# How to modify the FPS of a video when using cv::VideoCapture to read frames **URL:** \ **Category:** C++\ **Tags:** videoio\ **Created:** [May 19, 2021, 10:43pm UTC](https://forum.opencv.org/t/how-to-modify-the-fps-of-a-video-when-using-cv-videocapture-to-read-frames/3430 "2021-05-19T22:43:44Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Stephane\_Charette](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/stephane_charette/32/1369_2.png) [@Stephane\_Charette](https://forum.opencv.org/u/Stephane_Charette)\ **Post date:** [May 19, 2021, 10:43pm UTC](https://forum.opencv.org/t/how-to-modify-the-fps-of-a-video-when-using-cv-videocapture-to-read-frames/3430/1 "2021-05-19T22:43:44Z")
Using cv::VideoCapture to process videos. Wondering what the technique is for changing the frame rate when I write out a new video. I assume I could do something simple like drop every 2nd frame to cut from 60 FPS to 30 FPS. But is that the right way to do it? Does that mean if I want to go from 30 FPS to 20 FPS I drop every 3rd frame? Seems too simple. Is there any interpolation or re-interpretation of the frames that needs to happen between frames to make sure everything is smooth? ---
**Author:** ![Stephane\_Charette](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/stephane_charette/32/1369_2.png) [@Stephane\_Charette](https://forum.opencv.org/u/Stephane_Charette)\ **Post date:** [May 20, 2021, 3:11am UTC](https://forum.opencv.org/t/how-to-modify-the-fps-of-a-video-when-using-cv-videocapture-to-read-frames/3430/3 "2021-05-20T03:11:32Z")
So explain to me how it works then, if 2/3 \* 30 != 20. But regardless of the number, I suspect that simply dropping every Nth frame is not the way to do it. What is the correct way to resample or change the FPS for video files? ---
**Author:** ![mshabunin](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/mshabunin/32/15_2.png) [@mshabunin](https://forum.opencv.org/u/mshabunin)\ **Post date:** [May 20, 2021, 12:16pm UTC](https://forum.opencv.org/t/how-to-modify-the-fps-of-a-video-when-using-cv-videocapture-to-read-frames/3430/4 "2021-05-20T12:16:20Z")
There are different approaches, simpler where frames are dropped or duplicated and more complex where frames can be interpolated. This functionality is built in many specialized video-processing libraries (e.g. FFmpeg - [ChangingFrameRate – FFmpeg](https://trac.ffmpeg.org/wiki/ChangingFrameRate), [https://www.reddit.com/r/ffmpeg/comments/dai5f1/smart\_resampling\_with\_ffmpeg/](https://www.reddit.com/r/ffmpeg/comments/dai5f1/smart_resampling_with_ffmpeg/)). There are no ready-to-use functions for this in the OpenCV, but I believe it is possible to implement it using existing functionality. ---
**Author:** ![matti.vuori](https://avatars.discourse-cdn.com/v4/letter/m/e79b87/32.png) [@matti.vuori](https://forum.opencv.org/u/matti.vuori)\ **Post date:** [May 20, 2021, 3:09pm UTC](https://forum.opencv.org/t/how-to-modify-the-fps-of-a-video-when-using-cv-videocapture-to-read-frames/3430/5 "2021-05-20T15:09:22Z")
I guess it should be noted that OpenCV is a CV tool and the idea is not to produce things that are nice to watch for humans, but video frames that are easy for computers to do things with - and any interpolation might not help with that ---
**Author:** ![Stephane\_Charette](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/stephane_charette/32/1369_2.png) [@Stephane\_Charette](https://forum.opencv.org/u/Stephane_Charette)\ **Post date:** [May 20, 2021, 3:47pm UTC](https://forum.opencv.org/t/how-to-modify-the-fps-of-a-video-when-using-cv-videocapture-to-read-frames/3430/6 "2021-05-20T15:47:47Z")
To be very clear: I wasn’t in any way expecting OpenCV to do this for me. I just happen to use OpenCV for most of my image manipulation, and I’m decently good at it. Since I do use the `cv::VideoCapture` class to read video files, I figured I would ask if anyone happened to know the name or the technique used to alter the FPS. ---
**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\ **Post date:** [May 20, 2021, 8:12pm UTC](https://forum.opencv.org/t/how-to-modify-the-fps-of-a-video-when-using-cv-videocapture-to-read-frames/3430/7 "2021-05-20T20:12:32Z")
> [@Stephane\_Charette](#): > > the name or the technique used to alter the FPS. depends on what effect you want to achieve. say you have _one_ minute of video at _30 fps_. do you want - _two_ minutes at 15 fps: same number of frames, just played slower - one minute at _15 fps_: same duration but half the frames are dropped (not shown) - _two_ minutes at _30 fps_: double the frame count, interpolated frames are inserted ? ---
**Author:** ![Stephane\_Charette](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/stephane_charette/32/1369_2.png) [@Stephane\_Charette](https://forum.opencv.org/u/Stephane_Charette)\ **Post date:** [May 20, 2021, 8:14pm UTC](https://forum.opencv.org/t/how-to-modify-the-fps-of-a-video-when-using-cv-videocapture-to-read-frames/3430/8 "2021-05-20T20:14:21Z")
Let’s say I want my 1 minute @ 30 FPS video to be converted to 1 minute @ 25 FPS. ---
**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\ **Post date:** [May 20, 2021, 8:17pm UTC](https://forum.opencv.org/t/how-to-modify-the-fps-of-a-video-when-using-cv-videocapture-to-read-frames/3430/9 "2021-05-20T20:17:00Z")
> [@Stephane\_Charette](#): > > 1 minute @ 30 FPS video to be converted to 1 minute @ 25 FPS okay, you have the choice of - keeping 5 of every 6 frames, and dropping the 6th, which looks jittery/jerky - interpolating new frames at precise times, taking “adjacent” frames from the source and interpolating linearly, which causes time-varying exposure, i.e. weird motion blur - interpolating, but using using optical flow to calculate the frames (“state of the art”)