Ribs

Table Of Content

• Introduction
• Ribs generator usage
  • Technical elements
    • Supported file format
    • Requirements
  • Examples of usage
    • Installation
    • Usage parameters
    • Usage example
  • Interacting through Blender with the resulting geometries
• The Cave system
  • The basic building block
  • Two types of geometrical outputs: triangulation and point cloud
  • Modeling parameter: varying the stalactites length
  • Modeling parameter: The effect of the subdivision level on the geometry
  • Modeling parameter: composition into a grid system of caves
• The tunnel
• TODO

Introduction

Ribs is a generator of both 2-manifold triangulations and point cloud geometrical data. Through a set of geometric/topological parameters, Ribs finely controls its synthetic results conceived as data tools for the study, evaluation and calibration of algorithms working on 3D triangulation and/or point cloud datasets.

Ribs uses the Python wrappers of Blender's 3D modeling features to generate two datasets illustrating two kinds of geometries:

• a regular and human manufactured system of tunnels,
• a less smooth/regular cave system.

In addition to the surface and point cloud data, Ribs also provides a path dataset (a branched system of discrete 1D-lines) that is a manual approximation of the topological skeleton of the tunnel/cave system. Such skeleton can be used either for studying visualisation techniques (how to constrain the camera exploring the tunnel/cave system to remain "inside" the system?) or to study skeleton extraction algorithms.

Note: the repository name, Ribs, is a mock up name for Lyon fish-bones that was a source of inspiration for the tunnel system part of the project.

Ribs generator usage

Technical elements

Supported file format

PLY - Polygon File Format (see also Paul Bourke's PLY format definition) is the only file format supported by Ribs.

PLY was chosen because:

• it allows the representation of both 3D point clouds and/or triangulations,
• it is supported by many softwares among witch Point Cloud Library (PCL), Cloud Compare, MeshLab),
• its ASCII representation (as opposed to binary formats) allows for a simple parsing.

Requirements

• Python > 3.10
• Blender version > 4.0.2

Examples of usage

Installation

git clone https://github.com/VCityTeam/TT-Ribs.git  # This repository
cd TT-Ribs

cd Src
python3.10 -m venv venv
source venv/bin/activate
pip install -r requirements.txt

Usage parameters

The flags and parameter arguments are document by the following command

python Cave.py -h

As a quick summary of parameters acting on the topology/geometry:

• --subdivision SUBDIVISION

  The number of recursive Catmull-Clark subdivisions that are applied to the original low-resolution surface prior to the triangularisation of the result. This parameter acts on the number of produced vertices/triangles.

• --grid_size_x GRID_SIZE_X and --grid_size_y GRID_SIZE_Y

  The number of replications, along the X axis and along the Y axis respectively, of the basic cave building block. Those parameter act on the topology (genus and number of boundaries) of the resulting gridified cave system and as such induces a higher number of vertices/triangles.

Other parameters

• --outputdir OUTPUTDIR

  Target directory for the resulting PLY files.

Usage example

python Cave.py -v --subdivision 2 --grid_size_x 2

python Cave.py -v --subdivision 2 --grid_size_x 2

Open the resulting file (cave_*.ply) e.g. with https://point.love/

Interacting through Blender with the resulting geometries

If you wish to interact with the resulting geometries with the help of the blender UI (that is use commands of the form blender --python <some_script.py>), and because of this issue you will further need to define the following PYTHONPATH environnement variable

export PYTHONPATH=`pwd`:`pwd`/venv/lib/python3.10/site-packages

Using Blender UI with the constructed is achieved with e.g.

blender --python Cave.py -- -v --subdivision 2

or

blender --python Tunnel.py -- -v --subdivision 2

Warning

In the above commands mind the additional " -- " argument that is not required when running "outside" of the blender UI.

The Cave system

The basic building block

Two types of geometrical outputs: triangulation and point cloud

Ribs generates

Point cloud, subdivision=1

Triangulation, subdivision=1

Point cloud, subdivision=2

Triangulation, subdivision=2

With grid_size_x = grid_size_y = 1

Subdivisions	# Vertices	#Edges	#Faces	Point Cloud	Triangulation	Time
1	12k	25k	12k	1.3Mb	1.7Mb	7"
2	50k	101k	50k	5Mb	7Mb	14"
3	202k	405k	202k	19Mb	28Mb	43"
4	810k	1 619k	809k	77Mb	112Mb	3'00"
5	3 238k	6 476k	3 237k	305M	459M	11'46"

Modeling parameter: varying the stalactites length

The vertices sitting at the top of the stalactites have been regrouped within a "Change_size_stalactite" vertex group to which a SimpleDeform modifier, configured in Stretch mode, is applied.

Triangulation, subdivision=2

Varying the Factor of this Stretch mode modifier changes the stalactite height as depicted on the following pictures where the StretchFactor is respectively 0, 7, 14 and 21

Modeling parameter: The effect of the subdivision level on the geometry

Triangulation, subdivision=1 (without color rendering)

Triangulation, subdivision=2 (without color rendering)

Triangulation, subdivision=3 (without color rendering)

Triangulation, subdivision=4, with colors

Triangulation, subdivision=4 (without color rendering)

Triangulation, subdivision=4, with colors

Modeling parameter: composition into a grid system of caves

In order to generate datasets with ranging topologies and dataset sizes, Ribs allows for the composition (the result is guaranteed to be a single 2-manifold surface) of the basic building block into grids.

Here is an example of a 3x2 grid system of caves rendered with different tools (and point sizes).

MeshLab renderer on point cloud (subdivision=2, grid_size_x=3, grid_size_y=2)

Point.love renderer on point cloud (subdivision=2, grid_size_x=3, grid_size_y=2)

The tunnel

The flags and parameter arguments are document by the following command

python Tunnel.py -h

Subdivisions	# Vertices	#Edges	#Faces	Point Cloud	Triangulation	Time
1	35k	105K	70k	4.5Mb	5.8M	32"
2	140k	420k	280k	16Mb	21Mb	1'32"
3	560k	1 682k	1 121k	59Mb	83Mb	5'33"
4	2 243k	6 731k	4 487k	226Mb	331Mb	23'17"

TODO

• For Tunnel: document the existence of a wall painting (a group of three hunters) and its position within the cave.
• Document the existence and usage of Density_test object
• Document the existence and usage of Path object
• Try the apply_modifiers=True exporting option.