Intel i5 Kaby Lake – Hardware-Assisted Transcoding to VP9 and FLAC Using FFmpeg

There is no automatic way to do that. You have to look at the parameters of the original file and apply them to the output file.

In most cases, these will be the following:

• Container format (MP4, MKV, …)
• Video and audio codec (H.264, H.265, …)
• Audio-specific:
  • Number of audio channels
  • Audio sampling rate
  • Audio bitrate
• Video-specific:
  • Profile and Level (to ensure compatibility, see here)
  • Maximum bitrate limitations (e.g. for H.264)
  • Maximum video resolution, change via -filter:v scale or -s:v
  • Framerate, change via -filter:v fps -r
  • Chroma subsampling, change via -pix_fmt (e.g., -pix_fmt yuv420p should give you the best compatibility)
  • GOP size (distance between IDR-frames), set via -g
  • Other specific encoding settings

But even if you get that all right, some devices may require specific, proprietary information embedded in the bitstream.

As for the specific task of using x264, this is not going to be trivial. I'm not aware of a single script that'd take care of these tasks, which are usually done manually. For the most info about the encoding settings, on Unix/Linux or OS X, you can use mediainfo with some Bash tricks.

For example, for an x264-encoded video in an MP4 file:

mediainfo input.mp4 | grep "Encoding settings" | cut -d':' -f2- | tr '/' '\n' | sed 's/ //'

This will output a list of x264 options:

cabac=1
ref=3
deblock=1:-1:-1
analyse=0x3:0x113
me=hex
subme=7
psy=1
…

You could then manually pass these options to the x264 binary.

If you go through FFmpeg, that's a little more complicated though, as not all of x264's options can or should be mapped like this. Note that often a simple preset, tune and profile specification will do as well (as seen in x264 --fullhelp and the x264 encoding guide), and specifying the CRF level is enough.

And this is not even considering audio, where luckily, there aren't that many options.

Adjust the h264 stream to be streamable:

Apparently the h264 codec has a special mode that is required for it to be efficiently streamable and you enable it with: -bsf:v h264_mp4toannexb

Script

The script I use to setup a H264+AAC matroska streaming pipe is this:

#!/bin/bash 
# ----------------------------------------------------------------------------
# This script is a helper to transcode a video to H264+AAC with subtitles to a 
# Matroska (.mkv) container that is suitable for live streaming to a mobile 
# device. It will transcode media that is not H264 or that has too high 
# resolution. It will not upsample content. 
# 
# Other suitable containers (and reasons for not using them) include: 
# * ASF (Microsoft, proprietary) 
# * MPEG2 Transport Stream (Standard, only supports bitmap subtitles) 
# * WebM (Has no support for metadata) 
# * DivX (Can't contain H264) 
# * FLV (Proprietary Bad support on target device) 
# * MP4 (Only bitmap subtitles, didn't work for streaming with FFMPEG) 
# * OGG (No support for H264) 
# ----------------------------------------------------------------------------

# ----------------------------------------------------------------------------
# Video options 
# ----------------------------------------------------------------------------
LINES=720

# One of: ultrafast,superfast, veryfast, faster, fast, medium, slow, slower, 
# veryslow or placebo 
PRESET=ultrafast 

# One of: baseline, main, high, high10, high422 or high444 
PROFILE=high10 

# One of: film animation grain stillimage psnr ssim fastdecode zerolatency 
TUNE=zerolatency 

# ----------------------------------------------------------------------------
# Audio options 
# ----------------------------------------------------------------------------
AUDIO="-c:a libfaac -b:a 128k -ar 48000 -ac 2 -async 1"

SUBTITLES="-c:s copy"

# ----------------------------------------------------------------------------
# Read input video parameters 
# ----------------------------------------------------------------------------
IN_RESOLUTION=`/usr/bin/ffmpeg -i "${1}" 2>&1 | grep Video | \
    perl -lane 'print $1 if /(\d+x\d+)/'`
IN_CODEC=`/usr/bin/ffmpeg -i "${1}" 2>&1 | grep Video | \
    perl -lane 'print $1 if /Video: (\S+)/'`
IN_DIMS=(${IN_RESOLUTION//x/ })
V_TRANSCODE="-c:v libx264 -bsf:v h264_mp4toannexb -preset ${PRESET} \
    tune ${TUNE} -profile:v ${PROFILE}"
V_COPY="-c:v copy -bsf:v h264_mp4toannexb"

if [ "${IN_DIMS[1]}" > "${LINES}" ]; then
    SCALE="-filter:v scale=-1:${LINES} ${OPT_TRANSCODE}"
else
    if ["${IN_CODEC}" != "h264" ]; then
        VIDEO=$OPT_TRANSCODE
    else
        VIDEO=$V_COPY
    fi
fi

exec /usr/bin/ffmpeg -threads `nproc` -i "${1}" $VIDEO $AUDIO $SUBTITLES \
    -f matroska -y "${2}" &> /store/tmp/log

TODO:

Make it read subtitles from external files if found and add them to the matroska stream. Make it not transcode the audio stream if it is already in a suitable format.