[FFmpeg-user] Question for alphamerge

[Johnny]

Hi,


I'm using ffmpeg to produce a video with transparent background, there
are two videos, one has a pink background and a women walking around,
and the other is just a white-black luma clip of the first video, i want
to make the video to be transarent except the women, however, the output
format is always "Video: vp8, yuv420p", the yuv420p does not support
alpha channel, but i have no idea how to set the output video pixel
format, i tried -pix_fmt argb and others, all are failed, please help
me, thank you very much!

Here is my command:
$: ffmpeg -i original.webm -vf
"movie=alpha.webm[alpha];[in][alpha]alphamerge[out]" out.webm

Here is the out put:
ffmpeg version N-47062-g26c531c Copyright (c) 2000-2012 the FFmpeg
developers
built on Nov 25 2012 12:23:20 with gcc 4.7.2 (GCC)
configuration: --disable-static --enable-shared --enable-gpl
--enable-version3 --disable-pthreads --enable-runtime-cpudetect --enable-
ynth --enable-bzlib --enable-frei0r --enable-libass
--enable-libopencore-amrnb --enable-libopencore-amrwb
--enable-libfreetype --enable-
sm --enable-libmp3lame --enable-libnut --enable-libopenjpeg
--enable-libopus --enable-librtmp --enable-libschroedinger --enable-libspeex
nable-libtheora --enable-libutvideo --enable-libvo-aacenc
--enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx
--enable-libx264 --
le-libxavs --enable-libxvid --enable-zlib
libavutil 52. 9.100 / 52. 9.100
libavcodec 54. 77.100 / 54. 77.100
libavformat 54. 37.100 / 54. 37.100
libavdevice 54. 3.100 / 54. 3.100
libavfilter 3. 23.102 / 3. 23.102
libswscale 2. 1.102 / 2. 1.102
libswresample 0. 17.101 / 0. 17.101
libpostproc 52. 2.100 / 52. 2.100
Input #0, matroska,webm, from 'original.webm':
Metadata:
creation_time : 2012-12-14 09:14:51
Duration: 00:00:38.96, start: 0.000000, bitrate: 1515 kb/s
Stream #0:0(eng): Video: vp8, yuv420p, 800x1280, SAR 1:1 DAR 5:8, 25
fps, 25 tbr, 1k tbn, 1k tbc (default)
[libvpx @ 00bf8980] v1.1.0
Output #0, webm, to 'out.webm':
Metadata:
encoder : Lavf54.37.100
Stream #0:0(eng): Video: vp8, yuv420p, 800x1280 [SAR 1:1 DAR 5:8],
q=-1--1, 200 kb/s, 1k tbn, 25 tbc (default)
Stream mapping:
Stream #0:0 -> #0:0 (vp8 -> libvpx)
Press [q] to stop, [?] for help
frame= 974 fps=7.8 q=0.0 Lsize= 1025kB time=00:00:38.96 bitrate=
215.5kbits/s
video:1018kB audio:0kB subtitle:0 global headers:0kB muxing overhead
0.725502%


Best Regards & many thanks!
Johnny