1. Find and select the URDF file in the Project window (`Assets/URDF/niryo_one/niryo_one.urdf`). From the menu, click `Assets -> Import Robot from URDF`, or in the Project window, right click on the selected file and click `Import Robot from URDF`.
> Note: The file extension may not appear in the Project window. The niryo_one.urdf file will appear in the root of the `Assets/URDF/niryo_one` directory.
1. Keep the default Y Axis type and VHACD mesh decomposer in the Import menu and click `Import URDF`.
1. Keep the default Y Axis type in the Import menu and click `Import URDF`.
> Note: VHACD algorithm produces higher quality convex hull for collision detection than the default algorithm.
> Note: The world-space origin of the robot is defined in its URDF file. In this sample, we have assigned it to sit on top of the table, which is at `(0, 0.63, 0)` in Unity coordinates.
- Fill the DH parameters for the robot starting from first joint and click `Add DH parameter`.
- The added DH parameter will appear on the text box below.
## Convex Mesh Collider
Collider components in Unity define the shape of a body for detecting collisions. There are three types of [colliders](https://docs.unity3d.com/Manual/CollidersOverview.html) supported in Unity.
- Primitive Colliders: These represent basic shapes like Box, Circle and sphere.
- Compound Colliders: These colliders are formed by union of various Primitive Colliders
- Mesh Colliders: This uses a mesh similar in shape to visual mesh to create more accurate collision meshes.
The drawback for using [MeshColliders](https://docs.unity3d.com/Manual/class-MeshCollider.html) is that it cannot collide with other MeshColliders. We need to use a convex hull of the collision mesh to enable collision with other MeshColliders. In Unity these collision meshes are limited to 255 triangles. This can lead to poor performance in simulation as the volume of these collision meshes tend be greater than the visual meshes resulting in erratic behavior in Articulation Body.
To address this predicament we have integrated another algorithm to create Convex Hulls from a mesh. This is called Volumetric Hierarchical Approximate Convex Decomposition or VHACD whose details can be found [here](https://www.microsoft.com/en-us/research/uploads/prod/2019/09/a226-thul.pdf) and the source code for the algorithm can be found [here](https://github.com/kmammou/v-hacd). The difference in algorithms can be found below.
- Copy the generated `niryo_one.urdf` file to `Assets/URDF`
- Right click on the this file and select `Import Robot from URDF`
- Select the co-ordinate system in which the meshes were designed. Default mesh orientation is Y-up which is supported by Unity but some packages often use Z-up and X-up configuration. For more [information](https://docs.unity3d.com/Manual/HOWTO-FixZAxisIsUp.html).
- Select the Convex Mesh Decomposer you want to use for the imported robot. More information can be found [here](urdf_appendix.md##Convex-Mesh-Collider).
vidurvij-Unity