[gstreamer-bugs] [Bug 639994] New: videomixer2: added 'transparent' background option

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=639994
  GStreamer | gst-plugins-good | git

           Summary: videomixer2: added 'transparent' background option
    Classification: Desktop
           Product: GStreamer
           Version: git
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: Normal
         Component: gst-plugins-good
        AssignedTo: gstreamer-bugs at lists.sourceforge.net
        ReportedBy: dirjud at gmail.com
         QAContact: gstreamer-bugs at lists.sourceforge.net
      GNOME target: ---
     GNOME version: ---


Created an attachment (id=178783)
 View: https://bugzilla.gnome.org/attachment.cgi?id=178783
 Review: https://bugzilla.gnome.org/review?bug=639994&attachment=178783

patch

This patch adds a 'transparent' background option to videomixer2. This 
allows the videomixer2 element to output a valid alpha
channel when the inputs contain a valid alpha channel, so that
mixing to occur in multiple stages serially if desired.

The following pipeline shows an example of such a pipeline:

gst-launch videotestsrc background-color=0x000000 pattern=ball !
video/x-raw-yuv,format=\(fourcc\)AYUV ! videomixer2 background=transparent
name=mix1 ! videomixer2 name=mix2 ! ffmpegcolorspace ! autovideosink 
videotestsrc ! ideo/x-raw-yuv,format=\(fourcc\)AYUV ! mix2.

The first videotestsrc in this pipeline creates a moving ball on a
transparent background. It is then passed to the first videomixer2.
Previously, this videomixer2 would have forced the alpha channel to
1.0 and given a background of checker, black, or white to the
stream. With this patch, however, you can now specify the background
as transparent, and the alpha channel of the input will be
preserved. This allows for further mixing downstream, as is shown in
the above pipeline where the a second videomixer2 is used to mix in a
background of an smpte videotestsrc. So the result is a ball hovering
over the smpte test source. This could, of course, have been
accomplished with a single mixer element, but staged mixing is useful
when it is not convenient to mix all video at once (e.g. a pipeline
where a foreground and background bin exist and are mixed at the final
output, but the foreground bin needs an internal mixer to create
transitions between clips).

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