Configuring the simulation

Before you start running your own experiments there are few details to take into account at the time of configuring your simulation. In this document we cover the most important ones.

Fixed time-step

The time-step is the simulation-time elapsed between two steps of the simulation. In video-games, this simulation-time is almost always adjusted to real time for better realism. This is achieved by having a variable time-step that adjusts the simulation to keep up with real-time. In simulations however, it is better to detach the simulation-time from real-time, and let the simulation run as fast as possible using a fixed time-step. Doing so, we are not only able to simulate longer periods in less time, but also gain repeatability by reducing the float-point arithmetic errors that a variable time-step introduces.

CARLA can be run in both modes.

Variable time-step

The simulation tries to keep up with real-time. To do so, the time-step is slightly adjusted each update. Simulations are not repeatable. By default, the simulator starts in this mode

Fixed time-step

The simulation runs as fast as possible, simulating the same time increment on each step. To run the simulator this way you need to pass two parameters in the command-line, one to enable the fixed time-step mode, and the second to specify the FPS of the simulation (i.e. the inverse of the time step). For instance, to run the simulation at a fixed time-step of 0.1 seconds we execute

$ ./ -benchmark -fps=10

It is important to note that this mode can only be enabled when launching the simulator since this is actually a feature of Unreal Engine.


Do not decrease the frame-rate below 10 FPS.
Our settings are adjusted to clamp the physics engine to a minimum of 10 FPS. If the game tick falls below this, the physics engine will still simulate 10 FPS. In that case, things dependent on the game's delta time are no longer in sync with the physics engine. Ref. #695

Changing the map

The map can be selected by passing the path to the map as first argument when launching the simulator

# Linux
./ /Game/Carla/Maps/Town01
rem Windows
CarlaUE4.exe /Game/Carla/Maps/Town01

The path "/Game/" maps to the Content folder of our repository in "Unreal/CarlaUE4/Content/".

Running off-screen

In Linux, you can force the simulator to run off-screen by setting the environment variable DISPLAY to empty

# Linux

This launches the simulator without simulator window, of course you can still connect to it normally and run the example scripts. Note that with this method, in multi-GPU environments, it's not possible to select the GPU that the simulator will use for rendering. To do so, follow the instruction in Running without display and selecting GPUs.

No-rendering mode

It is possible to completely disable rendering in the simulator by enabling no-rendering mode in the world settings. This way is possible to simulate traffic and road behaviours at very high frequencies without the rendering overhead. Note that in this mode, cameras and other GPU-based sensors return empty data.

settings = world.get_settings()
settings.no_rendering_mode = True

Synchronous mode


Always run the simulator at fixed time-step when using the synchronous mode. Otherwise the physics engine will try to recompute at once all the time spent waiting for the client, this usually results in inconsistent or not very realistic physics.

The client-simulator communication can be synchronized by using the synchronous mode. When the synchronous mode is enabled, the simulation is halted each update until a tick message is received.

This is very useful when dealing with slow client applications, as the simulator waits until the client is ready to continue. This mode can also be used to synchronize data among sensors by waiting until all the data is received. Note that data coming from GPU-based sensors (cameras) is usually generated with a delay of a couple of frames respect to data coming from CPU-based sensors.

The synchronous mode can be enabled at any time in the world settings.

# Example: Synchronizing a camera with synchronous mode.

settings = world.get_settings()
settings.synchronous_mode = True

camera = world.spawn_actor(blueprint, transform)
image_queue = queue.Queue()

while True:
    timestamp = world.wait_for_tick()

    image = image_queue.get()

Other command-line options

  • -carla-port=N Listen for client connections at port N, streaming port is set to N+1.
  • -quality-level={Low,Epic} Change graphics quality level, "Low" mode runs significantly faster.
  • -carla-server-timeout=10000ms Set server timeout.
  • -no-rendering Disable rendering.
  • Full list of UE4 command-line arguments.