1 ArtiSynth Overview

1.5 Creating an application model

ArtiSynth applications are created by writing and compiling an application model that is a subclass of RootModel. This application-specific root model is then loaded and run by the ArtiSynth program.

The code for the application model should:

  • Declare a no-args constructor

  • Override the RootModel build() method to construct the application.

ArtiSynth can load a model either using the build method or by reading it from a file:

Build method

ArtiSynth creates an instance of the model using the no-args constructor, assigns it a name (which is either user-specified or the simple name of the class), and then calls the build() method to perform the actual construction.

Reading from a file

ArtiSynth creates an instance of the model using the no-args constructor, and then the model is named and constructed by reading the file.

The no-args constructor should perform whatever initialization is required in both cases, while the build() method takes the place of the file specification. Unless a model is originally created using a file specification (which is very tedious), the first time creation of a model will almost always entail using the build() method.

The general template for application model code looks like this:

package artisynth.models.experimental; // package where the model resides
import artisynth.core.workspace.RootModel;
... other imports ...
public class MyModel extends RootModel {
   // no-args constructor
   public MyModel() {
      ... basic initialization ...
   // build method to do model construction
   public void build (String[] args) {
      ... code to build the model ....

Here, the model itself is called MyModel, and is defined in the (hypothetical) package artisynth.models.experimental (placing models in the super package artisynth.models is common practice but not necessary).

Note: The build() method was only introduced in ArtiSynth 3.1. Prior to that, application models were constructed using a constructor taking a String argument supplying the name of the model. This method of model construction still works but is deprecated.

1.5.1 Implementing the build() method

As mentioned above, the build() method is responsible for actual model construction. Many applications are built using a single top-level MechModel. Build methods for these may look like the following:

   public void build (String[] args) {
      MechModel mech = new MechModel("mech");
      addModel (mech);
      ... create and add components to the mech model ...
      ... create and add any needed agents to the root model ...

First, a MechModel is created (with the name "mech" in this example, although any name, or no name, may be given) and added to the list of models in the root model. Subsequent code then creates and adds the components required by the MechModel, as described in Sections 3, 4 and 6. The build() method also creates and adds to the root model any agents required by the application (controllers, probes, etc.), as described in Section 5.

When constructing a model, there is no fixed order in which components need to be added. For instance, in the above example, addModel(mech) could be called near the end of the build() method rather than at the beginning. The only restriction is that when a component is added to the hierarchy, all other components that it refers to should already have been added to the hierarchy. For instance, an axial spring (Section 3.1) refers to two points. When it is added to the hierarchy, those two points should already be present in the hierarchy.

The build() method supplies a String array as an argument, which can be used to transmit application arguments in a manner analogous to the args argument passed to static main() methods. In cases where the model is specified directly on the ArtiSynth command line (using the -model <classname> option), it is possible to also specify arguments for the build() method. This is done by enclosing the desired arguments within square brackets [ ] immediately following the -model option. So, for example,

> artisynth -model projects.MyModel [ -foo 123 ]

would cause the strings "-foo" and "123" to be passed to the build() method of MyModel.

1.5.2 Making models visible to ArtiSynth

In order to load an application model into ArtiSynth, the classes associated with its implementation must be made visible to ArtiSynth. This usually involves adding the top-level class directory associated with the application code to the classpath used by ArtiSynth.

The demonstration models referred to in this guide belong to the package artisynth.demos.tutorial and are already visible to ArtiSynth.

In most current ArtiSynth projects, classes are stored in a directory tree separate from the source code, with the top-level class directory named classes, located one level below the project root directory. A typical top-level class directory might be stored in a location like this:


In the example shown in Section 1.5, the model was created in the package artisynth.models.experimental. Since Java classes are arranged in a directory structure that mirrors package names, with respect to the sample project directory shown above, the model class would be located in


At present there are three ways to make top-level class directories known to ArtiSynth:

Add projects to your Eclipse launch configuration

If you are using the Eclipse IDE, then you can add the project in which are developing your model code to the launch configuration that you use to run ArtiSynth. Other IDEs will presumably provide similar functionality.

Add the directories to the EXTCLASSPATH file

You can explicitly list class directories in the file EXTCLASSPATH, located in the ArtiSynth root directory (it may be necessary to create this file).

Add the directories to your CLASSPATH environment variable

If you are running ArtiSynth from the command line, using the artisynth command (or artisynth.bat on Windows), then you can define a CLASSPATH environment variable in your environment and add the needed directories to this.

All of these methods are described in more detail in the “Installing External Models and Packages” section of the ArtiSynth Installation Guide (available for Linux, Windows, and MacOS).

1.5.3 Loading and running a model

If a model’s classes are visible to ArtiSynth, then it may be loaded into ArtiSynth in several ways:

Loading by class path

A model may be loaded by directly by choosing File > Load from class ... and directly specifying its class name. It is also possible to use the -model <classname> command line argument to have a model loaded directly into ArtiSynth when it starts up.

Loading from the Models menu

A faster way to load a model is by selecting it in one of the Models submenus. This may require editing the model menu configuration files.

Loading from a file

If a model has previously been saved to a file, it may be loaded from that file by choosing File > Load model ....

These methods are described in detail in the section “Loading and Simulating Models” of the ArtiSynth User Interface Guide.

The demonstration models referred to in this guide should already be present in the models menu and may be loaded from the submenu Models > All demos > tutorial.

Once a model is loaded, it can be simulated, or run. Simulation of the model can then be started, paused, single-stepped, or reset using the play controls (Figure 1.2) located at the upper right of the ArtiSynth window frame.

Figure 1.2: The ArtiSynth play controls. From left to right: step size control, current simulation time, and the reset, skip-back, play/pause, single-step and skip-forward buttons.

Comprehensive information on exploring and interacting with models is given in the ArtiSynth User Interface Guide.