Here's the method using current versions of FFmpeg. It relies on the concat demuxer not rescaling the PTS of inputs after the first file, but simply applying a fixed offset. Let's say you have a 30 fps stream with a timescale of 15360 (typical of FFmpeg output). That means frame 0 has PTS 0 and frame 30 has PTS 15360. This would become a 45 fps stream if we could change the timescale to 23040 without affecting the PTS values.
Essentially, that's what the method below does.
1. Identify the source properties.
Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 1171 kb/s, 30 fps, 30 tbr, 15360 tbn (default) You want to note the source properties, especially resolution and tbn.
2a. (Optional) Change the timescale to something convenient, to make calculations simpler.
ffmpeg -i in.mp4 -c copy -an -video_track_timescale 30 in-v30.mp4 This gets us
Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 1171 kb/s, \ 30 fps, 30 tbr, 30 tbn (default If you do this step, the new timescale should be equal or an integral multiple of the original framerate.
2b. Calculate the timescale needed, so that for target framerate x, PTS of frame # x in the source should have the same value as the new tbn. If you carried out step 2a, this is very easy and it's simply the new framerate. So, for target fps 45, new tbn should be 45.
3. Generate dummy video.
ffmpeg -f lavfi -i color=s=1280x720:r=45:d=1 -profile:v main -video_track_timescale 45 0.mp4 All properties should be same like resolution, H.264 profile, pixel format, refs count..etc for best results.
4 Concat the videos.
First make a text file
file '0.mp4' file 'in-v30.mp4' Then, the concat
ffmpeg -f concat -i list.txt -c copy -video_track_timescale 45 45fps.mp4 The output file will have the 2nd video playing at 45 fps.
5. Now, cleave off the dummy preroll
ffmpeg -ss 1.1 -i 45fps.mp4 -c copy -avoid_negative_ts make_zero in45.mp4 and you have
Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 1757 kb/s, \ 45 fps, 45 tbr, 11520 tbn (default) I did say this was convoluted!