vlc-playing DVD won't display on external monitor

[Robert P. J. Day]

Quoting Mogens Kjaer <mk at crc.dk>:

> Robert P. J. Day wrote:
> ...
>>   then, connected an external (samsung WUXGA) flat-panel
>> monitor via DVI, and the entire laptop display is reflected
>> nicely on the monitor *except* for the actual DVD image in
>> the VLC window -- that just shows up as blank, either in
>> regular or fullscreen mode (in fullscreen mode, the entire
>> external display is blank, even while the movie is playing
>> on the laptop).
>
> On an old HP/Compaq EVO 800c with radeon I need to do a:
>
> yum install xvattr
> xvattr -a XV_SWITCHCRT -v 1
>
> to get video on an external monitor. Then the
> video is not shown on the laptop. To get it back,
> do a:
>
> xvattr -a XV_SWITCHCRT -v 0

   as i posted earlier, i got pretty much the same effect using
"xrandr", but i'm still unclear on what happened underneath.  a
colleague's explanation went something like, "seen that before ...
blah blah blah ... video card ...  blah blah blah ... not enough
horsepower ... blah blah blah ... can only drive one screen at a
time ... yadda yadda yadda."

   someone else suggested that the DVD image was bypassing the
framebuffer, which i didn't follow.  can someone clarify the
technical issues behind why i need to deactivate the attached
LCD display on the laptop to get DVD output on the external
display?  i'm convinced it works, and now i know how to do it.
i'd just like it to not be quite so much magic.  thanks.

rday