Some of the command options below are not equivalent in the CARLA packaged releases. Read the [Command line options](start_quickstart.md#command-line-options) section to learn more about this.
Vulkan is the default graphics API used by Unreal Engine, and CARLA. It consumes more memory, but performs faster and makes for a better frame rate. However, it is quite experimental, especially in Linux, and it may lead to some issues.
There is the option to change to OpenGL. Use a flag when running CARLA.
When working with the build version of CARLA, Unreal Engine needs to be set to use OpenGL. [Here][UEdoc] is a documentation regarding different command line options for Unreal Engine.
CARLA also allows for two different graphic quality levels. __Epic__, the default is the most detailed. __Low__ disables all post-processing and shadows, the drawing distance is set to 50m instead of infinite.
The simulation runs significantly faster in __Low__ mode. This is not only used when there are technical limitations or precision is nonessential. It may be useful to train agents under conditions with simpler data or regarding only close elements.
The images below compare both modes. The flag used is the same for Windows and Linux. There is no equivalent option when working with the build, but the UE editor has its own quality settings. Go to `Settings/Engine Scalability Settings` for a greater customization of the desired quality.
The issue that made Epic mode show an abnormal whiteness has been fixed. If the problem persists, delete `GameUserSettings.ini`. It is saving previous settings, and will be generated again in the next run. __Ubuntu path:__` ~/.config/Epic/CarlaUE4/Saved/Config/LinuxNoEditor/`__Windows path:__`<Package folder>\WindowsNoEditor\CarlaUE4\Saved\Config\WindowsNoEditor\`
This mode disables rendering. Unreal Engine will skip everything regarding graphics. This mode prevents rendering overheads. It facilitates a lot traffic simulation and road behaviours at very high frequencies. To enable or disable no-rendering mode, change the world settings, or use the provided script in `/PythonAPI/util/config.py`.
Unreal Engine needs for a screen in order to run. However, there is a workaround for remote servers with no display, or desktop users with a GPU not connected to any screen.
The simulator launches but there is no available window. It runs in the same way as normal mode. This mode tricks Unreal Engine into running in a "fake screen".
* In __no-rendering__, Unreal Engine does not render anything. Graphics are not computed.
* In __off-screen__, Unreal Engine is working as usual, rendering is computed. Simply, there is no display available. GPU sensors return data when off-screen, and no-rendering mode can be enabled at will.
This is __only possible in Linux while using OpenGL__. Unreal Engine crushes when Vulkan is running off-screen, and this issue is yet to be fixed by Epic.
To force the simulator run off-screen set the environment variable `DISPLAY` to empty and run CARLA using OpenGL.
The best way to run a headless CARLA and select the GPU is to [__run CARLA in a Docker__](build_docker.md).
This section contains an alternative tutorial, but this method is deprecated and performance is much worse. It is here only for those who Docker is not an option.
This tutorial only works in Linux and makes it possible for a remote server using several graphical cards to use CARLA on all GPUs. This is also translatable to a desktop user trying to use CARLA with a GPU that is not plugged to any screen. To achieve that, the steps can be summarized as:
__1.__ Configure the server to have Nvidia working with no display.
__2.__ Use VNC and VGL to simulate a display connected to any GPU.
__3.__ Run CARLA.
This tutorial was tested in Ubuntu 16.04 using NVIDIA 384.11 drivers.
* __[VGL](https://virtualgl.org/vgldoc/2_2_1/#hd004001)__: redirects 3D rendering commands from Unix and Linux OpenGL to the hardware in a dedicated server.
* __[TurboVNC 2.11](https://cdn.rawgit.com/TurboVNC/turbovnc/2.1.1/doc/index.html#hd005001)__: graphical desktop-sharing system to connect remotely to the server.
To disable the need of sudo when creating the `nohup Xorg` go to `/etc/X11/Xwrapper.config` and change `allowed_users=console` to `allowed_users=anybody`.
It may be needed to stop all Xorg servers before running `nohup Xorg`. The command for that could change depending on your system. Generally for Ubuntu 16.04 use:
