Welcome to iMSTK-Unity

iMSTK is a free & open source C++ toolkit for prototyping of interactive multi-modal surgical simulations. The iMSTK-Unity asset gives you access to its’ capabilities through the Unity authoring interface. While this asset is still under development you can already exercise a variety of parts of iMSTK inside of Unity. This guide will help you get acquainted with the architecture and workings of the plugin.

iMSTK-Unity

User Documentation

Introduction

This document describes the iMSTK-Unity interface as well as how to build and develop it.

Overview

iMSTK is a free & open source C++ toolkit for prototyping of interactive multi-modal surgical simulations. The iMSTK-Unity asset provides classes that allow the use of iMSTK features inside of unity, those are amongst others various rigid and deformable models, collision detection and response, geometric importers and filters.

The iMSTK-Unity classes utilizes a C# interface of iMSTK that is generated by using the SWIG tool. This interface in effect is wrapped around most C++ functions and forwards its call to the original C++ code. This allows you to use iMSTK in C# much the almost the same way as you would in C++.

Limitations

This is an early release, not all iMSTK classes are easily accessible through the unity layer. Additionally there may be some combinations that have not been tested. Some errors _may_ cause Unity to crash, save early and save often. And please report issue in the iMSTK-Unity issue tracker

While there is an OpenHaptics asset available at the store, iMSTK-Unity does not support that currently, if you need support for the 3D-Touch series of devices you will have to build iMSTK with the appropriate settings.

Project Structure

The files of the asset are split in the following directories:

• iMSTK/Scripts: This directory is for the runtime scripts of the plugin.

• iMSTK/Scripts/Editor: This directory is for the editor scripts of the plugin. These scripts implement custom editor functionality. Editor scripts may use runtime scripts but won’t be included in the player, which means you should not use functions from these in your own scripts

• iMSTK/Resources: This directory contains resources required during runtime.

• iMSTK/Editor/Resources: This directory contains resources required in the editor (such as style sheets or UI markup).

• iMSTK/Plugins: This directory is for libraries (dlls/so) that Unity needs to load.

Directories needed for the project but not relevant to the plugin:

• iMSTK/Models, Materials, & Textures: These directories are for various assets used by the Demo. They are not part of the plugin.

The asset on the store may not contain the latest version of iMSTK-Unity, if you want to be up to date you can check out the sources from gitlab. As the binary files for iMSTK aren’t included in the repository, you will also have to build iMSTK yourself when you go this route.

Setup for Development

When checking out iMSTK-Unity from gitlab you will first need to build iMSTK. iMSTK uses CMake for its build system. iMSTK is a superbuild meaning it builds, includes, and links to all of its dependencies. There is no need to go find them. To build iMSTK clone the iMSTK repository and build it using cmake. Turn on the IMSTK_BUILD_FOR_UNITY cmake switch. This will reduce the number of dependencies that are being built. It will also enable the generation of “.cs” files as well as “.dlls”/”.so” files in your install directory that are needed by the “.cs” code. These will be used in Unity. Optionally enable iMSTK_USE_VRPN and/or iMSTK_USE_OpenHaptics for device support.

You can install the binaries two different ways, for both methods you need to know the directory in which iMSTK was installed. If you didn’t change anything that directory will be called install and resides inside the directory that you defined in cmake to build imstk, e.g. C:Projectsimstkbuildinstall :

• Using `InstallImstk.bat` from the asset directory the format is

  InstallImstk.bat <path to imstk/install>

• Alternatively you can enable Developer Mode for iMSTK inside of Unity. This option ca be found in “Edit->Project Settings->iMSTK Settings”. When turned on, you will prompted to install iMSTK whenever you start Unity. After turned on you may restart Unity, when it prompts you, select yes and provide it your iMSTK install directory, located in <iMSTK build directory>/install. All the necessary files will be copied. If you don’t want to be asked to reinstall iMSTK toggle this option off again.

If you make changes in iMSTK you must reinstall it to Unity.

Note: There is a ‘Force Install’ button to install iMSTK into Unity at the instant it is clicked. This may no always work depending on if an iMSTK script (that utilizes a given dll) has run in the editor. In that case those dll’s cannot be unloaded from Unity.

Note: Make sure Unity is not running. Sometimes it can close but still be running in the background prevent installation.

Devices

The asset available from the asset store does not have OpenHaptics or VRPN enabled, to use external devices you will have to build iMSTK for yourself. After enabling enable iMSTK_USE_VRPN or iMSTK_USE_OpenHaptics respectively. This will create a version of iMSTK with device support enabled. When the build is done install iMSTK into the Unity Asset as described above.

Debugging

Debugging may be done in visual studios, even on the native side. You must attached the debugger as you would to an external process. With iMSTK open in visual studios, click debug->attach to process. Select the unity executable and make sure you’re set on native debugging.

Tutorial

In this tutorial we are going to assemble a simple scene containing a deformable object colliding with static scenery. The completed tutorial is available as the Tutorial scene in the scene folder.

