapture cap(2);
    Mat frame;

    float fps = 0.0;
    double prev = clock(); 
    while (true){
        double now = (clock()/(double)CLOCKS_PER_SEC);
        fps = (fps*FPS_SMOOTHING + (1/(now - prev))*(1.0 - FPS_SMOOTHING));
        prev = now;

        printf("fps: %.1f\n", fps);

        if (cap.isOpened()){
            cap.read(frame);
        }
        imshow("asdf", frame);
        if (waitKey(2) == 27){
            break;
        }
    }
}

Possible benchmark limitations:

• Camera frame rate
• Timer measuring precision
• Time spent in print formatting