[ale] A question for the X gurus out there.

[Michael B. Trausch]

On Tue, 2007-03-06 at 08:23 -0500, Geoffrey wrote:

> Michael B. Trausch wrote:
> > 
> > I am pretty sure that you can use Xinerama across two (or more)
> video
> > boards.  Xinerama is an extension which allows the dragging between
> two
> > different screens, though on all of the implementations that I know
> of
> > (including X.org's), OpenGL only works on one of the displays.  All
> > other displays will display a black area where OpenGL applications
> are
> > supposed to be.
> 
> Can't answer the question on OpenGL, but I do know that when properly
> configured, Xinerama will work across two separate video cards as well
> as a single card with multiple outputs.  I've done both.  I've not had
> the opportunity to do this with Nvidia cards, I've always used ATI
> cards.  I know that for the ATI cards, 3d acceleration is not
> supported when using xinerama.
> 


Yes.  Xinerama is the "desktop stretching" that Windows users are
familiar with.  It works for all sorts of cards; I have used my i810
series chipset on my laptop both with and without Xinerama in dual-head
mode.  I originally started using the non-Xinerama mode because of the
3D acceleration issues with Xinerama.  They seem to exist on every X
server other than Sun Microsystems' implementation.  The Wikipedia
article discusses it there in a decent enough amount of detail.


> > The multi-screen mode (the non-Xinerama mode) where you have two
> > different desktops is actually quite beneficial to use.  It is
> different
> > from the way the Windows world works, sure, but with the right
> software
> > (I use GNOME, Evolution, and Epiphany which all play nice in this
> > configuration) you can be quite productive with two different
> displays,
> > and run OpenGL applications on both of them.  Very nice.  Sure, you
> lose
> > the "extended" desktop, but I think that is a small price to pay.
> Your
> > primary display will be :0.0 and your secondary one will be :0.1,
> and so
> > forth, in this configuration.  (Xinerama just makes a really big
> desktop
> > on :0.0, and Ximerama-aware window managers will display panels on
> each,
> > if you configure them to do so.)
> 
> I'm a bit confused by your description above. So are you saying you
> can switch to :0.1 and you have another desktop?  You can do this by
> simply running multiple X servers.


When using multiple heads without using Xinerama, each screen has its
own subdisplay ("X11 screen").  The DISPLAY variable reflects this.  For
example, you can have multiple X servers, which will be on different
displays (the first number), or one X server that will manage multiple
screens (the second number).  In addition to having the ability to use
3D-accelerated applications without Xinerama, the XRANDR extension
works, permitting the control panel for systems like GNOME to
dynamically change the resolution on each of the displays (as well as
the rotation).  In other words, if you're using two X servers, they have
no relationship with each other (one would be on :0.0, and the other
would be on :1.0).  If you're using one X server managing two displays
and not using Xinerama, they are related to each other loosely (you can
move the mouse between them, and drag and drop between them, and
cut-and-paste between them, and so forth).  The primary display would be
on :0.0, and the secondary display would be on :0.1, assuming that
you're on display 0.


> I currently have a single desktop spread across two monitors (19" flat
> and 22" crt, works out nicely as the real estate on a 19" flat is 
> roughly equivalent to the 22" crt).  I can drag windows from one to
> the other and have windows span both.


Yep.  There is no problem with that, and some users will prefer that
(especially when they have come from the Windows world, because that is
the only way to run dual heads under Windows).


> > 
> > I would work with the non-Xinerama mode for a little while and see
> what
> > you think of it.  It feels a bit awkward at first if you have gotten
> > used to using more than one monitor on a Windows system.  However,
> there
> > are definitely advantages.
> 
> I think it sucks, but that's just me.
> 


I did too, at first.  But, it is nice to have semi-autonomous, related
displays that you can play with separately but together.  There are some
outstanding bugs with some more popular software (e.g., Firefox) that
don't work as you would expect in the non-Xinerama mode, but, using
Epiphany (which has the same rendering engine) fixes that problem.  It
is probably also possible to create a custom startup script for Firefox
that would do the job, too, but I haven't much looked into the
possibility.  I started using Epiphany and decided to keep with that (I
really like the integration with GNOME that it offers; Firefox doesn't
quite do that as well).

    -- Mike

--
Michael B. Trausch
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