Hi,
I'm trying to decode a vp8 encoded video with alpha channel. I clone ffmpeg
from git and configured with the following options:
./configure --prefix="${HOME}/ffmpeg_build_git" --pkg-config-flags=--static
--extra-cflags=-I"${HOME}/ffmpeg_build_git/include"
--extra-ldflags=-L"${HOME}/ffmpeg_build_git/lib"
--bindir="${HOME}/ffmpeg_build_git/bin" --enable-gpl --enable-yasm
--enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame
--enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx
--enable-nonfree --disable-doc --enable-vaapi --enable-vda --enable-vdpau
--enable-decoder=h264_vdpau --enable-hardcoded-tables
--enable-hwaccel=h264_vaapi --enable-hwaccel=h264_vdpau --enable-libx264
After that I run the command:
./ffmpeg -i ~/Desktop/png/filmato_01_alfa.png -c:v libvpx -pix_fmt yuva420p
-metadata:s:v:0 alpha_mode="1" output.webm
to encode a vp8 video, I obtain the following output:
ffmpeg version N-81641-ga50ca6b Copyright (c) 2000-2016 the FFmpeg developers
built with gcc 4.9.2 (Debian 4.9.2-10)
configuration: --prefix=/home/qmax/ffmpeg_build_git
--pkg-config-flags=--static
--extra-cflags=-I/home/qmax/ffmpeg_build_git/include
--extra-ldflags=-L/home/qmax/ffmpeg_build_git/lib
--bindir=/home/qmax/ffmpeg_build_git/bin --enable-gpl --enable-yasm
--enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame
--enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx
--enable-nonfree --disable-doc --enable-vaapi --enable-vda --enable-vdpau
--enable-decoder=h264_vdpau --enable-hardcoded-tables
--enable-hwaccel=h264_vaapi --enable-hwaccel=h264_vdpau --enable-libx264
libavutil 55. 29.100 / 55. 29.100
libavcodec 57. 55.101 / 57. 55.101
libavformat 57. 48.103 / 57. 48.103
libavdevice 57. 0.102 / 57. 0.102
libavfilter 6. 62.100 / 6. 62.100
libswscale 4. 1.100 / 4. 1.100
libswresample 2. 1.100 / 2. 1.100
libpostproc 54. 0.100 / 54. 0.100
Input #0, png_pipe, from '/home/qmax/Desktop/png/filmato_01_alfa.png':
Duration: N/A, bitrate: N/A
Stream #0:0: Video: png, rgba(pc), 1280x720 [SAR 1:1 DAR 16:9], 25 tbr, 25
tbn, 25 tbc
[libvpx @ 0x2d2ace0] v1.3.0
[webm @ 0x2d29080] Using AVStream.codec to pass codec parameters to muxers is
deprecated, use AVStream.codecpar instead.
Output #0, webm, to 'output.webm':
Metadata:
encoder : Lavf57.48.103
Stream #0:0: Video: vp8 (libvpx), yuva420p, 1280x720 [SAR 1:1 DAR 16:9],
q=-1--1, 200 kb/s, 25 fps, 1k tbn, 25 tbc
Metadata:
alpha_mode : 1
encoder : Lavc57.55.101 libvpx
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
Stream mapping:
Stream #0:0 -> #0:0 (png (native) -> vp8 (libvpx))
Press [q] to stop, [?] for help
frame= 1 fps=0.0 q=0.0 Lsize= 69kB time=00:00:00.00
bitrate=562384.0kbits/s speed=0.00144x
video:34kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing
overhead: 101.311569%
I would use ffmpeg to decode this video when i run:
~/ffmpeg_build_git/bin/ffmpeg -i ./output.webm -f null - -benchmark
ffmpeg version N-81641-ga50ca6b Copyright (c) 2000-2016 the FFmpeg developers
built with gcc 4.9.2 (Debian 4.9.2-10)
configuration: --prefix=/home/qmax/ffmpeg_build_git
--pkg-config-flags=--static
--extra-cflags=-I/home/qmax/ffmpeg_build_git/include
--extra-ldflags=-L/home/qmax/ffmpeg_build_git/lib
--bindir=/home/qmax/ffmpeg_build_git/bin --enable-gpl --enable-yasm
--enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame
--enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx
--enable-nonfree --disable-doc --enable-vaapi --enable-vda --enable-vdpau
--enable-decoder=h264_vdpau --enable-hardcoded-tables
--enable-hwaccel=h264_vaapi --enable-hwaccel=h264_vdpau --enable-libx264
libavutil 55. 29.100 / 55. 29.100
libavcodec 57. 55.101 / 57. 55.101
libavformat 57. 48.103 / 57. 48.103
libavdevice 57. 0.102 / 57. 0.102
libavfilter 6. 62.100 / 6. 62.100
libswscale 4. 1.100 / 4. 1.100
libswresample 2. 1.100 / 2. 1.100
libpostproc 54. 0.100 / 54. 0.100
Input #0, matroska,webm, from './output.webm':
Metadata:
encoder : Lavf57.48.103
Duration: 00:00:00.04, start: 0.000000, bitrate: 14059 kb/s
Stream #0:0: Video: vp8, yuv420p, 1280x720, SAR 1:1 DAR 16:9, 25 fps, 25
tbr, 1k tbn, 1k tbc (default)
Metadata:
alpha_mode : 1
[null @ 0x27019e0] Using AVStream.codec to pass codec parameters to muxers is
deprecated, use AVStream.codecpar instead.
Output #0, null, to 'pipe:':
Metadata:
encoder : Lavf57.48.103
Stream #0:0: Video: wrapped_avframe, yuv420p, 1280x720 [SAR 1:1 DAR 16:9],
q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc (default)
Metadata:
alpha_mode : 1
encoder : Lavc57.55.101 wrapped_avframe
Stream mapping:
Stream #0:0 -> #0:0 (vp8 (native) -> wrapped_avframe (native))
Press [q] to stop, [?] for help
frame= 1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.04 bitrate=N/A speed= 5.8x
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing
overhead: unknown
bench: utime=0.004s
bench: maxrss=18300kB
The input stream has yuv420p pixel format rather than yuva420p.
By analyzing the source code in libavcodec/libvpxdec.c in the function
vpx_decode there is a reference to the alpha channel that is set in:
planes[3] =
ctx->has_alpha_channel ? img_alpha->planes[VPX_PLANE_Y] : NULL;
linesizes[3] =
ctx->has_alpha_channel ? img_alpha->stride[VPX_PLANE_Y] : 0;
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