The V-HACD library decomposes a 3D surface into a set of "near" convex parts.
Why do we need approximate convex decomposition?
Collision detection is essential for realistic physical interactions in video games and computer animation. In order to ensure real-time interactivity with the player/user, video game and 3D modeling software developers usually approximate the 3D models composing the scene (e.g. animated characters, static objects...) by a set of simple convex shapes such as ellipsoids, capsules or convex-hulls. In practice, these simple shapes provide poor approximations for concave surfaces and generate false collision detection.
A second approach consists in computing an exact convex decomposition of a surface S, which consists in partitioning it into a minimal set of convex sub-surfaces. Exact convex decomposition algorithms are NP-hard and non-practical since they produce a high number of clusters. To overcome these limitations, the exact convexity constraint is relaxed and an approximate convex decomposition of S is instead computed. Here, the goal is to determine a partition of the mesh triangles with a minimal number of clusters, while ensuring that each cluster has a concavity lower than a user defined threshold.
Installing the Package
A 2.8x port of the blender addon tool can be found here: https://github.com/andyp123/blender_vhacd, please go there and follow the instructions provided in that repository. The in-repo included 2.7x version of the tool will not function for Blender 2.8+.
- Clone this Github repository
- You may either rebuild the binaries for your machine or use the provided binaries. Here we will assume that you will use the provided binaries in the v-hacd/bin directory.
- Copy the Python script in v-hacd/add-ons/blender/object_vhacd.py to your Blender addons directory. For Blender 2.78 this directory will be "Blender Foundation/Blender/2.78/scripts/addons/". You're at the right place if you see other scripts prefixed with object_.
- The addon must be enabled before use. After copying the script to the addons directory, open Blender and navigate to File > User Preferences > Add-ons. Object: V-HACD will be featured on the list. Check its mark to enable the addon and save user settings.
Using the Addon
- After you have enabled the addon, the V-HACD menu will appear in the object menu when an object is selected.
- Go to this menu and select a preset (or leave the path presets).
- Select your VHACD path by selecting the "Open File" button next to the VHACD path, which should currently be blank.
- Navigate to the directory in which you cloned this repository and find the appropriate executable in the bin folder for your operating systme.
- If you have a video card that supports OpenCL, this will be bin.
- If you don't have a video card or your video card does not support OpenCL, the appropriate executable for you will be bin-no-ocl (no OpenCL).
- Select the V-HACD button at the button of the panel. You will be presented with some options.
- Modify the options as desired and select "OK."
- Note that the processing may take some time. Increasing voxel resolution will particularly increase runtime.
|Parameter name||Description||Default value||Range|
|resolution||maximum number of voxels generated during the voxelization stage||100,000||10,000-64,000,000|
|depth||maximum number of clipping stages. During each split stage, all the model parts (with a concavity higher than the user defined threshold) are clipped according the "best" clipping plane||20||1-32|
|planeDownsampling||controls the granularity of the search for the "best" clipping plane||4||1-16|
|convexhullDownsampling||controls the precision of the convex-hull generation process during the clipping plane selection stage||4||1-16|
|alpha||controls the bias toward clipping along symmetry planes||0.05||0.0-1.0|
|beta||controls the bias toward clipping along revolution axes||0.05||0.0-1.0|
|gamma||maximum allowed concavity during the merge stage||0.00125||0.0-1.0|
|pca||enable/disable normalizing the mesh before applying the convex decomposition||0||0-1|
|mode||0: voxel-based approximate convex decomposition, 1: tetrahedron-based approximate convex decomposition||0||0-1|
|maxNumVerticesPerCH||controls the maximum number of triangles per convex-hull||64||4-1024|
|minVolumePerCH||controls the adaptive sampling of the generated convex-hulls||0.0001||0.0-0.01|
Using V-HACD inside PyBullet
PyBullet is an easy to use Python module for physics simulation for robotics, games, visual effects and machine learning, with a focus on sim-to-real transfer. From version 2.6.7, PyBullet includes V-HACD embedded:
pip3 install pybullet --upgrade --user #now in a script you can use import pybullet as p import pybullet_data as pd import os p.connect(p.DIRECT) name_in = os.path.join(pd.getDataPath(), "duck.obj") name_out = "duck_vhacd2.obj" name_log = "log.txt" p.vhacd(name_in, name_out, name_log, alpha=0.04,resolution=50000 )