Writing Video in OSX with OpenCV

09 Dec 2015

Tutorial doesn’t work!

The tutorial says we can record a webcam video with following code. (which doesn’t work)

#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>

int main()
{
  VideoCapture capture(0);

  int w = capture.get(CV_CAP_PROP_FRAME_WIDTH);
  int h = capture.get(CV_CAP_PROP_FRAME_HEIGHT);
  int fps = 30;
  VideoWriter writer("out.mp4", CV_FOURCC('X','2','6','4'), fps, Size(w,h), true);

  Mat frame;
  while (true)
  {
    capture >> frame;
    writer << frame;
    imshow("frame", frame);
    if (waitKey(20) == 27) break;
  }
}

It doesn’t work on default OpenCV library for OSX. The program runs without error, but produces an empty file. I tried to change the fourcc values into 'M','J','P','G' and change the extension to .avi or even set fourcc to -1. But all these tries didn’t work.

According to a Stackoverflow Thread OSX version of OpenCV does not have a working video writer. Ouch! Instead, this thread suggests me another way.

Saving images and stitching them using ffmpeg

Save images

First, modify the code a little bit.

#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <stdio.h>

int main()
{
  VideoCapture capture(0);

  int w = capture.get(CV_CAP_PROP_FRAME_WIDTH);
  int h = capture.get(CV_CAP_PROP_FRAME_HEIGHT);
  int fps = 30;
  VideoWriter writer("out.mp4", CV_FOURCC('X','2','6','4'), fps, Size(w,h), true);

  Mat frame;
  char buf[100];
  int i = 0;
  while (true)
  {
    capture >> frame;
    i ++;
    sprintf(buf, "frames/%03d.jpg", i); // Make sure directory 'frames' exists.
    imwrite(buf, frame);
    imshow("frame", frame);
    if (waitKey(20) == 27) break;
  }
}

Then image frames will be saved sequentially. With ffmpeg, we can combine these image files into one video file.

Install ffmpeg

brew install ffmpeg

H264 codec are automatically installed with this command.

Run ffmpeg

Finally, combine image files into single video file.

ffmpeg -i "%03d.jpg" -c:v libx264 -r 30 out.mp4

-i specifies the input file names, in printf() format.
-c specifies the codec we will use. This one is H264 codec.
-r specifies the frame rate (FPS).

For details, consult man ffmpeg