[FFmpeg-user] How to add alpha channel to normal png image

[Paul B Mahol]

On 6/28/15, qw <applemax82 at 163.com> wrote:
> At 2015-06-28 17:11:03, "Paul B Mahol" <onemda at gmail.com> wrote:
>>Dana 28. 6. 2015. 09:31 osoba "qw" <applemax82 at 163.com> napisala je:
>>>
>>> Hi,
>>>
>>>
>>> PNG format supports alpha channel, but some png images don't have alpha
>>channel. How to add alpha channel to those png images and set alpha value?
>>>
>>>
>>
>>See colorkey filter documentation.
>>
> I use the below command to run colorkey filter, and ffmpeg report there is
> no colorkey filter:
> ffmpeg -i ./image.png -vf colorkey=green ./output.png
>
>
> ffmpeg version 2.7.1 Copyright (c) 2000-2015 the FFmpeg developers
>   built with gcc 4.4.7 (GCC) 20120313 (Red Hat 4.4.7-11)
>   configuration: --enable-gpl --enable-libx264
>   libavutil      54. 27.100 / 54. 27.100
>   libavcodec     56. 41.100 / 56. 41.100
>   libavformat    56. 36.100 / 56. 36.100
>   libavdevice    56.  4.100 / 56.  4.100
>   libavfilter     5. 16.101 /  5. 16.101
>   libswscale      3.  1.101 /  3.  1.101
>   libswresample   1.  2.100 /  1.  2.100
>   libpostproc    53.  3.100 / 53.  3.100
> Input #0, png_pipe, from './transparent_3.png':
>   Duration: N/A, bitrate: N/A
>     Stream #0:0: Video: png, rgba(pc), 256x256, 25 tbr, 25 tbn, 25 tbc
> [AVFilterGraph @ 0x1d1e900] No such filter: 'colorkey'
> Error opening filters!
>
>
> which version of ffmpeg has colorkey filter?
>

Git version.