lavfi: add alphaextract and alphamerge filters

These filters are designed for storing and transmitting video sequences
with alpha using higher-efficiency codecs such as x264 which don't
natively support an alpha channel. 'alphaextract' takes an input stream
with an alpha channel and returns a video containing just the alpha
component as a grayscale value; 'alphamerge' takes an RGB or YUV stream
and adds an alpha channel recovered from a second grayscale stream.

Signed-off-by: Steven Robertson <steven@strobe.cc>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/doc/filters.texi b/doc/filters.texi
index 4a6c092e59..2b5cc875f5 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -1101,6 +1101,31 @@ build.
 
 Below is a description of the currently available video filters.
 
+@section alphaextract
+
+Extract the alpha component from the input as a grayscale video. This
+is especially useful with the @var{alphamerge} filter.
+
+@section alphamerge
+
+Add or replace the alpha component of the primary input with the
+grayscale value of a second input. This is intended for use with
+@var{alphaextract} to allow the transmission or storage of frame
+sequences that have alpha in a format that doesn't support an alpha
+channel.
+
+For example, to reconstruct full frames from a normal YUV-encoded video
+and a separate video created with @var{alphaextract}, you might use:
+@example
+movie=in_alpha.mkv [alpha]; [in][alpha] alphamerge [out]
+@end example
+
+Since this filter is designed for reconstruction, it operates on frame
+sequences without considering timestamps, and terminates when either
+input reaches end of stream. This will cause problems if your encoding
+pipeline drops frames. If you're trying to apply an image as an
+overlay to a video stream, consider the @var{overlay} filter instead.
+
 @section ass
 
 Draw ASS (Advanced Substation Alpha) subtitles on top of input video