Xinerama

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Xinerama is an extension to the X Window System which allows applications and window managers to use the two (or more) physical displays as one large virtual display [1].

The calculation for display_aspect, including the support for Xinerama, is done in mythtv/libs/libmythtv/videoout_xv.cpp.

Basicly, it takes the total display size in mm, and scales from the total display resolution to the size of the view display resolution.

An example with my system

Script.png /etc/X11/XF86Config-4

Section "ServerFlags"
Option "Xinerama" "true"
EndSection
Section "ServerLayout"
Identifier "Default Layout"
Screen "Screen 0"
Screen "Screen 1" RightOf "Screen 0"
InputDevice "Generic Keyboard"
InputDevice "Configured Mouse"
EndSection

From /var/log/XFree86.0.log:

...
(II) NVIDIA(0): Virtual screen size determined to be 1600 x 1200
(--) NVIDIA(0): Display dimensions: (400, 300) mm
(--) NVIDIA(0): DPI set to (101, 101)
...
(II) NVIDIA(1): Virtual screen size determined to be 1280 x 960
(--) NVIDIA(1): Display dimensions: (350, 260) mm
(--) NVIDIA(1): DPI set to (92, 93)

From mythfrontend:

Total desktop width=2880, height=1200, numscreens=2
2005-07-21 20:41:57.335 Using screen 0, 1600x1200 at 0,0

X reports to mythfrontend:

723mm 302mm
dpi 101 101

videoout_xv.cpp calculation:

w_mm = w_mm * gui_w / w;
w_mm = 723 * 1600 / 2880 = 401
h_mm = 302 * 1200 / 1200 = 302
display_aspect = 1.33 = 4/3
Correct in my case.

The possible things that may go wrong include:

Setting NoTwinViewXineramaInfo:
Option "NoTwinViewXineramaInfo" "boolean"
When in TwinView, the NVIDIA X driver normally provides
a Xinerama extension that X clients (such as window
managers) can use to to discover the current TwinView
configuration. Some window mangers can get confused by
this information, so this option is provided to disable
this behavior. Default: TwinView Xinerama information
is provided.

Forcing a dots per inch, as shown from 'ps aux | grep dpi':
/usr/bin/X11/X -dpi 100 -nolisten tcp

(--) VIA(0): Virtual size is 720x576 (pitch 720)
(**) VIA(0): *Mode "720x576": 26.8 MHz (scaled from 0.0 MHz), 30.5 kHz, 50.3 Hz
(II) VIA(0): Modeline "720x576" 26.80 720 752 848 880 576 588 593 605
(**) VIA(0): Default mode "640x480": 25.2 MHz (scaled from -0.0 MHz), 31.5 kHz, 60.0 Hz
(II) VIA(0): Modeline "640x480" 25.20 640 656 752 800 480 490 492 525 -hsync -vsync
(++) VIA(0): DPI set to (100, 100)

Using a incorrect DisplaySize in /etc/X11/XF86Config-4, correct example:
DisplaySize 183 146 # (99.93 dpi:100.21 dpi for 720x576)
dpi = resolution / ( mm / 25.4 )
dpi = 720 / ( 183 / 25.4) = 99.93 dpi

If you still can't get this to work, please post your /etc/X11/XF86Config-4, /var/log/XFree86.0.log and mythfrontend log on the mythtv-users mail list.

Paul Andreassen