Extending desktop using xrandr causes display corruption

krasl · 12-30-2014, 11:13 PM

Hello! I have been searching for some time to set up a virtual display to share with VNC in order to extend my FC21 desktop to a second device (android tablet). I finally found this post (https://bbs.archlinux.org/viewtopic.php?id=180904) and it got me started, but I am running into display corruption.
Here's the scenario:

Laptop with 1280x800 main display "LVDS"
Android tablet with 1280x80 display and VNC viewer app.
Want to extend my laptop display to the tablet via VNC.

I almost have it working. I used xrandr to enable one of the unused display ports (VGA-0) on my laptop:

Code:

xrandr --addmode VGA-0 1280x800
xrandr --output VGA-0 --mode 1280x800 --left-of LVDS

Then I enable x11vnc server:

Code:

x11vnc -clip 1280x800-1280+0

This would work perfectly, but as soon as I issue the second xrandr command, the right third of my main LVDS display is corrupted, and the left two-thirds is "squashed". However, I can still drag/drop windows between the main display and the VNC-connected tablet! and the mouse pointer is ablet to move between the two displays as well.

I tried placing the virtual screen "below" the main screen, and this works perfectly without screen corruption, but is not intuitive for use, and also GNOME crashes after a short time of usage, requiring the whole process to be repeated to re-establish the multi-monitor setup.

Does anyone have suggestions for avoiding the screen corruption? FYI The BIOS does not have options for increasing the amount of display memory.

Thanks!

allend · 12-31-2014, 07:12 PM

Have you tried this?

Code:

xrandr --output LVDS --auto --output VGA-0 --auto --left-of LVDS

Works for me.

krasl · 12-31-2014, 08:50 PM

Hi@allend, thanks for your reply. I tried this but it doesn't enable VGA-0 output. After executing this command, then running xrandr with no arguments shows VGA-0 disabled. I have to first add the mode (xrandr --addmode VGA-0 1280x800) then assign it with (xrandr --output LVDS --auto --output VGA-0 --mode 1280x800 --left-of LVDS). And this is when the display corruption occurs.

I have also noticed that if I change the size of the framebuffer using xrandr --fb, if I start at a width greater than 2000, I get the bar of corrupted display on the main LVDS. Then as I resize the screen and approach 2000px width, the bar shrinks until at 2000px width, it is gone. So is this indicative of an issue or incorrect allocation of video ram? It's seeming to be overwriting used video ram addresses and causing that corruption.

thanks!

allend · 01-01-2015, 06:17 PM

From 'man xrandr'

Quote:

Forces to use a 1024x768 mode on an output called VGA:
xrandr --newmode "1024x768" 63.50 1024 1072 1176 1328 768 771 775 798 -hsync +vsync
xrandr --addmode VGA 1024x768
xrandr --output VGA --mode 1024x768

You can use cvt to generate what is required for the --newmode option for xrandr.

Code:

bash-4.3$ cvt 1280 800
# 1280x800 59.81 Hz (CVT 1.02MA) hsync: 49.70 kHz; pclk: 83.50 MHz
Modeline "1280x800_60.00"   83.50  1280 1352 1480 1680  800 803 809 831 -hsync +vsync