[MPlayer-users] Change the colour of the green screen
phil_rhodes at rocketmail.com
Mon May 26 22:18:46 CEST 2008
> I was under the impression that the green frames come about when
> graphics cards are set to "sync on Green" in windows.
The usage of "sync on green" I'm aware of refers to imposing the sync pulse
on the green channel of an analogue RGB video signal, such as VGA, and has
nothing to do with the green flashes occasionally visible on overlay video.
There are several sources of corruption in this sort of situation. The one
we've been working on is the green background that's commonly - but not
always - placed into windows which will later receive overlay video, and
which are used to effectively chroma-key the overlay into the rest of the
desktop. It's commonly full bright green, RGB 0,255,0 but I've seen very
dark green and very dark purple used for the same purpose. This overlay
backdrop can sometimes become momentarily visible if the windows is drawn
before the graphics board's overlay hardware has started up.
mplayer, and other video playback software, is capable of producing coloured
flashes in other situations, particularly given various combinations of
operating system, hardware, and drivers. Zero pixel values in a YUV display
can sometimes create a darkish green. I'm not quite sure how this can be
correct given that zero Y ought to be black regardless of U and V, but it
can certainly happen. Issues may also occur when switching from RGB to YUV
overlay video, which is down to the card or the card's driver clearing
everything down to black before resizing framebuffers, and so forth, which
they sometimes don't do. Of course, the error may also be encoded into the
video, whether as a sloppy bit of editing, or as a simple transcription
error in a file.
All this said, I have never found a situation where I have not been able to
make mplayer create a seamless display where arbitrary playlists and
arbitrary re-launches of the playback software segue neatly through black.
Frequently it is necessary to use the brightness controls to rapidly fade
video out at the end of file playback, since black levels in files can
differ and this creates a soft fade in brightness rather than a hard chop. I
have also had to alternate the files I want to play with a specifically
created black-video file which ensures the overlay framebuffer(s) are filled
with black pixel values before and after playback.
So, try combinations of -vo and -vf options, -colorkey, black-file playback,
and try either putting all your files (and black-video files) in a playlist,
or relaunching mplayer every time.
To the developers, I would encourage a continued consideration of things
like this - if in doubt, clear everything to black, and be careful how
things are built and set up to avoid glitches where possible. This sort of
thing is critical to mplayer's continued usefulness in this sort of field.
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