Models in Drake

Introduction

This document discusses how physical objects (i.e., robots and other objects such as furniture) are represented in Drake.

The representations of physical objects are modeled in Drake declaratively using either URDF (Unified Robot Description Format) or SDF (Simulation Description Format), which are both XML formats.

The target audience for this document is for those who want to model a physical object for use in Drake. You should have some understanding of XML formats, and be able to run Drake. You don’t need to know C++. This document provides enough reference material for someone to edit or create a representation of a simple object and its environment, by defining the terminology and providing an overview of what’s in a model file. This document is not meant as a tutorial for creating URDF/SDF files, which you can find elsewhere. For deeper understanding of polygonal modeling and advanced topics like meshes and textures, we try to provide links to further information, but you may have to do more of your own research and education.

In this document, we call the URDF or SDF representation of the object a model. The model of an object will describe the object’s physical properties, including visual, contact, and dynamic properties. Visual properties describe the appearance of an object, such as its shape and texture. Dynamic properties describe how the object moves and have parameters like mass and spatial inertia. Contact properties describe how an object behaves when in contact with other objects and include numerous collision parameters.

These properties allow you to do simulation, visualization, motion planning, and control.

Terminology

Mesh - a polygon mesh which is a collection of vertices, edges, and faces that describes the surface of a 3D object. For a mesh to represent the fine details of a surface, a complex mesh is required. Processing a mesh gets more expensive as the mesh becomes more complex. Therefore, it is important to use a mesh with just enough detail to serve the purpose. To be compatible with Drake, all of the faces in the mesh must be triangles. Many modeling tools have a simple tool for “triangulating” the mesh.

SDF- Simulation Description Format. An XML format that describes objects and environments for robot simulators, visualization, and control. SDF is OSRF’s successor to URDF. Unlike URDF, SDF allows multiple objects to be modeled in a single SDF file, and allows you to model the pose of the robot in the world. Relative to URDF, SDF is being more actively maintained.

Texture - the digital representation of an object’s surface. A texture may include color, brightness, transparency, reflectivity, and other aspects. The texture is mapped onto a preexisting surface.

URDF- Unified Robot Description Format, an XML format for representing one model of a robot or other object. Unlike SDF, only one object can be modeled per file, and pose cannot be modeled. Drake’s URDF extends the official ROS URDF.

File formats for Drake model components

Model file format

Drake’s models can be defined in either URDF or SDF file formats, but SDF is newer and thus preferred. They are similar formats. Both are XML-based.

Mesh file format

Drake can process the following mesh file format (file extension):

  • OBJ - a 3D geometry definition file format.

If Drake processes an SDF or URDF file and finds a mesh file name extension that is not a type that it can process (currently only OBJ), it will see if a file exists with the same path and name but with an “obj” extension.

The following are mesh file formats that are not processed by Drake, but may be found in the Drake repository and may be used to find OBJ versions:

  • DAE (digital asset exchange) - can include textures. An issue exists to add Drake support for DAE files.
  • STL (STereo Lithography) - describes only the surface geometry of a three-dimensional object without any representation of color, texture or other common CAD model attributes. An issue exists to add Drake support for STL files.
  • WRL- an obsolete mesh format that is superseded by a new format called X3D. Allows you to specify surface color, textures, shininess, transparency, and other parameters. Vertices and edges for a 3D polygon can be specified.

The content of a Model file

The best way to learn how to create a model is to examine and change an existing, working model. This tutorial walks through editing a model in Drake.

For detailed information on the elements of a model, see the URDF or SDF references for more information.

Modelling the shape of an object

To model a shape for visual purposes, use URDF’s or SDF’s <visual> tag. To model a shape for the purpose of determining contact between your model and other objects, use URDF’s or SDF’s <collision> tag. (While both SDF and URDF use those same tags, note that the structure and content of those tags differ.)

To explain why you might want to define the collision element differently than the visual element, let’s explore the different ways of defining shapes.

An object’s shape can be modelled using a 3D scanner, which produces a polygon mesh. Meshes of geometrically complicated objects contain many polygons. The detail (density) of the mesh is great for a realistic visual display. However when a mesh is used for a collision element, collision algorithms must process all the polygons that are close to the target, which can be slow. For these reasons it is often desirable to use something simpler than a mesh for the collision model. It’s a tradeoff between accuracy of the shape and processing time.

The <visual> tag is used in visualization programs like drake-visualizer (in the Director external). Drake does not process the visual tag, unless you have something specific in your code that will process it, like RgbdCamera. Regardless of what program is processing the visual data, the processing time of visual elements is generally not an issue.

In addition to modelling shapes with meshes, you can also model shapes with geometric primitives, such as cylinders, spheres, or boxes. Geometric primitives are far less complex than meshes and so require far less processing time.

In the iiwa14.urdf robotic arm example, we use meshes for the visual geometry elements, and cylinders for the collision elements, except for the last links where meshes are used. The idea is that for links of the arm, we don’t need exact shapes for collision, where we don’t expect to need precision. Even if we were to use an “elbow” to shove an object out of the way, we probably don’t need exact accuracy.

But there are cases where we need the precision of a mesh. For example, extra modeling precision may be needed to simulate a robot arm’s end effector reaching into tight spaces or performing dexterous manipulation tasks. In these cases, if it’s taking too long to process the original mesh, then the mesh needs to be simplified. The next section describes some options.

Simplifying a Mesh

Q: What is the easiest way to use something simpler for a collision model?

A: If you are starting with a mesh of an object (e.g., from a 3D scanner), you may be able to use a primitive geometric shape (e.g., cylinder, box, or sphere) instead of the mesh, as described above. To do this, consider a tool like Gazebo, which provides a visual editor of model files, and allows you to modify primitive geometric shapes in your model.

Q: A single geometric shape is not accurate enough for my needs. What else can I do?

A: You can use multiple overlapping geometric shapes to define slightly more complex collision elements.

Q: I need something much more accurate than these simple shapes. How do I simplify a mesh?

A: Sometimes you really need the accuracy of a mesh.

An existing complex mesh can be simplified using a tool like Blender, a free and open source 3D creation suite.