Getting individual frames using CV_CAP_PROP_POS_FRAMES in cvSetCaptureProperty

Question

I am trying to jump to a specific frame by setting the CV_CAP_PROP_POS_FRAMES property and then reading the frame like this:

cvSetCaptureProperty( input_video, CV_CAP_PROP_POS_FRAMES, current_frame );
frame = cvQueryFrame( input_video );

The problem I am facing is that, OpenCV 2.1 returns the same frame for the 12 consecutive values of current_frame whereas I want to read each individual frame, not just the key frames. Can anyone please tell me what's wrong?

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I did some research and found out that the problem is caused by the decompression algorithm.

The MPEG-like algorithms (including HD, et all) do not compress each frame separately, but save a keyframe from time to time, and then only the differences between the last frame and subsequent frames.

The problem you reported is caused by the fact that, when you select a frame, the decoder (ffmpeg, likely) automatically advances to the next keyframe.

So, is there a way around this? I don't want only key frames but each individual frame.

Answer 1

Due to this limitation in OpenCV, it may be wise to to use FFMPEG instead. Moviepy is a nice wrapper library.

# Get nth frame from a video
from moviepy.video.io.ffmpeg_reader import FFMPEG_VideoReader
cap = FFMPEG_VideoReader("movie.mov",True)
cap.initialize()
cap.get_frame(n/FPS)

Performance is great too. Seeking to the nth frame with get_frame is O(1), and a speed-up is used if (nearly) consecutive frames are requested. I've gotten better-than-realtime results loading three 720p videos simultaneously.

Answer 2

I don't know whether or not this would be precise enough for your purpose, but I've had success getting to a particular point in an MPEG video by grabbing the frame rate, converting the frame number to a time, then advancing to the time. Like so:

cv::VideoCapture sourceVideo("/some/file/name.mpg");
double frameRate = sourceVideo.get(CV_CAP_PROP_FPS);
double frameTime = 1000.0 * frameNumber / frameRate;
sourceVideo.set(CV_CAP_PROP_POS_MSEC, frameTime);