Scene detection and concat makes my video longer (FFMPEG) - ffmpeg

Question

I'm encoding videos by scenes. At this moment I got two solutions in order to do so. The first one is using a Python application which gives me a list of frames that represent scenes. Like this:
285
378
553
1145
...
The first scene begins from the frame 1 to 285, the second from 285 to 378 and so on. So, I made a bash script which encodes all this scenes. Basically what it does is to take the current and previous frames, then convert them to time and finally run the ffmpeg command:
begin=$(awk 'BEGIN{ print "'$previous'"/"'24'" }')
end=$(awk 'BEGIN{ print "'$current'"/"'24'" }')
time=$(awk 'BEGIN{ print "'$end'"-"'$begin'" }')
ffmpeg -i $video -r 24 -c:v libx265 -f mp4 -c:a aac -strict experimental -b:v 1.5M -ss $begin -t $time "output$count.mp4" -nostdin
This works perfect. The second method is using ffmpeg itself. I run this commands and gives me a list of times. Like this:
15.75
23.0417
56.0833
71.2917
...
Again I made a bash script that encodes all these times. In this case I don't have to convert to times because what I got are times:
time=$(awk 'BEGIN{ print "'$current'"-"'$previous'" }')
ffmpeg -i $video -r 24 -c:v libx265 -f mp4 -c:a aac -strict experimental -b:v 1.5M -ss $previous -t $time "output$count.mp4" -nostdin
After all this explained it comes the problem. Once all the scenes are encoded I need to concat them and for that what I do is to create a list with the video names and then run the ffmpeg command.
list.txt
file 'output1.mp4'
file 'output2.mp4'
file 'output3.mp4'
file 'output4.mp4'
command:
ffmpeg -f concat -i list.txt -c copy big_buck_bunny.mp4
The problem is that the "concated" video is longer than the original by 2.11 seconds. The original one lasts 596.45 seconds and the encoded lasts 598.56. I added up every video duration and I got 598.56. So, I think the problem is in the encoding process. Both videos have the same frames number. My goal is to get metrics about the encoding process, when I run VQMT to get the PSNR and SSIM I get weird results, I think is for this problem.
By the way, I'm using the big_buck_bunny video.

Answer 1

The probable difference is due to the copy codec. In the latter case, you tell ffmpeg to copy the segments, but it can't do that based on your input times.
It has to find first the previous I frames (a frame that can be decoded without any reference to any previous frame) and starts from here.
To get what you need, you need to either re-encode the video (like you did in the 2 former examples) or change the times to stop at I frames.
To assert I getting your issue correctly:
You have a source video (that's encoded at variable frame rate, close to 18fps)
You want to split the source video via ffmpeg, by forcing the frame rate to 24 fps.
Then you want to concat each segment.
I think the issue is mainly that you have some discrepancy in the timing (if I divide the frame index by the time you've given, I getting between 16fps to 18fps). When you are converting them in step 2, the output video segment time will be 24fps. ffmpeg does not resample in the time axis, so if you force a video rate, the video will accelerate or slow down.
There is also the issue of consistency for the stream:
Typically, a video stream must start with a I frame, so when splitting, FFMPEG has to locate the previous I frame (when using copy codec, and this changes the duration of the segment).
When you are concatenating, you could also have the issue of consistency (that is, if the segment you are concatenating does end with a I frame, and the next one starts with a I frame, it's possible FFMPEG drops either one, although I don't remember what is the current behavior now)
So, to solve your issue, if I were you, I would avoid step 2 (it's bad for quality anyway). That is, I would use ffmpeg to split the segments of interest based on the frame number (that's the only value that's not approximate in your scheme) in png or ppm frames (or to a pipe if you don't care about keeping them) and then concat all the frames by encoding them at the last step with the expected rate set to totalVideoTime / totalFrameCount.
You'll get a smaller and higher quality final video.
If you can't do what I said for whatever reason, at least for the concat input, you should use the ffconcat format:
ffconcat version 1.0
file segment1
duration 12.2
file segment2
duration 10.3
This will give you the expected duration by cutting each segment if it's longer
For selecting by frame number (instead of time as time is hard to get right on variable frame rate video), you should use the select filter like this:
-vf select=“between(n\,start_frame_num\,end_frame_num),setpts=STARTPTS"

Answer 2

I suggest checking the input and output frame rate and make sure they match. That could be a source of the discrepancy.