Radiant Media Player

Working with FFmpeg

Mobile applications documentation



Last updated on June 14, 2017


Set up

This article describes how to produce MP4 and WebM video files compatible with Radiant Media Player and HTML5 video in general. For this guide with use the latest FFmpeg version available as of June 2017 (N-86442-g8aa6060) on Ubuntu Desktop LTS 16.04 64-bits.

Documentation sections:


Installing FFmpeg

General instruction for installing FFmpeg can be found here.

Guidelines for compiling FFmpeg can be found here.

Enabling some encoding/decoding options with FFmpeg can trigger licensing requirements depending on your use-case. Radiant Media Player does not provide licensing for encoding/decoding media content.


On-demand video transcoding to MP4 with AVC (H.264) video and AAC-LC audio - keyframes aligned renditions

In this section we will review how to produce MP4 files that will work with HTML5 video on devices where H.264 video and AAC-LC audio are supported. Specifically we will produce keyframes aligned video files at different bitrates and resolutions. This is a requirement for proper adaptive bitrate streaming in DASH and HLS.

We use a 16:9 1920x1080 24fps (a.k.a. True HD 1080p24) MOV input video file named input.mov. If you need a sample to play around, can get one from blender.org site.

ffmpeg -i input.mov -s 640x360 -c:v libx264 -b:v 650k -r 24 -x264opts keyint=48:min-keyint=48:no-scenecut -profile:v main -preset fast -movflags +faststart -c:a libfdk_aac -b:a 128k -ac 2 out-low.mp4

Explanation:

  • -i input.mov: this is our input video file
  • -s 640x360: we tell FFmpeg to resize our input file to 640x360 while transcoding
  • -c:v libx264: we tell FFmpeg to use x264 as the video encoding library
  • -b:v 650k: the target video bitrate should be 650 kbps
  • -r 24: we want a constant framerate at 24 fps (which is the same as our source video file in this case)
  • -x264opts keyint=48:min-keyint=48:no-scenecut: we should have one keyframe every 48 frames (every 2 seconds). The keyframe injection should be constant
  • -profile:v main: we want H.264 main profile which is supported by most devices on the market while offering good transcoding quality and options
  • -preset fast: we use a fast preset for x264 transcoding
  • -movflags +faststart: the file should be web ready (moov box before mdat box)
  • -c:a libfdk_aac: we use libfdk_aac as our audio encoding library
  • -b:a 128k: the target audio bitrate should be 128 kbps
  • -ac 2: we want a stereo (2 audio channels) output
  • out-low.mp4: our output file should be a MP4 file named out-low.mp4

Once we have created our first rendition we can create others at different resolutions and bitrates.

ffmpeg -i input.mov -s 960x540 -c:v libx264 -b:v 1400k -r 24 -x264opts keyint=48:min-keyint=48:no-scenecut -profile:v main -preset fast -movflags +faststart -c:a libfdk_aac -b:a 128k -ac 2 out-med.mp4
ffmpeg -i input.mov -s 1280x720 -c:v libx264 -b:v 2500k -r 24 -x264opts keyint=48:min-keyint=48:no-scenecut -profile:v main -preset fast -movflags +faststart -c:a libfdk_aac -b:a 128k -ac 2 out-high.mp4
ffmpeg -i input.mov -s 1920x1080 -c:v libx264 -b:v 5500k -r 24 -x264opts keyint=48:min-keyint=48:no-scenecut -profile:v main -preset fast -movflags +faststart -c:a libfdk_aac -b:a 128k -ac 2 out-max.mp4

Here is a suggestive encoding table for H.264/AAC-LC (16:9 aspect ratio at 24fps up to 4K resolution) for adaptive bitrate streaming:

  • 480x270 @ 350 kbps video with 128 kbps audio (bits/pixel: 0.113)
  • 640x360 @ 650 kbps video with 128 kbps audio (bits/pixel: 0.118)
  • 960x540 @ 1400 kbps video with 128 kbps audio (bits/pixel: 0.112)
  • 1280x720 @ 2500 kbps video with 128 kbps audio (bits/pixel: 0.113)
  • 1920x1080 @ 5500 kbps video with 128 kbps audio (bits/pixel: 0.111)
  • 2560x1440 @ 10000 kbps video with 128 kbps audio (bits/pixel: 0.113)
  • 3840x2160 @ 22000 kbps video with 128 kbps audio (bits/pixel: 0.111)

Generally a 0.1 bits/pixel ratio is good enough (but that may depend on content type) for H.264 encoding. A 0.11 bits/pixel ratio gives us a little more quality at an acceptable increase of video bitrate.

