How to re-encode H.264 video with minimal quality loss

First of all, install a more recent version of FFmpeg – grab a static build from the download page.

The use of vpre presets (which is a way to set default values for ffmpeg settings, not encoder settings) is not really necessary; you usually want to use the -preset options defined by the encoders.

The reasons you get low output quality are the following, for your two cases respectively:

1. In the first case you use crf 25, a Constant Rate Factor that's going to give you worse quality than the default for the x264 encoder (which is 23). The CRF controls the quality. Try setting a lower CRF, maybe 20, 18, etc. Here, lower means better quality, but it'll increase the file size. A change of 6 in the CRF gives you twice/half the original average bitrate, roughly speaking.

   You'll have to set a lower CRF because of the generation loss. You're encoding something that was already encoded, so you're again throwing away visual information. That's never good, but if you have to, you're going to have to set a higher quality so as not to remove too much information from the input video.

2. In the second case you're trying to set a constant bit rate of 2 MBit/s. Your input video roughly has the same bit rate. Now, x264 delivers much better visual quality than an MPEG-4 Visual encoder for the same bitrate, but due to the generation loss, again, you might want to use an even higher bitrate than the original – otherwise you'll end up compressing away too much information.

   Furthermore, constant bitrate encoding might result in some passages looking good, but other parts of the video looking worse. If you don't let the encoder freely choose the amount of bits it wants to spend on something, you're going to sacrifice quality at the expense of knowing the target file size.

   x264 does have a constant bit rate mode, but it's considered inferior to the other encoding methods. Actually, two-pass encoding isn't meant to target optimal quality, so scratch that.

That all being said, try something along the following:

ffmpeg -i input.avi -c:v libx264 -crf 19 -preset slow -c:a libfdk_aac -b:a 192k -ac 2 out.mp4

If libfdk_aac is not available, use this instead:

ffmpeg -i input.avi -c:v libx264 -crf 19 -preset slow -c:a aac -b:a 192k -ac 2 out.mp4

The main quality control knob will be your CRF setting. Experiment with that and use a lower value if you need better quality.

You can also choose the veryslow preset, which will give you better compression, but the encoding will obviously take longer.

If you can't manage to get a decent quality file at a reasonable file size, then you're out of luck. Better keep the original file as-is, without re-encoding. There's no magic "keeping the same quality" tool when you're compressing something that's already compressed.

I had similar issues with the concat filter and I think it's due to the differing time bases used for the inputs.

I overcame it with the concat protocol method.

I think it's showing variable framerate for your trim-only output because with a 1/90000 base time a 60Hz video will have expect a frame every 1500, but the concat filter may have smushed two videos together with a 1530 cycle gap between last and first frames (58.824 fps) and somewhere else 1440 (62.500 fps). Also, for whatever reason, ffmpeg decided to set the output time base to 1/15360.

When you cropped, it reevaluated the base time for the whole run-length of the output, which is why you get the constant frame rate. Notice that your first video shows a tbn of 15360, while the second is 90k.

In my case, I noticed that with the concat filter, the PTS/DTS values were being set in terms of the first input's time base even when the time base for the other input video streams weren't the same. With the first video using 1/25 and the second using 1/90000, running this on the output (which had a new time base of 1/12800):

ffprobe -hide_banner -show_frames -i output.mp4 2>&1 | grep -A 21 video | grep ts=

At the transition point between videos this happens:

pkt_pts=253952
pkt_dts=253952
pkt_pts=254464
pkt_dts=254464
pkt_pts=254976
pkt_dts=254976
pkt_pts=255488
pkt_dts=255488
pkt_pts=256000
pkt_dts=923443200
pkt_pts=925286400
pkt_dts=925286400
pkt_pts=923443200
pkt_dts=1045094400
pkt_pts=952934400
pkt_dts=1046937600
pkt_pts=954777600
pkt_dts=1048780800
pkt_pts=956620800
pkt_dts=1050624000