# OpenCV VideoCapture: Howto get specific frame correctly?

I am trying to get at specific frame from a video file using OpenCV 2.4.11. I have tried to follow the documentation and online tutorials of how to do it correctly and have now tested two approaches:

1) The first method is brute force reading each frame using the video.grab() until I reach the specific frame (timestamp) I want. This method is slow if the specific frame is late in the video sequence!

string videoFile(videoFilename);
VideoCapture video(videoFile);
double videoTimestamp = video.get(CV_CAP_PROP_POS_MSEC);
while (videoTimestamp < targetTimestamp)
{
videoTimestamp = video.get(CV_CAP_PROP_POS_MSEC);

// Grabe frame (but don't decode the frame as we are only "Fast forwarding")
video.grab();
}
// Get and save frame
if (video.retrieve(frame))
{
char txtBuffer[100];
string imgName = txtBuffer;
imwrite(imgName, frame);
}


2) The second method I uses the video.set(...). This method is faster and doesn't seem to be any slower if the specific frame is late in the video sequence.

string videoFile(videoFilename);
VideoCapture video2(videoFile);
videoTimestamp = video2.get(CV_CAP_PROP_POS_MSEC);
video2.set(CV_CAP_PROP_POS_MSEC, targetTimestamp);
while (videoTimestamp < targetTimestamp)
{
videoTimestamp = video2.get(CV_CAP_PROP_POS_MSEC);

// Grabe frame (but don't decode the frame as we are only "Fast forwarding")
video2.grab();
}
// Get and save frame
if (video2.retrieve(frame))
{
char txtBuffer[100];
string imgName = txtBuffer;
imwrite(imgName, frame);
}


Problem) Now the issue is that using the two methods does end up with the same frame number of the content of the target image frame is not equal?!?

I am tempted to conclude that Method 1 is the correct one and there is something wrong with the OpenCV video.set(...) method. But if I use the VLC player finding the approximate target frame position it is actually Method 2 that is closest to a "correct" result?

As some extra info: I have tested the same video sequence but in two different video files being encoded with respectively 'avc1' MPG4 and 'wmv3' WMV codec.

Using the WMV file the two found frames are way off?

Using the MPG4 file the two found frames are only slightly off?

Is there anybody having some experience with this, can explain my findings and tell me the correct way to get a specific frame from a video file?

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