How to improve the output video quality with ffmpeg and h264_videotoolbox flag?

Question

Currently I am using this command to convert

ffmpeg -i <srcfile> -vcodec libx264 -profile:v main -level 3.1 -preset slower -crf 18 -x264-params ref=4 -acodec copy -movflags +faststart <outfile>

to convert some dashcam footage for viewing on an iOS device.

The above command took about 30 min to complete on a 2017 Macbookpro with 16G of RAM.

I want to speed it. One thing I tried is to harness the GPU in the computer. Therefore I added the flag -c:v h264_videotoolbox

It sped up by a lot. I can complete the conversion in 1 min.

However, when I inspected the output, the GPU version suffers from banding and blurry.

Here is a screenshot. CPU version on the left and GPU version on the right.

To highlight the difference, here are the parts of the videos

1. Trees in reflections

2. corrugated iron sheet wall

Is there any switch that I can manipulate to make the GPU version clearer?

Answer 1

This is a simplistic H.264 encoder compared to x264, so you're not going to get the same quality per bitrate. h264_videotoolbox is optimized for speed and does not support -crf.

You can view some options specific to this encoder with ffmpeg -h encoder=h264_videotoolbox, but as they are probably already set to "auto" (I didn't confirm via source code and I don't have the hardware to try it) these additional options may not make much of a difference.

So you'll just have to increase the bitrate, such as with -b:v 8000k.

Or continue to use libx264 with a faster -preset.

Answer 2

I see the question's been answered and nearly two years ago. Jumping in for others who might stumble on the thread. I get great results with VideoToolbox as encoder, using either GPU or software to accelerate, depending which machine I am using.

As already mentioned, setting a constant bitrate, and adjusting it upward is key to producing a result that is nearly indistinguishable from a large source file. A constant bitrate is as effective as two-pass encoding for high-quality output, when paired with other key parameters, and is much quicker than two-pass.

May seem counter-intuitive, but a computer running on all threads, full throttle, to encode a video won't give you best results. Several researchers have demonstrated that quality actually goes down if all cpu threads are engaged in encoding; better to use fewer threads and even throttle ffmpeg with a 3rd party app (encoding does not slow down significantly, in my experience). So limit threads on newer multithread desktops and laptops.

Common practice for target bitrates (seen on Netflix, Amazon) vary with resolution, naturally: at least 5,000kbps for 1080p; 3,500 for 720p. For a noticeable improvement in video quality, the encoder bitrate should be set to at least 1.5 times those common practice bitrates: ie, 7,500 for 1080p, 5,250 for 720p. Similarly for 4K GoPros or dash cams.

Often I work with large movie files from my bluray library, and create slimmed-down versions that are 1/3 to 1/2 the size of the original (20G original gives way to a file of 8-10GB with no perceptible loss of quality. Also: framerate. Maintaining the same framerate from source to slimmed-down file is essential, so that parameter is important. Framerate is either 24fps, 25fps, or 30fps for theatrical film, European tv, and North American tv, respectively. (Except in transferring film to a tv screen, 24fps becomes 23.976fps, in most cases.) Of course 60fps is common for GoPro-like cameras, but here 30fps would be a reasonable choice.

It is this control of framerate and bitrate the keeps ffmpeg in check, and gives you predictable, repeatable results. Not an errant, gigantic file that is larger than the one you may have started with.

I work on a Mac, so there may be slight differences on the command line, and here I use VideoToolbox as software encoder, but a typical command reads:

ffmpeg -loglevel error -stats -i source.video -map 0:0 -filter:v fps\=24000/1001 -c:v h264_videotoolbox -b:v 8500k -profile 3 -level 41 -coder cabac -threads 4 -allow_sw:v 1 -map 0:1 -c:a:0 copy -disposition:a:0 default -map 0:6 -c:s:0 copy -disposition:s:0 0 -metadata:g title\=“If you want file title in the metadata, goes here” -default_mode passthrough ‘outfile.mkv’

   -loglevel error (to troubleshoot errors)
   -stats (provides progess status in terminal window)
   -i infile (source video to transcode)
   -map 0:0 (specify each stream in the original to map to output)
   -filter:v fps\=24000/1001 (framerate of 23.976, like source)
   -c:v h264_videotoolbox (encoder)
   -b:v (set bitrate, here I chose 8500k) 
   -profile 3 -level 41 (h264 profile high, level 4.1)
   -coder cabac (cabac coder chosen)
   -threads 4  (limit of 4 cpu threads, of 8 on this laptop)
   -allow_sw:v 1  (using VideoToolbox software encoding for accleration; GPU is not enabled)
   -map 0:1 -c:a:0 copy -disposition:a:0 default (copies audio stream over, unchanged, as default audio)
   -map 0:6 -c:s:0 copy -disposition:s:0 0 (copies subtitle stream over, not as default ... ie, will play subtitles automatically)
   -metadata:g (global metadata, you can reflect filename in metadata)
   -default_mode passthrough (allow audio w/o further processing) 
    outfile (NOTE: no dash precedes filename/path. Chose mkv format to
   hold my multiple streams; mp4 or other formats work just fine ... as
   long as contents are appropriate for format.)

Answer 3

In addition of llogan's answer I'd recommend set 'realtime' property to zero (this can increase quality in motion scenes)

Answer 4

As llogan says, bitrate option is good parameter in this situation.

ffmpeg -i input.mov -c:v h264_videotoolbox -b:v {bitrate} -c:a aac output.mp4

if you want to set 1000kb/s bitrate, command is like this

ffmpeg -i input.mov -c:v h264_videotoolbox -b:v 1000k -c:a aac output.mp4