Feel free to play around with the numerous audio and video encoding settings that provide FFmpeg. An AAC encoding guide is available here. A H.264 encoding guide is available here.


On-demand video transcoding to MP4 with HEVC (H.265) video and HE-AACv2 audio - keyframes aligned renditions

Now that you know about everything on H.264/AAC-LC encoding we can go ahead with H.265/HE-AACv2 encoding. H.265 claims it can reduce file size up to 50% versus H.264 without loss of quality (in reality H.265 works well at lower and higher bitrate but may not be as ground breaking as it wants to be especially considering the licensing cost). HE-AACv2 is a newer version of AAC which works well for low bitrate audio encoding.

ffmpeg -i input.mov -s 640x360 -c:v libx265 -b:v 390k -r 24 -x265-params "keyint=48:min-keyint=48:no-open-gop:no-scenecut" -preset fast -movflags +faststart -c:a libfdk_aac -profile:a aac_he_v2 -b:a 96k -ac 2 out-low.mp4

Here is a suggestive encoding table for H.265/HE-AACv2 (16:9 aspect ratio at 24fps up to 4K resolution) for adaptive bitrate streaming:

  • 480x270 @ 220 kbps video with 96 kbps audio (bits/pixel: 0.071)
  • 640x360 @ 390 kbps video with 96 kbps audio (bits/pixel: 0.071)
  • 960x540 @ 880 kbps video with 96 kbps audio (bits/pixel: 0.071)
  • 1280x720 @ 1550 kbps video with 96 kbps audio (bits/pixel: 0.070)
  • 1920x1080 @ 3500 kbps video with 96 kbps audio (bits/pixel: 0.070)
  • 2560x1440 @ 6200 kbps video with 96 kbps audio (bits/pixel: 0.070)
  • 3840x2160 @ 14000 kbps video with 96 kbps audio (bits/pixel: 0.070)

Generally a 0.07 bits/pixel ratio is an acceptable conservative ratio for H.265 encoding. You can go down to a 0.06 bits/pixel ratio for smaller bitrate.

A H.265 encoding guide is available here.


On-demand video transcoding to WebM with VP9 video and opus audio

Alternatively to the patent-encumbered H.26*/AAC codecs we can use the open and royalty free VP9 video & opus audio codecs. VP9/opus work great on most modern browsers but with the significant caveat that macOS, iOS and most Microsoft browsers do not support them :(

Arguably VP9 (slightly) outperforms H.264 for video encoding. H.265 does outperform VP9 encoding however. The future release of the AV1 codec may just match H.265 current capabilities. In all cases, aside obvious licensing considerations for H.26* and AAC, adoption is going to be an issue. HEVC has slow adoption as of mid 2017 (though this may change with Apple announcing support for HEVC on iOS 11 and macOS High Sierra at WWDC 2017). We can bet that companies holding HEVC patents are going to be reluctant to sail with AV1 even if it becomes as good as HEVC ... to be continued.

But enough with the chit-chat and let us do some encoding:

ffmpeg -i input.mov -s 640x360 -c:v libvpx-vp9 -minrate 480k -maxrate 480k -b:v 480k -r 24 -c:a libopus -b:a 96k -ac 2 out-low.webm

A guide to VP9 encoding with FFmpeg is available here. In the above example we used a constant bitrate (CBR) encoding approach. I am a big fan of CBR (or more precisely CVBR - constrained variable bitrate) encoding because as some of you may know UVBR (unconstrained variable bitrate) may cause issues at decoding time especially on lower end devices or in poor network conditions. Achieving CBR or CVBR with VP9 encoding is quite straight forward - for H.264 it can be done but you are going to need to do some research about the bufsize setting (help for H.264 CBR encoding can be found here).

Here is a suggestive encoding table for VP9/Opus (16:9 aspect ratio at 24fps up to 4K resolution) for adaptive bitrate streaming:

  • 480x270 @ 270 kbps video with 96 kbps audio (bits/pixel: 0.087)
  • 640x360 @ 480 kbps video with 96 kbps audio (bits/pixel: 0.087)
  • 960x540 @ 1070 kbps video with 96 kbps audio (bits/pixel: 0.086)
  • 1280x720 @ 1900 kbps video with 96 kbps audio (bits/pixel: 0.086)
  • 1920x1080 @ 4300 kbps video with 96 kbps audio (bits/pixel: 0.086)
  • 2560x1440 @ 7600 kbps video with 96 kbps audio (bits/pixel: 0.086)
  • 3840x2160 @ 17000 kbps video with 96 kbps audio (bits/pixel: 0.085)

Generally a 0.85 bits/pixel ratio is an acceptable semi-conservative ratio for VP9 encoding.

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