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I have a bunch of videos with some statistics of what is happening inside a video. One such piece of information is given in terms of time of the video in seconds up to one decimal point.

To get the FPS of a video, I am using ffmpeg -i

But when I manually compute one particular frame's time using given FPS, it does not match.

For example, ffmpeg outputs FPS = 30.

I look at the video statistics, the frame at the 156.8 = 2.368 has to be 4704'th frame. I open the video using 'skvideo', read all the frames, and view the 4704'th frame. It is some frame around time 2.12?. I checked multiple such instances in multiple videos and this is a common behavior.

I do not understand why this is so and how can I get around the problem?

As such I am not bounded by ffmpeg. Skvideo is being used to read the videos. I tried opencv, as of now it does not work with VideoCapture, and reinstalling it is costly for me time wise. But I guess 'opencv/skvideo' should not matter, one can count the frames manually as well.

So, in the solution, I am looking out for -

  1. Given timestamps of inside of a video, how can I find a frame of that particular time location?

  2. In case someone might have already worked on this, this is related to THUMOS dataset. I am on Ubuntu 16.04

EDIT_1

Actually I can be more specific as it is a publicly available data. The time bounds are of an important activity. For example, in a video, when does basketball dunk occurs? It is given in pairs - [start end]. Some videos have multiple activities, some have only one.

Here is a sample video, and following are the activity times. 

[[ 16.5, 20.8], [ 26.6, 32.2], [ 34.8, 42.1], [ 47.8, 50.0], [ 58.1, 62.9], [ 65.6, 67.2], [ 68.5, 74.0], [ 76.4, 78.3], [ 78.7, 79.8], [ 80.8, 82.1], [ 85.0, 87.3], [ 90.1, 91.4], [ 98.5, 100.3]]

I also tried checking manually, 32.87 FPS "almost" works for few videos but not for all. and almost means it is off by ~ 10 frames. This is a huge difference for my task, and I need exact frame.

Also, there has to be some way, because it can be visually observed with multiple video players that times in the dataset are correct.

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  • Most of the video codec's are relying on incomplete (difference images, etc.) images that are calculated on playblack time. Depending on how your footage is interpreted, this can be interpolated. So you want to make sure, that in all domains you are interpreting the footage as it is. Do you have an example video file where you see this problem happening? Commented Dec 4, 2017 at 1:48
  • @user1767754, sample video and more information added Commented Dec 4, 2017 at 2:01
  • do video players play the videos with right fps? Maybe recording or video editing just was erroneous. Commented Dec 4, 2017 at 6:37

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If your aim is to extract a frame corresponding to a timestamp, use

ffmpeg -i in.mp4 -vf select='lt(t,TS)*lt(TS-t,1/FPS)' -vsync 0 frame.png

TS should be replaced with your timestamp and FPS with framerate e.g. 'lt(t,78.7)*lt(78.7-t,1/30)'

Frame indexes are unreliable if the video is variable framerate as yours appear to be.

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A simple frame accurate way to get a frame at a given timestamp using ffmpeg is use the ffmpeg seek -ss option. If that's what you are looking for, you can find the description here - https://trac.ffmpeg.org/wiki/Seeking

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