CARLA includes now a recording and replaying API, that allows to record a simulation in a file and later replay that simulation. The file is written on the server side only, and it includes which **actors are created or destroyed** in the simulation, the **state of the traffic lights** and the **position** and **orientation** of all vehicles and pedestrians.
All data is written in a binary file on the server. We can use filenames with or without a path. If we specify a filename without any of `\`, `/` or `:` characters, then it is considered to be only a filename and will be saved on folder **CarlaUE4/Saved**. If we use any of the previous characters then the filename will be considered as an absolute filename with path (for example: `/home/carla/recording01.log` or `c:\records\recording01.log`).
* **duration**: Amount of seconds we want to play. When the simulation reaches the end, then all actors remaining will have autopilot enabled automatically. The purpose of this parameter is to allow users to replay a piece of a simulation and then let all actors start driving in autopilot again.
Please note that all vehicles at the end of the playback will be set in autopilot to let them continue driving by themselves, and all pedestrians will be stopped at their current place (we plan to set autopilot for pedestrians also, to walk at random places). This behaviour let's you for example replay a piece of simulation and test how they continue after some changes in the environment.
From the previous log, we can retrieve the information regarding the **date** and the **map** where the simulation was recorded.
Each frame will display information about any event that could happen (create or destroy an actor, collisions). When creating actors, it outputs for each of them its corresponding **Id** together with some other additional information. This **Id** is the one we need to specify in the **camera** attribute when replaying if we want to follow that actor during the replay.
At the end, we can see as well the **total time** of the recording and also the number of **frames** that were recorded.
In simulations with a **hero actor**, the collisions are automatically saved, so we can query a recorded file to see if any **hero actor** had collisions with some other actor. Currently, the actor types we can use in the query are these:
The collision query needs to know the type of actors involved in the collision. If we do not want to specify it, we can specify **a** (any) for both. These are some examples:
So, if we want to see what happened on that recording for the first collision where the hero actor was colliding with a vehicle, we could use this API call. So for example:
There is another API function to get information about actors that have been blocked by an obstacle, not letting them follow their way. That could be helpful for finding incidences. The API call is:
The result is sorted by duration, so the actor that is blocked for more time comes first. By checking the vehicle with Id 173 at time 36 seconds, it is evident that it stopped for 336 seconds. To check the cause of it , it would be useful to check how it arrived to that situation by replaying a few seconds before the second 36:
* **show_recorder_file_info.py**: This will show all the information recorded in file. It has two modes of detail, by default it only shows the frames where some event is recorded, the second is showing info about all frames (all positions and trafficlight states).
* **show_recorder_actors_blocked.py**: This will show all the actors that are blocked or stopped in the recorder. We can define the *time* that an actor has not been moving and *travelled* distance by the actor thresholds to determine if a vehicle is considered as blocked or not.
+ `-f`: Filename
+ `-t`: Minimum seconds stopped to be considered as blocked (optional)
+ `-d`: Minimum distance to be considered stopped (optional)
