Video reading 2 frames less

[ggian]

I started using mexopencv 2 days ago. Congratulations and thank you for your work. The problem I have is that when a read a video it reads 2 frames less than normal video length. I tested it with both MATLAB VideoFileReader (normal length) and mexopencv (2 frames less).
The code I use is

cap = cv.VideoCapture(video_path);
for i=1:cap.get('FrameCount') 
        img = cap.read(); 
        if isempty(img), break; end 
        imshow(img) 
        drawnow
        disp(i);
end 

e.g. A video with 1446 frames goes until 1444 frames.

thanks

[amroamroamro]

Here is a quote from MATLAB's VideoReader:

NumberOfFrames will be removed in a future release.

Some files store video at a variable frame rate, including many Windows Media Video files.
For these files, VideoReader cannot determine the number of frames until you read the
last frame.

To count the number of frames in a variable frame rate file,
use the read method to read the last frame of the file.

Which is why both VideoReader and vision.VideoFileReader suggest iterating over frames one-by-one, instead of querying the count at the beginning...

The same thing applies to OpenCV, the FrameCount property is sometimes off, and the only way to exactly determine the number of frames is to read all the video frames:

vid = cv.VideoCapture('video.mp4');
i = 0;
while true
    frame = vid.read();
    if isempty(frame), break; end
    i = i + 1;
end
display(i)

[amroamroamro]

Also I wouldn't exclude the possibility of a bug in OpenCV (or one of the underlying video drivers it uses). There are a couple of reports online regarding this...

If that's the case, we can't do much about it in mexopencv, and it should be reported upstream.