[FFmpeg-user] Bad Quality Webm video with alpha

[Rafael Mancini]

>
> 2018-09-25 1:14 GMT+02:00, Rafael Mancini <rafaelbmancini at gmail.com>:
>
> > I'm trying to generate a WebM video with alpha for a HTML5 template.
> > Already tried different compression settings and I'm still getting poor
> > quality results for complex videos with lots of animated objects.
> >
> > The Input video is made in After Effects. Its a short 8 seconds video.
> I'm
> > using a PNG sequence with alpha, 24fps in Full HD.
> >
> > I'm using the following command line:
> >
> > ffmpeg -r 24 -i "InputFileName_%04d.png" -r 24 -b:v 10000k -bufsize
> 10000k
> > -force_key_frames expr:gte(t,n_forced*1) OutputFileName.webm
>
> Complete, uncut console output missing.
> (Did you confirm that this output format actually helps you?
> Afair, not many applications can read it.)
>
> The second "-r" is not a good idea, please try first without
> "-force_key_frames",
> you can still add it later, and please try with constant quality instead of
> bitrate first.
>
> Carl Eugen
>

So, after altering the command line I got a much better result with this:

*ffmpeg -r 24 -i "input 1920x1080.mov" -c:v libvpx-vp9 -crf 30 -b:v 0
"output 1920x1080.webm"*

The quality is a lot better, but still not exactly what I wanted... very
close, but not quiet. The file size is good: 4.4MB.

This is very usefull to generate videos for HTM5 with alpha channel, so you
can create interaction layers on top of any template.

I wish I could set the -crf value to 20. That's where I got a very good
result, but the file gets too big: +30MB.

Below is the complete output. Any ideas on how to increase quality and
decrease file size?

*ffmpeg version N-92043-g0f36ad514c Copyright (c) 2000-2018 the FFmpeg
developers*
*  built with gcc 8.2.1 (GCC) 20180813*
*  configuration: --enable-gpl --enable-version3 --enable-sdl2
--enable-fontconfig --enable-gnutls --enable-iconv --enable-libass
--enable-libbluray --enable-libfreetype --enable-libmp3lame
--enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg
--enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr
--enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack
--enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2
--enable-libzimg --enable-lzma --enable-zlib --enable-gmp
--enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc
--enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom
--enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid
--enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2
--enable-avisynth*
*  libavutil      56. 19.101 / 56. 19.101*
*  libavcodec     58. 31.100 / 58. 31.100*
*  libavformat    58. 18.102 / 58. 18.102*
*  libavdevice    58.  4.104 / 58.  4.104*
*  libavfilter     7. 32.100 /  7. 32.100*
*  libswscale      5.  2.100 /  5.  2.100*
*  libswresample   3.  2.100 /  3.  2.100*
*  libpostproc    55.  2.100 / 55.  2.100*
*Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input 1920x1080.mov':*
*  Metadata:*
*    major_brand     : qt*
*    minor_version   : 537199360*
*    compatible_brands: qt*
*    creation_time   : 2018-09-27T18:58:25.000000Z*
*  Duration: 00:00:08.04, start: 0.000000, bitrate: 731670 kb/s*
*    Stream #0:0(eng): Video: qtrle (rle  / 0x20656C72), bgra(progressive),
1920x1080, 730627 kb/s, SAR 1:1 DAR 16:9, 24 fps, 24 tbr, 24 tbn, 24 tbc
(default)*
*    Metadata:*
*      creation_time   : 2018-09-27T18:58:25.000000Z*
*      handler_name    : Apple Video Media Handler*
*      encoder         : Animation*
*      timecode        : 00:00:00:00*
*    Stream #0:1(eng): Data: none (tmcd / 0x64636D74), 0 kb/s (default)*
*    Metadata:*
*      creation_time   : 2018-09-27T18:58:25.000000Z*
*      handler_name    : Time Code Media Handler*
*      timecode        : 00:00:00:00*
*Stream mapping:*
*  Stream #0:0 -> #0:0 (qtrle (native) -> vp9 (libvpx-vp9))*
*Press [q] to stop, [?] for help*
*[libvpx-vp9 @ 00000155dfeccec0] v1.7.0-825-g7b925825a*
*Output #0, webm, to 'output 1920x1080.webm':*
*  Metadata:*
*    major_brand     : qt*
*    minor_version   : 537199360*
*    compatible_brands: qt*
*    encoder         : Lavf58.18.102*
*    Stream #0:0(eng): Video: vp9 (libvpx-vp9), yuva420p, 1920x1080 [SAR
1:1 DAR 16:9], q=-1--1, 24 fps, 1k tbn, 24 tbc (default)*
*    Metadata:*
*      creation_time   : 2018-09-27T18:58:25.000000Z*
*      handler_name    : Apple Video Media Handler*
*      timecode        : 00:00:00:00*
*      encoder         : Lavc58.31.100 libvpx-vp9*
*    Side data:*
*      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1*
*frame=  193 fps=1.8 q=0.0 Lsize=    4480kB time=00:00:08.00
bitrate=4586.9kbits/s speed=0.0758x*
*video:1756kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB
muxing overhead: 155.090347%*