[FFmpeg-user] "Extracting" Alpha Channel
dflett at bigpond.net.au
Thu Jul 28 13:47:59 CEST 2011
From: ffmpeg-user-bounces at ffmpeg.org [mailto:ffmpeg-user-bounces at ffmpeg.org]
On Behalf Of Stefano Sabatini
Sent: Thursday, 28 July 2011 19:13
To: FFmpeg user questions and RTFMs
Subject: Re: [FFmpeg-user] "Extracting" Alpha Channel
On date Thursday 2011-07-28 12:00:36 +1000, Dan Flett encoded:
> In television graphics or vision mixing, it's often necessary to "extract"
> the alpha channel from a graphics file and turn it into a separate
> "Key" or "Matte" clip.
> The Key video is a grayscale representation of the alpha channel -
> black is fully transparent and white is fully opaque.
> It is possible to do this the using "lut" filter (I use lutrgb):
> -vf "[in] format=rgba, split [T1], fifo,
> [T2] overlay [out]; [T1] fifo, lutrgb=r=maxval:g=maxval:b=maxval [T2]"
> This filter first converts the input video to the rgba colorspace.
> It then splits the input video into 2 streams (as per the turorial in
> the libavilfter docs). The "background" stream is set to a fully
> black frame by setting the r,g,b values to "minval". The foreground
> stream is set to a fully white frame by setting the r,g,b values to
> "maxval". In both streams the alpha channel is untouched.
> At the end of the first stream, the "overlay" filter overlays the
> white stream over the black one, using the untouched alpha channel as the
> The result is a grayscale representation of the alpha channel - the
> actual rgb video content is thrown away.
Another interesting possibility (simpler, faster) would be to implement a
decomposer and a composer filter, for splitting RGBA channels into separate
gray video streams and re-composing them.
How about being able to do something like this?
Basically, being able to specify that the y (luma) channel output is equal
to the a (alpha) channel of the input?
Being able to swap color channels around would be good:
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