Now I think I got what you are trying to achieve and why you use a fps of 15. Sorry that I didn't get it earlier. You want to record 10 hours real time and shrink the whole video to 5 minutes as massive timelapse. You don't want to cut scenes out of it using a regular video editor. Instead you just want to postprocess the whole video uncut and simply shrink it to 5 minutes. You want to show how a plant grows during one day in a 5 minute video or something like this.
There are video editors out there who can adjust the fps of a video without doing much computation. We just need to know at which fps we need to record to get our final fps if we speed up the video afterwards by patching a higher fps into it.
Let's compute a bit.
We have a source video with length s seconds. (that's our 10 hour long video)
We want to know the recording frame rate r to configure OBS accordingly.
We want a destination video with length t seconds (that's our timelapse video)
We want a destination frame rate of 60 (I made this up; this seems a nice rate for a smooth video)
We can compute the number of frames (n) in the destination video:
n = t * 60 [1]
We want the original video contain the same number of frames to not have to cut any frames away:
n = s * r
or
r = n / s [2]
[1] in [2]:
r = t * 60 / s
Suppose we have a source video of 10 hours (s = 36000 seconds) and a destination video of 5 minutes (t=300 seconds):
r = 300 * 60 / 36000 = 18000 / 36000 = 1/2
That means we need a frame rate of 1/2 in the source video. That's one frame every 2 seconds or 0.5 fps. We are able to configure this in OBS.
To get this frame rate, do this:
- go to Settings->Video and set the fps dropdown box to "Fractional FPS Value".
- set Numerator to 1 and Denominator to 2. This makes our 1/2.
Be careful at stopping a recording. This will take up to 1 minute (at least at my system it does this). This is due to the low frame rate and the encoder - don't ask why. The stopping time gets longer the lower the fps is. Don't go lower than 1/5 fps if you want to get faster timelapse.
To get a good video quality, you can set really high quality encoder settings, because your machine has to compress only one frame in 2 seconds. Use:
- Output mode: advanced
- on Recording tab: format mkv, Encoder x264, Rate Control CRF, CRF 10, Preset Veryfast. Keyframe interval: 1. This means every frame will be a keyframe, because we have 1 frame every 2 seconds. This makes seeking and cutting very fast, if you want to also cut some frames out.
Now get a video editor that has a filter to adjust the frame rate but not add or remove frames. I used Avidemux for this:
http://avidemux.sourceforge.net/
We now have our 10 hour source video with frame rate 0.5
We want a video of 60 frames.
Add the video filter called "change fps" in avidemux.
Unfortunately, we cannot enter a fps of lower than 1 in that certain filter, but the source fps is somewhat ignored. Just enter 1 fps as source fps and 60 fps as destination fps. The resulting video will have the same amount of frames as the original video but will be played with 60 fps.
WHY THIS WILL WORK ON A LOW END PC:
We record with OBS exactly the amount of frames that will be in the final video. The original video is as small as possible due to this, no frames are thrown away. Avidemux will recode the video wile changing the fps, but there are also video file editors that only patch a new fps and leave the video data unchanged. You might look for these, if even the minimal recoding Avidemux does is too much for your machine.
