opencv video reading slow framerate - c++

Question

I am trying to read a video with OpenCV in C++, but when the video is displayed, the framerate is very slow, like 10% of the original framerate.
The whole code is here:
// g++ `pkg-config --cflags --libs opencv` play-video.cpp -o play-video
// ./play-video [video filename]
#include <iostream>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
using namespace std;
using namespace cv;
int main(int argc, char** argv)
{
// video filename should be given as an argument
if (argc == 1) {
cerr << "Please give the video filename as an argument" << endl;
exit(1);
}
const string videofilename = argv[1];
// we open the video file
VideoCapture capture(videofilename);
if (!capture.isOpened()) {
cerr << "Error when reading video file" << endl;
exit(1);
}
// we compute the frame duration
int FPS = capture.get(CV_CAP_PROP_FPS);
cout << "FPS: " << FPS << endl;
int frameDuration = 1000 / FPS; // frame duration in milliseconds
cout << "frame duration: " << frameDuration << " ms" << endl;
// we read and display the video file, image after image
Mat frame;
namedWindow(videofilename, 1);
while(true)
{
// we grab a new image
capture >> frame;
if(frame.empty())
break;
// we display it
imshow(videofilename, frame);
// press 'q' to quit
char key = waitKey(frameDuration); // waits to display frame
if (key == 'q')
break;
}
// releases and window destroy are automatic in C++ interface
}
I tried with a video from a GoPro Hero 3+, and with a video from my MacBook's webcam, same problem with both videos. Both videos are played without problem by VLC.
Thanks in advance.

Answer 1

Try reducing the waitKey frame wait time. You are effectively waiting for the frame rate time (i.e. 33 ms), plus all the time it takes to grab the frame and display it. This means that if capturing the frame and displaying it takes over 0ms (which it does), you are guaranteed to be waiting for too long. Or if you really want to be accurate, you could time how long that part takes, and wait for the remainder, e.g. something along the lines of:
while(true)
{
auto start_time = std::chrono::high_resolution_clock::now();
capture >> frame;
if(frame.empty())
break;
imshow(videofilename, frame);
auto end_time = std::chrono::high_resolution_clock::now();
int elapsed_time = std::chrono::duration_cast<std::chrono::milliseconds>(end_time - start_time).count();
//make sure we call waitKey with some value > 0
int wait_time = std::max(1, elapsed_time);
char key = waitKey(wait_time); // waits to display frame
if (key == 'q')
break;
}
The whole int wait_time = std::max(1, elapsed_time); line is just to ensure that we wait for at least 1 ms, as OpenCV needs to have a call to waitKey in there to fetch and handle events, and calling waitKey with a value <= 0 tells it to wait infinity for a user input, which we don't want either (in this case)