python opencv cv2.cv.CV_CAP_PROP_FRAME_COUNT get wrong numbers

Question

import os
import cv2
path='/home/nlpr4/video-data/UCF-101/GolfSwing/v_GolfSwing_g24_c06.avi'
cap=cv2.VideoCapture(path)
video_length=int(cap.get(cv2.cv.CV_CAP_PROP_FRAME_COUNT))

success=True
count=0
while success:
    success,image=cap.read()
    if success==False:
        break
    count=count+1

print video_length,count

output:

149 
146

why the two numbers different? what's wrong?

Answer 1

The get() for CAP_PROP_FRAME_COUNT is never supposed to be accurate! If you check the opencv source code. You can find this:

int64_t CvCapture_FFMPEG::get_total_frames() const
{
    int64_t nbf = ic->streams[video_stream]->nb_frames;

    if (nbf == 0)
    {
        nbf = (int64_t)floor(get_duration_sec() * get_fps() + 0.5);
    }
    return nbf;
}

This means it will first look into the stream header for nb_frames, which you can check with ffprobe. If there is no such field, then there is no better way to get frame number than directly decoding the video. The opencv did a rough estimation by get_duration_sec() * get_fps() + 0.5 which surely not mean for accuracy.

Thus, to obtain the correct frame number you have to decode and read through the entire stream, or you have to ask the video generator to generate correct stream header with nb_frames field.

Answer 2

CV_CAP_PROP_FRAME_COUNT gives the property of 'number of frames', that comes from the video header. The other number is basically "How many frames can I read from this video file?".

If the video contains frames that cannot be read/decoded (e.g., due to being corrupted), OpenCV skips those frames (after trying to read them) and gives you the next valid frame. So the difference between your two numbers is the number of frames that couldn't be read.

Also, if your video header is corrupted and/or cannot be parsed by the underlying codecs that OpenCV uses, then those numbers can be different too.