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zminton edited this page Sep 11, 2018 · 16 revisions

1. Overview

The goal of the Trolley Mod is to create a simple, effective means of setting up trolley problems in the CARLA simulator. These can be used to record human decisions when confronted with ethical dilemmas on the road, which later could teach the AI of a self-driving car what is the socially accepted decision when confronted with a similar situation.

In the case of the mod, the trolley problems are set up such that there are two victims or group of victims on each side of the road in CARLA’s Unreal Engine 4 (UE4) maps. When playing the Trolley Mod, the car will accelerate automatically, and the user can only swerve left or right. Thus, the human playing the simulation has no choice but to hit something. Upon conclusion of the collision, the choices that the driver made are recorded into a text file for later analysis. That way, raw data will exist for uses such as teaching an AI how to make decisions, compiling sociological information, etc. From this point forward, each individual Trolley problem will be called a scenario. A sequence of scenarios is what constitutes a Trolley Mod simulation. The following text will demonstrate an example implementation of a Trolley Mod simulation to give the user an idea on where to begin.

2. How to set up a Trolley Mod simulation

First, it is important to understand how the Trolley Mod spawns object models into a level in the Unreal Engine 4. It takes an input text file named TestVictims.txt, which contains a list of tuples; the syntax for these tuples are discussed in greater detail later. Each tuple specifies a victim—such as a pedestrian, a car, or some other object—and provides any other ancillary data that describes the object. This text is parsed so the there is a reference to a UE4 Actor class in memory. The remaining text is converted into whatever data type is appropriate—such as integers—to represent the ancillary data. Other functions then take this parsed information and spawns the desired Actor into the level opened in the UE4 editor. For more information on navigating the editor, view this link.

There are three types of objects that can be spawned automatically with the Trolley Mod: pedestrians, vehicles, and prop objects. These correspond to the Walker, the CarlaWheeledVehicle, and StaticActor classes respectively. The first two are native to the CARLA plugin, while StaticActor is a product of the Trolley Mod. The mod adjusts these classes so each class instance stores new information pertaining to it. All three classes share two variables:

  1. TestNum: an integer that tracks what scenario is being run.
  2. GroupMemberNames: a delineated string that lumps together the names of all victims in the scenario that the class instance is a part of. It is formatted in the form of Victim1^Victim2^Victim3...

Walker Actors have additional information:

  1. Age: an integer.
  2. Gender: a string.
  3. GroupNumber: an integer that tracks on the map if a Walker is part of a group. If it equals 0 or isn’t included in an input string, the Walker is considered solitary. Otherwise, all Walkers that share the same nonzero GroupNumber is considered to be part of the same group. For example, if three Walkers on the map share GroupNumber = 1, then all three of those Walkers are considered part of the same group. An important note: proximity has nothing to do with whether or not the script interprets a Walker as part of a group—only the GroupNumber.
  4. GroupSize: an integer that simply records the size of the group the Walker resides in. If it isn’t part of a group, the size is 1.
  5. SpecialTrait: a UE4 Name that stores a user-specified trait that makes the Walker unique, such as being pregnant, disabled, etc. It is stored using the Tags array native to all UE4 Actors; this page is a good reference for basic information of Tags. It can be left blank if desired.

The input text file should be in the following format: TestNumber|Victim_1|Victim_2

Note that there are no spaces whatsoever in the text. For each victim specifically, the following format is used:

  1. Pedestrians (Walker classes): ClassName,Gender,Age,GroupNumber,SpecialTrait
    • If the Walker has no special trait: ClassName,Gender,Age
    • If the Walker has a special trait: ClassName,Gender,Age,0,SpecialTrait
    • To spawn a Walker group, chain them together in the string using “^” like so: ClassName,Gender,Age,GroupNumber,SpecialTrait^ClassName,Gender,Age,GroupNumber,SpecialTrait. There is no limit to the number of Walkers that can be spawned in a cluster using this method. However, to ensure all of these Walkers are treated as a group, make sure they share the same GroupNumber, otherwise the output data will be affected!
  2. Vehicles (CarlaWheeledVehicle classes): ClassName
  3. Objects (StaticActor class): ClassName,StaticMesh
    • ClassName is "StaticActor" for all cases.
    • StaticMesh affects the kind of object that will be spawned in. To elaborate, the StaticActor class is a child of the AStaticMeshActor class, which contains a UStaticMesh component. StaticMesh specifies the appearance and physics that the StaticActor will gain. For example, to spawn in a traffic cone the input text may be StaticActor,Prop_ConstructionCone. Or, if a trash can is needed, StaticActor,Prop_Trashcan. See the “StaticMeshes list” page for the compilation of currently stored static meshes.

Now, for an example assume that three scenarios need to be created with a male and a pregnant female pedestrian, a female pedestrian with a car, a group of three pedestrians with a car, and finally a male pedestrian with a traffic cone:

1|WalkeMaleShortsShirtShoesVar1_C,Male,30|WalkerFemaleDressShoesVar1_C,Female,25,0,Pregnant 2|WalkerFemaleSkinnyjeansTshirtBootsVar1_C,Female,21|AudiA2_C 3|WalkeMaleShortsShirtShoesVar1_C,Male,30,1,Deaf^WalkerFemaleDressShoesVar1_C,Female,25,1^WalkerFemaleSkinnyjeansTshirtBootsVar1_C,Female,21,1|AudiA2_C 4|WalkeMaleShortsShirtShoesVar1_C,Male,45|StaticActor,Prop_ConstructionCone

Each numbered line of the text file represents a scenario, where each scenario is separated by a new line. To reiterate, do not use any spaces in the text.

It is important that all values of the victim are spelled correctly, because there are instances when the names must resolve class references in the Unreal Engine 4. If the text has a syntax error, then its Actor may not spawn into the level. A useful reference to ascertain all of the available generated classes can be created using the GetVictimTypes Blutility; this will print a text file called VictimsList.txt with all of the correct ClassNames for Walkers and CarlaWheeledVehicles. To ensure the victim names are correct, simply copy and paste the ClassNames from the VictimsList file into your scenario’s input file. For more information on UE4 Blutilities in general, use this link. More information on Blueprints can be found here.

Once the TestVictims.txt input file is complete, ensure that it is saved in the .../UnrealEngine_4.18/carla/Unreal/CarlaUE4 directory, which is the CARLA simulation’s home directory. The scenarios should now be ready for generation into a map.

3. Generating scenarios in a map

CARLA comes with two maps—Town1 and Town2—which is sufficient for the Trolley Mod to perform. It is highly advisable not to modify these maps without making copies of them first. To do so, navigate to the “File” tab in the upper left corner of the Editor, click it, and then select “Open level...”. After the level is open, click the “File” tab again and select “Save current as...”. Rename the level and then save; there will now be a copy of the original map under this new name.

Assuming the Town1 and Town2 maps were unmodified before saving a copy, there should be numerous Actors already in the new map comprising of the classes WalkerStartSpawnPoint, WalkerSpawnPoint, and PlayerStart. To ensure the Trolley Mod works correctly, all Actors from these classes should be deleted from the map. The most efficient way of doing so is using the World Outliner in the Editor.

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