Start Webots

Webots needs a X11 connection to a X-server to run. So there are 2 options to start Webots:

• GUI on your local display: In the jumax access tutorial, at step 11, you added the ForwardX11 yes line in your SSH config file. This allows to forward your X display to the remote computer. This way, Webots will get a X11 connection to a X-server running on your local machine. You can launch Webots in the $WEBOTS_HOME directory with the following command: ./webots. Webots will start with the GUI. This is only possible on jumaxbuild1 and jumaxbuild2. Display forwarding is not possible on jumax-cpu. Webots automatically starts with the GPU acceleration if GCC and libGLU modules are loaded..

• Headless (without GUI): The other option is to run Webots headless.

  • jumaxbuild: On jumaxbuild, the DISPLAY environment variable can simply be changed to a X server running on the remote machine:

    1. export DISPLAY=:0
    2. Run Webots headless with: ./webots --no-rendering --batch /path/to/your/world/file.wbt
       You can also add --stdout and --stderr options to redirect the webots logs to the terminal.
    3. To switch back to the local display, use the following value for the DISPLAY variable: export DISPLAY=localhost:10.0

    In headless mode, Webots is also started with GPU acceleration by default if GCC and libGLU modules are loaded.

  • jumax-cpu: The alternative on jumax-cpu (which doesn't have a X server) is to use Virtual Network Computing (VNC), using the vncserver package installed on Jumax. VNC allows to remotely control and display another computer. Here we will use the package to create a virtual display for Webots. To do that, follow these steps:

    1. In your user1@jumaxbuild1:~/ directory, execute the following command: vncserver.

    2. VNCserver will ask you for a username and a password. Choose user information that you will remember.

    3. Once you finish the configuration, type cd .vnc and edit the xstartup file with vim: vim xstartup. This file is used to define applications you want to run when creating a display. You need to add the following line after the unset lines:

       /home/jusers/user1/jumax/webots/webots --no-rendering --batch /path/to/your/world/file.wbt
       Don't forget to replace user1 by our username. You can also add --stdout and --stderr options to redirect the webots logs to the VNC log file (see step v.).

    4. Then, you can start a new virtual display using vncserver command. It will start Webots headless at the same time.

    5. The logs generated are saved in .vnc/jumaxbuild1:DISPLAY#.log

    6. To see the list of existing displays, use the following: vncserver -list. To kill an existing display: vncserver -kill :DISPLAY#

Redirecting the GUI to the local computer has some performance impact compared to running headless. The following table shows the simulation speed ratio of a Webots simulation rendering a camera of size 1000x1000 on jumaxbuild. Running headless is a little faster than with GUI. If the window is minimized, the performance is also increased.

GUI with no rendering	GUI minimized	Headless
10.1	11.3	12.0