[FFmpeg-trac] #7941(undetermined:closed): Transparency broken when encoding animated webp

[FFmpeg]

#7941: Transparency broken when encoding animated webp
-------------------------------------+-------------------------------------
             Reporter:  Nirel d      |                    Owner:  (none)
                 Type:  defect       |                   Status:  closed
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  git-master   |               Resolution:  invalid
             Keywords:  webp alpha   |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
Comment (by Mario Zechner):

 I've encountered the bug as well. It is indeed an FFmpeg issue (latest
 git-master) and can be reproduced by encoding a webm file with transparent
 background to an animated webp file file via e.g.

 {{{
 ❯ ./ffmpeg -c:v libvpx-vp9 -i in.webm -c:v libwebp -loop 0 out.webp

 ffmpeg version N-105642-g538be75a69 Copyright (c) 2000-2022 the FFmpeg
 developers
   built with Apple clang version 13.0.0 (clang-1300.0.29.30)
   configuration: --pkg-config-flags=--static --disable-ffplay --disable-
 ffprobe --disable-doc --disable-htmlpages --disable-manpages --disable-
 podpages --disable-txtpages --disable-libxcb --disable-lzma --disable-sdl2
 --enable-libvpx --enable-libwebp
   libavutil      57. 21.100 / 57. 21.100
   libavcodec     59. 21.100 / 59. 21.100
   libavformat    59. 17.101 / 59. 17.101
   libavdevice    59.  5.100 / 59.  5.100
   libavfilter     8. 27.100 /  8. 27.100
   libswscale      6.  5.100 /  6.  5.100
   libswresample   4.  4.100 /  4.  4.100
 [libvpx-vp9 @ 0x7f8e19412980] v1.11.0
     Last message repeated 1 times
 Input #0, matroska,webm, from 'out.webm':
   Metadata:
     ENCODER         : Lavf59.17.101
   Duration: 00:00:00.84, start: 0.000000, bitrate: 2777 kb/s
   Stream #0:0: Video: vp9 (Profile 0), yuva420p(tv, progressive), 511x699,
 SAR 1:1 DAR 511:699, 25 fps, 25 tbr, 1k tbn
     Metadata:
       alpha_mode      : 1
       ENCODER         : Lavc59.21.100 libvpx-vp9
       DURATION        : 00:00:00.840000000
 File 'out.webp' already exists. Overwrite? [y/N] y
 [libvpx-vp9 @ 0x7f8e197043c0] v1.11.0
 Stream mapping:
   Stream #0:0 -> #0:0 (vp9 (libvpx-vp9) -> webp (libwebp))
 Press [q] to stop, [?] for help
 [libvpx-vp9 @ 0x7f8e197043c0] v1.11.0
 Output #0, webp, to 'out.webp':
   Metadata:
     encoder         : Lavf59.17.101
   Stream #0:0: Video: webp, yuva420p(tv, progressive), 511x699 [SAR 1:1
 DAR 511:699], q=2-31, 200 kb/s, 25 fps, 1k tbn
     Metadata:
       alpha_mode      : 1
       DURATION        : 00:00:00.840000000
       encoder         : Lavc59.21.100 libwebp
 frame=   21 fps=0.0 q=-0.0 Lsize=     775kB time=00:00:00.80
 bitrate=7927.0kbits/s speed=0.848x
 video:775kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB
 muxing overhead: unknown
 }}}

 You can find the input webm file here: https://marioslab.io/dump/in.webm

 The bug is located in line 137 in webpenc.c. This sets the disposal method
 of the ANMF chunk to 0, which means the previous canvas contents are
 retained. For transparent frames, that is incorrect. The simplest fix is
 to set the disposal method to 1, like this:

 {{{
 git patch --- a/libavformat/webpenc.c
 +++ b/libavformat/webpenc.c
 @@ -134,7 +134,7 @@ static int flush(AVFormatContext *s, int trailer,
 int64_t pts)
                  avio_wl24(s->pb, pts - w->last_pkt->pts);
              } else
                  avio_wl24(s->pb, w->last_pkt->duration);
 -            avio_w8(s->pb, 0);
 +            avio_w8(s->pb, 0x1);
          }
          avio_write(s->pb, w->last_pkt->data + skip, w->last_pkt->size -
 skip);
          av_packet_unref(w->last_pkt);
 }}}

 This fixes the issue, but may lead to bigger webp files in case of non-
 transparent input frames.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/7941#comment:19>
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