This tutorial explains how to create more accurate collision boundaries for vehicles (relative to the original shape of the object). These are used by raycast-based sensors such a the LIDAR to retrieve more accurate data. New colliders can be integrated into CARLA so that all the community can benefit from these. Find out more about how to contribute to the content repository [here](cont_contribution_guidelines.md).
There are two approaches to create the new colliders.
*__Manual approach__ — This approach requires some basic 3D modelling skills, as the collider mesh will be generated manually.
*__Automatic approach__ — This approach follows the tutorial created by the contributor __Yan Kaganovsky / yankagan__ to create a mesh with no need of manual modelling.
* [__Manual approach__](#manual-approach)
* [1-Export the vehicle FBX](#1-export-the-vehicle-fbx)
* [2-Generate a low density mesh](#2-generate-a-low-density-mesh)
* [3-Import the mesh into UE](#3-import-the-mesh-into-ue)
* [4-Add the mesh as collider](#4-add-the-mesh-as-collider)
* [5-Configure the collider](#5-configure-the-collider)
First of all, the original mesh of the vehicle is necessary to be used as reference. For the sake of this tutorial, let's choose the mesh of a CARLA vehicle.
__1.1__ open CARLA in UE and go to `Content/Carla/Static/Vehicles/4Wheeled/<model_of_vehicle>`.
__1.2__ Press `right-click` on `SM_<model_of_vehicle>` to export the vehicle mesh as FBX.
### 2-Generate a low density mesh
__2.1__ Open a 3D modelling software and, using the original mesh as reference, model a low density mesh that stays reliable to the original.
__2.2__ Save the new mesh as FBX. Name de mesh as `sm_sc_<model_of_vehicle>.fbx`. E.g. `sm_sc_audiTT.fbx`.
!!! Note
As for the wheels and additional elements such as roofs, mudguards, etc. the new mesh should follow the geometry quite accurately. Placing simple cubes will not do it.
### 3-Import the mesh into UE
__3.1__ Open CARLA in UE and go to `Content/Carla/Static/Vehicles/4Wheeled/<model_of_vehicle>`.
__3.2__ Press `right-click` to import the new mesh `SM_sc_<model_of_vehicle>.fbx`.
### 4-Add the mesh as collider
__4.1__ Go to `Content/Carla/Blueprints/Vehicles/<model_of_vehicle>` and open the blueprint of the vehicle named as `BP_<model_of_vehicle>`.
__4.2__ Add a new element of type `Static mesh`. This will create an empty object in the blueprint.
This tutorial is based on a [contribution](https://bitbucket.org/yankagan/carla-content/wiki/Home) made by __yankagan__! The contributor also wants to aknowledge __Francisco E__ for the tutorial on [how to import custom collisions in UE](https://www.youtube.com/watch?v=SEH4f0HrCDM).
__1. Use a collection of static meshes__ and move them around as if they were part of the vehicle. This requires significant redesign of CARLA and creates new problems such as how certain features such as suspension or wheels should be animated.
__2. Create a custom collision boundary__ that describes the mesh more accurately and can be used with skeletal meshes.
The later method will be discussed in this tutorial. It requires some knowledge of 3D modelling, but it is accessible for newcomers. The following image shows a more detailed collider created with this tutorial.
*__Build CARLA from source__ on [Linux](build_linux.md) or [Windows](build_windows.md).
*__Blender 2.80 or newer__ from the [official site](https://www.blender.org/download/) for free (open-source 3D modelling software).
*__VHACD Plugin for Blender__ following the using the instructions in [here](https://github.com/andyp123/blender_vhacd). This plugin automatically creates an approximation of a selected object using a collection of convex hulls. [Read more](https://github.com/kmammou/v-hacd).
!!! Note
This [series](https://www.youtube.com/watch?v=ppASl6yaguU) and [Udemy course](https://www.udemy.com/course/blender-3d-from-zero-to-hero/?pmtag=MRY1010) may be a good introduction to Blender for newcomers.
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## Create custom colliders for a Skeletal Mesh
Here is a brief summary of the steps to follow.
__Step 1.__ *(UE)* — Add collision boundaries for the wheels.
__Step 2.__ *(UE)* — Export the skeletal mesh asset of a vehicle to an FBX file.
__Step 3.__ — Import the FBX file into Blender.
__Step 4.__ *(Blender)* — Add convex hull meshes to form the new collision boundary (UE requirement for computational efficiency). This is the hardest step.
__Step 5.__ *(Blender)* — Export the custom collision boundaries into an FBX file.
__Step 6.__ *(UE)* — Import the new FBX into CARLA as an Unreal asset file (static mesh).
__Step 7.__ *(UE)* — Import the custom collider into the physics asset for the specific vehicle, so that it is used for computations.
__Step 8.__ *(UE)* — Create constraints that connect the different joints and define the physics of all parts.
Below you will find 2 clips that gives an example of this process but before you watch it let me explain a bit about the logic that goes into this process:
If you select the entire car and create the collision boundary with the VHACD Plugin you will get a very ugly approximation and the wheels will contains sharp edges which will mess-up the drive on the road (it will make them collide with the road every time it's not straight). It's important that the wheels have smooth boundaries around them. If you used convex decomposition on the car's body the mirror would still not look right.
For computer vision applications, we need some important features of the vehicles, such as side mirrors, wheels that look right, etc. It is with this in mind that I break down the vehicle into several parts.
Generally the steps are:
Cut out the bottom parts of the wheels, the side mirrors and the top part of the car's body to create the first boundary using the VHACD tool
Cut out the bottom half of the car to create the second boundary (top part of the car) using the VHACD tool
Create separate boundaries for side mirrors using the VHACD tool
Be very careful about naming the objects!! Each boundary should have "UCX_" at the beginning and the rest of the name has to be EXACTLY the same as the original mesh
This clip: https://youtu.be/aJPyskYjzWo explains this simple step. Follow it exactly. Select only the original car object + all the newly added objects for collision. Make sure that in the export menu you check "selected objects" and select only "Mesh".