For the purpose of this guides component names will be written in a mono spaced font e.g. SimulationManager, whereas the names of GameObjects or EditorFields will be in italics e.g. HeartModel

• Create a new scene in Unity

• Create a new gameobject and add a SimulationManager component to it

  The SimulationManager is a required component for all simulations using iMSTK-Unity, it controls initialization and updates and also lets you set a number of global variables. Set up the manager parameters as shown in the picture

• Create an empty GameObject, add a PBDModel component and name it HeartModel.

  PBDModel is one of the classes that iMSTK-Unity offers to you it implements the PBD 1 and XPBD 2 methods for simulating deformable objects.

  We will add geometry separately but you can also use the items under the menu Imstk\GameObject to add very simple objects with algorithmically generated meshes.

  The PBDModel has a large number of parameters but we will focus only on a few. This will eventually become a deformable object but there is still a lot of work to do. For more information look at the Imstk Documentation

  We will come back to this later.

• From the Models folder add model named heart under the HeartModel GameObject

  This will serve as the visual representation of what the simulation is doing, to make it look better get the flesh material from the Materials folder and assign it as well.

  With the heart GameObject selected add a GeometryFilter component to it and drag the MeshFilter component, the name will be Heart_mesh_surface (Mesh Filter) into the Mesh field of the GeometryFilter. The dropdown should say Unity Mesh. This makes this mesh available to iMSTK-Unity.

  For every kind of geometry you want to use in the PBD model, iMSTK-Unity needs a GeometryFilter. This maps the Unity type to something that iMSTK can understand. It can also be used to define fixed shapes like Plane or Capsule. For meshes the source of an Geometry filter can either be a Unity mesh in the scene, a mesh asset, or (especially for tetrahedral meshes) an asset imported by the geometry importer of imstk-Unity.

  You can use the check box named Show Handles to verify that the mesh is in the correct location

• Lets look at PBDModel component again, and set up some parameters

  First Check the Distance Stiffness, Volume Stiffness and Use Realtime checkboxes and fill in the properties as you see in the image below. This sets the material properties of this object.

• Add a GeometryFilter to use in the simulation

  We will need a mesh to use as the geometry for calculating the physical behavior of our object. Add a GeometryFilter to the HeartModel object. As we will use a tetrahedral mesh the method to a assign the mesh is slightly different. First Select Tetrahedral Mesh in the dropdown box. Then click on the o icon to the right of the Mesh input field. This will bring up an input dialog. Select the “Assets” tab, and double click the item name heart_mesh.

  As you can see the GeometryFilter component can be used for meshes in the scene or just assets of the project.

• Now we will set the shapes that are being used for simulation and visualization.

  The PBDModel uses three different geometries

  • Visual Geometry is the geometry that is being shown on the screen, this is usually some texture mesh

  • Physics Geometry is the geometry that is being used in the simulation

  • Collision Geometry is the geometry that is being used to determine collisions with other objects.

  The tetrahedral mesh that was set up in the previous step will be used for as the Physics Geometry the other mesh from earlier will be used for the two other geometries.

  First Drag the Geometry Filter that you just created into the Physics Geometry of the PBDModel component. The drag the heart GameObject from the hierarchy view to both the Visual Geometry and the Collision Geometry fields.

  

  As we used different geometries for visualization and simulation we need a way to keep those in the same state, this is the responsibility of the GeometryMap component. Drag the HeartModel Gameobject into the Parent Geom slot. Then drag the heart GameObject into the Child Geom slot. Additionally make sure the Force One One is checked.

  

This concludes the setup for the PBDModel object.

You should be able to run the scene now but as there are no other objects to interact with the heart will just succumb to gravity and drop on the ground.

• Lets add a plane for collisions

  We will go an alternate route for this object. In the hierarchy view add a normal Unity Plane from and move the plane to a position of 0.0, -2.5, and 0.0. Add a Geometry Filter and select Plane in the drop down menu. The default settings for the plan will work, the position and normal will be calculated from the transform. Even though the visual mesh of the plan is finite in the editor, with regards to iMSTK this plane is infinite.

  To enable the plane to interact with other imstk objects we need to set up a model for it as well. Add a StaticModel component to the Plane object and drag the GeometryFilter component into the Collision Geometry field. A StaticModel represents an object that participates in collision but doesn’t react.

• Add the interaction between the Heart and the Plane

  Imstk needs to know about which objects can interact with each other, in this case we want the heart and the plane to collide with each other. Add a CollisionInteraction to the plane object. Drag the HeartModel into the slot named Model 1 and drag the plane itself into the slot named Model 2. The dropdown named Collision Type can be left in the setting Auto. At the moment there is most likely no need to select the Collision Type yourself, the automatic mechanism should be sufficient.

• This concludes the tutorial scene setup, press play you should see the deformable object hit the plane and bounce slightly on it.

Please note that at the moment iMSTK-Unity is very sensitive to misconfigurations and may cause Unity to crash, we are working to improve the error handling and stability both on the Unity asset side and inside the iMSTK sources

Example Scenes

PbdClothCollision

A scene with a freely moving deformable item (Cloth), shows how to set up a deformable (PBDCloth) with and various static obstacles.

