Downscaling 4K iPhone video to 1080

ffmpegiphonevideo

I am trying to downscale a huge 4K video taken with an iPhone 14 so that I can further process it with Powerdirector's mobile app for Android. I spent hours fiddling with both Handbrake and ffmpeg converting the source video, but the result was always too jerky, especially when the camera was moving, as if the number of frames per second was too low. Is there maybe a special codec that I need?

Here are the characteristics of the source video: MPEG-4, 26.3 Mb/s variable, 32.587 FPS, AVC High 5.2, 3840×2160, YUV, BT.709.

Apparently, Powerdirector works best with MPEG-2, and 1080 should be sufficient for my purpose. Can anyone help me find the correct command line for doing that conversion with ffmpeg? Since the video is almost an hour long, I would like to run the command for the first 30 seconds of the video only and then check the result.

Best Answer

The check after 30 seconds:

ffmpeg -i [inputfile] -t 30 -c:v mpeg2video -crf 22 -c:a mp3 -vf scale=1920:1020 [outfile].mpg

To encode the whole file:

ffmpeg -i [inputfile] -c:v mpeg2video -crf 22 -c:a mp3 -vf scale=1920:1020 [outfile].mpg

Notes

Since your original video was in 4K, it was likely encoded in HEVC. Going from HEVC to mpeg2 is a major quality loss as HEVC as 4x more efficient than mpeg2

Related Solutions

HLS video segmenting complications. How to create a transport stream with ffmpeg

Try:

ffmpeg -i in.mp4 -acodec copy -vcodec copy out.ts

Ffmpeg also has a segmenter. See http://ffmpeg.org/ffmpeg.html#segment_002c-stream_005fsegment_002c-ssegment

Reduce video frame rate without dropping useful frames

I managed to fix my file. I found the mpdecimate filter, which drops duplicate or near duplicate frames from a video stream. First I tried:

ffmpeg -loglevel debug -i orig.mp4 -an -vf "mpdecimate" test.mp4

I added -loglevel debug to get more info during transcoding. The info showed what mpdecimate was doing with the frames, which was lots and lots of this pattern:

lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:44032 pts_time:3.44 drop_count:1
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:44544 pts_time:3.48 drop_count:2
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:45056 pts_time:3.52 drop_count:3
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:45568 pts_time:3.56 drop_count:4
781>=hi keep pts:46080 pts_time:3.6 drop_count:-1
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:46592 pts_time:3.64 drop_count:1
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:47104 pts_time:3.68 drop_count:2
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:47616 pts_time:3.72 drop_count:3
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:48128 pts_time:3.76 drop_count:4
821>=hi keep pts:48640 pts_time:3.8 drop_count:-1
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:49152 pts_time:3.84 drop_count:1
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:49664 pts_time:3.88 drop_count:2
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:50176 pts_time:3.92 drop_count:3
lo:0<217 lo:0<-2147483648 lo:0<-2147483648 drop pts:50688 pts_time:3.96 drop_count:4
793>=hi keep pts:51200 pts_time:4 drop_count:-1

I.e., it was consistently dropping 4 frames in a row as duplicates and keeping every 5th. This showed that the original frame rate was 1/5th of what it was now. I was lucky! Since the video's current 25 fps was a multiple of the original frame rate, there shouldn't be a problem with temporal misalignment of the kept and dropped frames. Therefore I retranscoded the original video with nothing more complex than -vf "fps=5", and the output seemed to be fine. Just to be sure, I piped the output through the mpdecimate filter, and it detected all remaining frames as a "keep". So the original file was not so messed up as I thought.

Best Answer

Notes

Related Solutions

HLS video segmenting complications. How to create a transport stream with ffmpeg

Reduce video frame rate without dropping useful frames

Related Question