CARLA Server
Build
-----
Some scripts are provided for building and testing CarlaServer on Linux
$ ./Setup.sh
$ make
$ make check
The setup script downloads and compiles all the required dependencies. The
Makefile calls CMake to build CarlaServer and installs it under "Util/Install".
Protocol
--------
All the messages are prepended by a 32 bits unsigned integer (little-endian)
indicating the size of the coming message.
Three consecutive ports are used,
* world-port (default 2000)
* measurements-port = world-port + 1
* control-port = world-port + 2
each of these ports has an associated thread that sends/reads data
asynchronuosly.
World thread
Server reads one, writes one. Always protobuf messages.
[client] RequestNewEpisode
[server] SceneDescription
[client] EpisodeStart
[server] EpisodeReady
...repeat...
Measurements thread
Server only writes, first measurements message then the bulk of raw images.
[server] Measurements
[server] raw images
...repeat...
Every image is an array of
[frame_number, width, height, type, FOV, color[0], color[1], ...]
of types
[uint64, uint32, uint32, uint32, float32, uint32, uint32, ...]
where FOV is the horizontal field of view of the camera as float, each color is
an [FColor][fcolorlink] (BGRA) as stored in Unreal Engine, and the possible
types of images are
type = 0 None (RGB without any post-processing)
type = 1 SceneFinal (RGB with post-processing present at the scene)
type = 2 Depth (Depth Map)
type = 3 SemanticSegmentation (Semantic Segmentation)
The measurements message is explained in detail [here](measurements.md).
[fcolorlink]: https://docs.unrealengine.com/latest/INT/API/Runtime/Core/Math/FColor/index.html "FColor API Documentation"
Control thread
Server only reads, client sends Control message every frame.
[client] Control
...repeat...
In the synchronous mode, the server halts execution each frame until the Control
message is received.
C API
-----
The library is encapsulated behind a single include file in C,
["carla/carla_server.h"][carlaserverhlink].
This file contains the basic interface for reading and writing messages to the
client, hiding the networking and multi-threading part. Most of the functions
have a time-out parameter and block until the corresponding asynchronous
operation is completed or the time-out is met. Set a time-out of 0 to get a
non-blocking call.
A CarlaServer instance is created with `carla_make_server()` and should be
destroyed after use with `carla_server_free(ptr)`.
[carlaserverhlink]: https://github.com/carla-simulator/carla/blob/master/Util/CarlaServer/include/carla/carla_server.h
Design
------
The C API takes care of dispatching the request to the corresponding server.
There are three asynchronous servers each of them running on its own thread.
Conceptually there are two servers, the _World Server_ and the _Agent Server_.
The _World Server_ controls the initialization of episodes. A new episode is
started every time it is requested to the World Server by a RequestNewEpisode
message. Once the episode is ready, the World Server launches the Agent Server.
The _Agent Server_ has two threads, one for sending the streaming of the
measurements and another for receiving the control. Both agent threads
communicate with the main thread through a lock-free double-buffer to speed up
the streaming of messages and images.
The encoding of the messages (protobuf) and the networking operations are
executed asynchronously.