Converting video to WebM with ffmpeg/avconv

to_webm.sh

#!/bin/sh
#
# -*- varribas archive -*-
#
## Converting video to WebM with ffmpeg/avconv
# https://superuser.com/questions/556463/converting-video-to-webm-with-ffmpeg-avconv
#
# The following information is taken from the FFmpeg vpx (WebM) Encoding Guide
# and gives a basic overview of WebM encoding. As the libvpx encoder for FFmpeg
# hasn't really been documented before, any suggestions for improvement are
# welcome, both here and on the FFmpeg Wiki.
#
# Constant Bit Rate
# First of all, libvpx offers constant bitrate and variable bitrate encoding modes.
# Constant bitrate should be avoided whenever possible (unless you target a
# specific file size or streaming scenario), since the average quality per
# file size will be worse. Still, you could try setting a constant bitrate
# if nothing else works for you, e.g. with 1 MBit/s:
#
# ffmpeg -i input.mp4 -c:v libvpx -minrate 1M -maxrate 1M -b:v 1M output.webm
#
# Look at the output and increase or decrease the bit rate to your liking
# (or file size constraints). For example, you can use 500K or 2M, et cetera.
#
# You have to specify -minrate, -maxrate and the bitrate -b:v in order for the
# encoder to use CBR. They all have to have the same value—otherwise it'll
# choose a different target bitrate instead and do VBR encoding, but with
# bad quality.
#
# Variable Bit Rate
# You need to play around with the options if you want VBR encoding, which
# gives you a better overall quality, since the encoder can freely choose
# how many bits to assign to a frame.
#
# In libvpx, you have four parameters to control VBR encoding. All are
# optional, but you should set them regardless:
#
# -qmin – the minimum quantization parameter (default 4)
# -qmax – the maximum quantization parameter (default 63)
# -b:v  – the target bit rate setting. If not set, the encoder will
#         choose ~1000 kBit/s as a default, but only when the -crf
#         option is used.
# -crf  – the overall quality setting. If not set, the encoder will
#         do "normal" VBR, trying to reach the target bitrate in
#         within the qmin/qmax bounds. If set, the encoder will
#         use CQ mode, and the target bitrate will become the
#         maximum allowed bitrate. The CRF value is 10 by default.
#
# Important: If neither -b:v nor -crf are set, the encoder will use a default
# bitrate of 256 kBit/s and your result will look bad.
#
# Per the WebM Wiki, -qmin should be set from 0–4, and -qmax from 50–63.
# These Q values are quantization parameters, and lower generally
# means "better quality". If you set the bounds from 0 to 63, this means
# the encoder has free choice of how to assign the quality.
#
# Here are different approaches on how to control the quality:
# * You can choose a lower -qmax value, and leave -qmin at something
#   between 0 and 4. From a quick test it seems 20–30 are sane values here
#   if you really want higher quality. Note that the target bit rate here is
#   still very low and we only force it to be higher because we artificially
#   restrict the encoder—doesn't seem like the best approach.
# ffmpeg -i input.mp4 -c:v libvpx -qmin 0 -qmax 20 output.webm
#
# * A better way: You can set the -crf option and a maximum bit rate,
#   then let libvpx do its job. To get better quality, simply choose
#    a lower -crf value:
# ffmpeg -i input.mp4 -c:v libvpx -crf 10 -b:v 2M output.webm
#
# * You can additionally specify -qmin and -qmax again, which seems like the
#   ideal approach, because you also tell it not to drop below a certain
#   quality (e.g., 50 here):
# ffmpeg -i input.mp4 -c:v libvpx -qmin 0 -qmax 50 -crf 10 -b:v 2M output.webm
#
# Audio
# In all of the above examples, Vorbis audio should be used, by setting
# -c:a libvorbis. The quality can also be set to VBR, with the -q:a option.
#
# ffmpeg -i input.mp4 … -c:a libvorbis -q:a 4 output.webm
#
# Values range from 0 to 10, where higher means better quality. 4 should be a
# reasonable default and corresponds to roughly 128 kBit/s for stereo audio.
#
# Do not use the -c:a vorbis option, as this is the FFmpeg-native encoder
# with very bad quality.


set -e
set -u

input=$1
output=${1%.*}.webm

avconv -i $input -c:v libvpx -qmin 0 -qmax 50 -crf 10 -b:v 2M -c:a libvorbis -q:a 6 $output