Convert mov with Alpha to VP9 Webm with Alpha Using ffmpeg

Question

I am trying to convert a mov with alpha transparency to webm with alpha transparency, as seen here. I followed the steps explained here to no avail.

From this answer I was able to remove all the black in the video, thus making it transparent but this is not what I need as I already have a transparent mov and would like to convert that to transparent webm format.

ffmpeg -i input.mp4 -c:v libvpx -vf "colorkey=0x000000:0.1:0.1,format=yuva420p" out.webm

This is the ffprobe output of the video I would like to convert to webm with transparency.

built with Apple LLVM version 7.0.2 (clang-700.1.81)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/2.8.4 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-libfreetype --enable-libtheora --enable-libvorbis --enable-libvpx --enable-librtmp --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libass --enable-ffplay --enable-libspeex --enable-libschroedinger --enable-libfdk-aac --enable-libopus --enable-frei0r --enable-libopenjpeg --disable-decoder=jpeg2000 --extra-cflags=-I/usr/local/Cellar/openjpeg/1.5.2_1/include/openjpeg-1.5 --enable-nonfree --enable-vda
  libavutil      54. 31.100 / 54. 31.100
  libavcodec     56. 60.100 / 56. 60.100
  libavformat    56. 40.101 / 56. 40.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 40.101 /  5. 40.101
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.101 /  1.  2.101
  libpostproc    53.  3.100 / 53.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input.mov':
  Metadata:
    major_brand     : qt
    minor_version   : 0
    compatible_brands: qt
    creation_time   : 2016-01-17 16:04:07
    encoder         : Mac OS X v? (AVF 1046.9.1, CM 1731.15.20, x86_64)
    encoder-eng     : Mac OS X v? (AVF 1046.9.1, CM 1731.15.20, x86_64)
  Duration: 00:00:06.63, start: 0.000000, bitrate: 63966 kb/s
    Stream #0:0(eng): Video: prores (ap4h / 0x68347061), yuva444p10le(bt470bg/smpte240m/bt709), 1920x1080, 63963 kb/s, 25.03 fps, 25 tbr, 600 tbn, 600 tbc (default)
    Metadata:
      creation_time   : 2016-01-17 16:04:07
      handler_name    : Core Media Data Handler
      encoder         : Apple ProRes 4444

I've also tried the following command which didn't work for me either.

ffmpeg -y -i input.mov -c:v libvpx-vp9 -b:v 2000k -pass 1 -an -f webm output.webm

I'm using version 2.8.4 of ffmpeg on a Mac, installed with brew. 2.8.5 is the latest version.

score 18 · Answer 1 · answered Aug 23 '16 at 23:26

18

Since 2016-07-13, it's possible to encode VP9/webm videos with alpha channel (VP9a). However, the command you use here will create a VP8a video. Assuming you got a copy of ffmpeg compiled after that date, all you need is change the libvpx to libvpx-vp9. You don't need the yuva420p conversion either (is selected by default).

answered Aug 23 '16 at 23:26

cdlvcdlv

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`yuva420p` does not seem to be selected by default in my case. Upd.: though it appears to be a [ffprobe bug](https://trac.ffmpeg.org/ticket/8344) since in browser transparency does work – Klesun Jul 12 '23 at 04:56

score 16 · Accepted Answer · answered Jan 18 '16 at 14:04

16

Try

ffmpeg -i input.mov -c:v libvpx -pix_fmt yuva420p out.webm

answered Jan 18 '16 at 14:04

Gyan

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How can I set a bit rate to this? – Enijar Jan 26 '16 at 17:15
2

Add `-b:v 2000k` for 2mbps avg rate. – Gyan Jan 26 '16 at 17:18
That's VP8, not VP9. – oldmud0 Mar 19 '18 at 00:13
8

Yes, at time of answer, VP9 didn't support alpha. Change to `libvpx-vp9` – Gyan Mar 19 '18 at 06:09
3

Transparency encoding with auto_alt_ref does not work i got this error – Iftikar Urrhman Khan Jan 15 '20 at 14:42
2

@IftikarUrrhmanKhan try adding `-auto-alt-ref 0` right before the output – xuhcc Jun 03 '20 at 14:48
@TheForgot3n1 If you want to debug, open a new Q with full command & log. – Gyan Jul 08 '22 at 04:43

Elijah · Answer 3 · 2019-08-31T14:08:59.477

5

All of the other solutions resulted in a video of subpar quality. Please ensure that you define the bitrate to your liking. I changed mine from 1M to 2M and was satisfied.

ffmpeg -i "Model 1 V1.mov" -f webm -c:v libvpx -b:v 2M -acodec libvorbis -auto-alt-ref 0 model1v3.webm -hide_banner

If you're using After Effects or Premiere Pro, there's also this plugin, which can be used in Adobe Media Encoder.

edited Aug 31 '19 at 14:08

answered Aug 27 '19 at 19:17

Elijah

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score 1 · Answer 4 · answered Jan 18 '16 at 14:00

1

WebM-alpha is only defined for VP8. It doesn't work at all for VP9 right now.

answered Jan 18 '16 at 14:00

Ronald S. Bultje

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score 0 · Answer 5 · answered Jul 10 '22 at 10:42

0

This is the command that worked for me for VP8.

ffmpeg -i input.mov -c:v libvpx -pix_fmt yuva420p -b:v 2M -auto-alt-ref 0 output.webm

Accepted option did not work.

answered Jul 10 '22 at 10:42

TheForgot3n1

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Convert mov with Alpha to VP9 Webm with Alpha Using ffmpeg

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