Xinerama

["Yaakov (Cygwin Ports)" <yselkowitz at users dot sourceforge dot net>]

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Jon,

Looking further into Xinerama, I think I brushed it off too early.  It
is *very* widely supported[1], and while XRandR is supposed to replace
it, actual implementation and usage may be some time off.

I also think that there may be some potential flaws in windowed
multiplemonitor mode that Xinerama support would fix.  While the current
behaviour makes sense in multiwindow mode, I'm not sure that it does in
windowed mode.  Since X window managers have no sense of what is really
happening in that case, placing a dialog in the centre of the X display
could easily land on the seam between two monitors.  (I know, I tried
it.)  With Xinerama support, however, they would have the necessary
information to DTRT.

I think that the implementation may be simpler than we thought.  To see
the situation as it stands:

1) Build and install xineramaproto;
2) Remove the --disable-xinerama flag from xorg-server and rebuild;
3) Build and install libXinerama;
4) Remove --without-xinerama from xdpyinfo and rebuild.

For this example, on my single-head system I launched XWin with:

X +xinerama -nodecoration -screen 0 640x480 -screen 1 640x480+640+0

This gives :0.0 and :0.1 side-by-side.  If you have a multihead system,
you could just as easily run:

X +xinerama -nodecoration -screen 0 @1 -screen 1 @2

Now, let's see what xdpyinfo has to say:

$ xdpyinfo -ext XINERAMA | tail
[snip]
XINERAMA version 1.1 opcode: 128
  head #0: 640x480 @ 0,0
  head #1: 640x480 @ 0,0

AFAICS that implies that the X geometry overlaps.  Indeed, if you run an
X client, both screens show exactly the same thing, but only screen 0
accepts input.  Compare this to -xinerama, where each screen is
completely independent from the other, as if you were running :0 and :1.

On Linux, after defining the Screens, you must define their relationship
in ServerLayout, e.g.[2]:

Section "ServerLayout"
    Identifier  "Simple Layout"
    Screen "Screen 2"
    Screen "Screen 1" RightOf "Screen 2"
    InputDevice "Mouse1" "CorePointer"
    InputDevice "Keyboard1" "CoreKeyboard"
EndSection

So if we were to add additional parameters to the -screen flag, e.g.:

X +xinerama -nodecoration -screen 0 640x480 -screen 1 640x480+640+0
- -rightof 0

Perhaps there is a better syntax, but the point would be to set the
geometry offset correctly so that the upper left corner of screen 1
would be 640,0.  (Hopefully the input on screen 1 would work correctly
then as well.)

Does this make any sense?


Yaakov
Cygwin/X


[1] http://gentoo-portage.com/x11-libs/libXinerama/RDep#ptabs
[2] http://tldp.org/HOWTO/Xinerama-HOWTO/editxf86config.html
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