Over the summer, I worked on putting together a script which took a x264 video/mp3 stream and broke it up into the different streams so that it would work via MSE-DASH. (Based heavily on the instructions on the webmproject.org website) Those same scripts have ceased to work, turning a 6GB video into several 25 Gb videos. I kept up with updates of ffmpeg and so I don't know when it stopped working, but I am guessing it was due to the way that their DASH Webm implementation was updated.
I found new method which works better, but still has a major problem with one stream. I was hoping someone could explain how this encoding works so that I could understand the underlying cause.
#!/bin/bash
COMMON_OPTS="-map 0:0 -an -threads 11 -cpu-used 4 -cmp chroma"
WEBM_OPTS="-f webm -c:v vp9 -keyint_min 50 -g 50 -dash 1"
ffmpeg -i $1 -vn -acodec libvorbis -ab 128k audio.webm &
ffmpeg -i $1 $COMMON_OPTS $WEBM_OPTS -b:v 500k -vf scale=1280:720 -y vid-500k.webm &
ffmpeg -i $1 $COMMON_OPTS $WEBM_OPTS -b:v 700k -vf scale=1280:720 -y vid-700k.webm &
ffmpeg -i $1 $COMMON_OPTS $WEBM_OPTS -b:v 1000k -vf scale=1280:720 -y vid-1000k.webm &
ffmpeg -i $1 $COMMON_OPTS $WEBM_OPTS -b:v 1500k -vf scale=1280:720 -y vid-1500k.webm
The transcode is not yet complete, but you can see where this is headed:
-rw-r--r-- 1 user staff 87M Jan 4 23:27 audio.webm
-rw-r--r-- 1 user staff 27M Jan 4 23:42 vid-1000k.webm
-rw-r--r-- 1 user staff 285M Jan 4 23:42 vid-1500k.webm
-rw-r--r-- 1 user staff 15M Jan 4 23:42 vid-500k.webm
-rw-r--r-- 1 user staff 20M Jan 4 23:42 vid-700k.webm
The 1500k variant is disproportionately larger than the other streams.
The other problem is that when I use a shorter video, lets say eight or nine minutes, the above configuration runs as expected and everything is perfect. I don't know where the limit for this is since each test costs a lot of processing power and time, but if it's less than ten minutes, it works and if its longer than an hour, it produces massive files.
If you want to keep the video bitrate constrained to some value it's not enough to use just
-b:v
. Depending on the source (high movement, scene complexity) the actual bitrate will go higher.You need to specify a
-maxrate
and-bufsize
too.Ie. to get a variable bitrate (VBR) video constrained at around 1500k - the peak can be higher depending on the size of the VBV - you would use:
For constant bitrate (CBR) the best result are achieved with a single frame VBV. Example for 25fps (1500 / 25 = 60):