Collision Models

1. summary Creating and editing collision models
2. labels Type-User

Collision Model Editing

For users seeking to edit the collision models of built-in or new robots, this is a guide to creating a collision model of any complexity.

Assigning a Model to a Robot

Robots in JBSim, as defined in the Robots file, can define their collision models using the "model" attribute. So, the generic CBC2 robot might have a line like:

cbc2.model = generic

This will tell JBSim to look at the _"models/generic.txt"_ file for collision model information.

Inside a Collision Model

Each collision model is a plain text file. Try opening one of the built-in models, and you will see something like this:

# Collision model for Create

# How it works:
# ...

base = ellipse, -165, -165, 330, 330
base.0 = subtract, rect, -165, -165, 40, 330

This model declares a 330mm by 330mm ellipse (a circle with radius 165 mm), and removes a 40mm wide rectangular section from the back.

Collision Model Details

• Collision models have units of millimeters, and have the origin at the robot's center of turn.
• A file can contain an arbitrary number of models.
• A *model* is defined by one or more directives.
• A *directive* is a line of the form model.# = _data_ where the numbers follow in increasing order starting from 0.
• Each model begins with a directive that lacks a number which defines the base geometry for that model. This directive has no operation.
• Each subsequent directive must have an *operation* which dictates how the shape referenced in that line will be used. The operation can be one of: *add*, *subtract*, *intersect*, or *xor*.
• The data in a directive is a sequence of comma separated values.
• All directives must continue with a shape declaration and optional parameters. Allowable shapes are *ellipse*, *rect* angle, and *poly* gon. Another model in the file can be referenced by name as well.
• If the shape is *ellipse* or *rect*, four parameters must be provided: the x and y coordinates of the upper left, and the width and height of the shape.
• If the shape is *poly*, an even number of parameters representing x and y coordinates of points must be provided. The polygon will be closed automatically.
• Directives are applied in numerical order, _not_ the order in the file.
• The *base* model is the model that will define your robot. In many cases, it is the only model that need be defined.
• A model cannot have a name that matches a primitive shape.
• There is _no_ "negative area": making a model which subtracts something from empty area will leave the empty area unchanged, and including this model as a part of another will not subsequently erase the subtracted area from the main model.
• All items in model files are _case sensitive_.

Example Models

# A hollow, square box with side length 200mm and hole size 100mm

base = rect, -100, -100, 200, 200
base.0 = subtract, rect, -50, -50, 100, 100

Equivalently:

# A hollow, square box with side length 200mm and hole size 100mm
#  This representation uses a sub-model

hole = rect, -50, -50, 100, 100

base = rect, -100, -100, 200, 200
base.0 = subtract, hole