Solution

Create a virtual screen beside the primary monitor. Then share it via x11vnc on display :0 1. Finally, access from iPad using the RealVNC viewer.

Procedures

  1. Check existing monitors.

    xrandr --listmonitors

    Monitors: 2
 0: +*DP-4 3840/597x2160/336+0+0  DP-4
 1: +DP-6 1080/476x1920/268+3840+0  DP-6

  2. Let the Linux system recognize the monitor DP-1, even though it is not physically connected, by adding the following two lines to the Screen section in /etc/X11/xorg.conf.

    Section "Screen"
    Option         "ConnectedMonitor" "DP-4,DP-6,DP-1"
    Option         "ModeValidation" "NoDFPNativeResolutionCheck,NoVirtualSizeCheck,NoMaxPClkCheck,NoHorizSyncCheck,NoVertRefreshCheck,NoWidthAlignmentCheck"
EndSection
  3. Restart the display manager sudo systemctl restart lightdm.service.

  4. Use xrandr to check the list of modes supported by DP-1 and select the maximum one that matches my iPad.

    Originally, I wanted to use the following command to generate a new mode, which matches the resolution of my iPad, and apply it to DP-1.

    cvt 2048 1536 60

    # 2048x1536 59.95 Hz (CVT 3.15M3) hsync: 95.45 kHz; pclk: 267.25 MHz
Modeline "2048x1536_60.00"  267.25  2048 2208 2424 2800  1536 1539 1543 1592 -hsync +vsync

    xrandr --newmode "2048x1536_60" 267.25  2048 2208 2424 2800  1536 1539 1543 1592 -hsync +vsync
xrandr --addmode DP-1 2048x1536_60

    However, the last command failed and it seems only existing modes of DP-1 can be used.

  5. Make DP-1 on the left of the primary monitor. Both of them are on the default display :0.

    xrandr --output DP-1 --mode 2048x1536_60 --left-of DP-4
  6. In KDE display settings, we can actually select the desired mode 2048x1536 for DP-1 with 60 Hz refresh rate, even though the previous command xrandr --addmode DP-1 2048x1536_60 failed.

  7. Set a password for the VNC server.

    x11vnc -storepasswd

  8. Start the VNC server x11vnc.

    x11vnc -clip 2048x1536+0+0 -multiptr -nevershared -forever -nopw -rfbport 5901 -rfbauth ~/.vnc/passwd -display :0 &

    N.B. -multiptr is mandatory because I want to move the mouse cursor on Linux and view its motion on iPad.

  9. Remember to enable port 5901 in iptable rules.

img

Footnotes

1 x11vnc is a VNC server to allow remote access to an existing X session. tightvncserver cannot be used, because it only supports creating a new X11 display, so that the virtual screen cannot be integrated the current desktop.