PbdClothScene

A cloth constrained on the top, look at for how to set up boundary conditions on a PbdObject

PbdThread

Sets up a line mesh that can be used as a thread

RbdScene

Simple example of two rigid spheres colliding with each other and the scenery. Uses geometric shapes rather than meshes, static colliders and collisions between dynamic objects

Tutorial

Scene that is used in the tutorial, uses a deformable model colliding with a plane

Devices

The scenes in the devices folder can only be used with VRPN or OpenHaptics built in. This means you have to build iMSTK and install it into the asset as described in Setup for Development

RbdController

Sets up a OpenHapticsDevice with a RbdModel and a RigidController to show how these pieces interactive

RbdControllerVRPN

Uses the VRPNDevice in the same scene as the RBDController

TissueContact and PbdThinTissueContact

Demonstrates the use of a rigid with haptics interacting with two kinds of simulated tissue

1

13. Müller, B. Heidelberger, M. Hennix, and J. Ratcliff, “Position based dynamics,” Journal of Visual Communication and Image Representation, vol. 18, no. 2, pp. 109–118, Apr. 2007, doi: 10.1016/j.jvcir.2007.01.005.

2

13. Macklin, M. Müller, and N. Chentanez, “XPBD: position-based simulation of compliant constrained dynamics,” in Proceedings of the 9th International Conference on Motion in Games, Burlingame California, Oct. 2016, pp. 49–54. doi: 10.1145/2994258.2994272.

Usage

For minimal usage of iMSTK-Unity, two things must be added to a Unity Scene.

• A GameObject with a SimulationManager attached to it

• A GameObject with a Model attached to it.

Commonly a PhysicsGeometry is also needed on the Model GameObject.

Component Structure

While the C# wrapper supports almost all iMSTK classes, there is a subset that is made available as unity components, this can be assembled in the editor to create simulations using iMSTK inside of Unity.

Infrastructure

SimulationManager

SimulationManager is a component responsible for managing the simulation. There may only exist one. It also controls the construction, initialization, and destruction of iMSTKBehaviour to ensure execution ordering: Simulation Manager created -> iMSTK objects created and internally initialized -> iMSTK objects externally initialized -> SimulationManager Start -> Updates -> iMSTK objects cleaned up -> SimulationManager cleaned up.

This component is required to be in the scene for simulations to run. It is created before any other iMSTK components on any GameObject. It implements the start, stop, pause, and other global scene related tasks.

iMSTKBehaviour

An extension of MonoBehaviour to provide different callbacks for special construction, initialization, and destruction ordering.

GeometryFilter

Similar to a MeshFilter in Unity. It provides an input and output geometry. It may take in any iMSTK geometry, as well as a Unity Mesh (one can also drag/drop a MeshFilter to it). These are instances of geometries used in all of iMSTK unity scripts.

GeometryMap

Allows the use of separate meshes for the deformable, visual and collision representation. Will move the vertices of the target mesh according to matching points on the source mesh. The points do not have to completely coincide.

RigidController

Object used between a device handled by the user and a RbdModel. It utilizes a mass spring system to correct for latency in the system. It corrects for problems with haptics in simulation systems. By manipulating the spring parameters the haptic response can be tuned to the behavior of the computer and the simulated system.

BoundaryCondition

Use this behavior to mark vertices on a deformable object as fixed. This means they won’t move but are still part of the overall system. In general this will mean that the object will be attached to the points selected. As the shape assigned can be any mesh this is an easy way to fix an object in space.

Importers

iMSTK-Unity provides a custom Unity importer to import geometry using iMSTK. This may import point, line, surface, tetrahedral, & hexahedral meshes (vtk, vtu, stl, ply, veg, …). If the mesh imported is a point, line, or surface mesh then it will be imported as a Unity Mesh object. Anything else not supported by Unity, is loaded as an iMSTKUnity Geometry Object. When a volumetric mesh (such as a tetrahedral mesh) is imported the accompanying surface is extracted and provided as an additional asset.

Editor

iMSTK-Unity provides extensions to the Unity editor. These extensions include:

• Custom inspectors for the models and geometry components.

• A global settings menu.

• Menu Items for quick creation of GameObject with iMSTK items already setup.

• Editors/windows for various operations

Models

PbdModel

Use this to represent deformable objects. Position Based Dynamics (PBD), is used to model the deformation. This model supports Lines (1D), Surface Meshes (2D) and, Tetrahedral Meshes (3D) dynamical models see the iMSTK Documentation for more information on constraints and models. Visual, physics and collision geometry can be assigned separately. If you do a separate map will be necessary to update the various meshes.

RbdModel

Use this to represent moveable rigid object like forceps or scalpels. Visual and collision geometry can be assigned separately, this behavior will update the transform of the GameObject with each update. For more information set the iMSTK Documentation

StaticModel

Use this to represent un-moveable rigid objects like the ground plane or other obstacles.

Interactions

CollisionInteraction

Use this behavior to set up collisions between two objects, in general this behavior can detect what the type of the two objects is that are interacting (mode Auto). But you can also select the algorithm that should be used.

PbdObjectInteraction

A collision interaction specific to PBD models, it allows setting of values specific to interactions with PBDs.

PbdRigidGraspingInteraction

An interaction that allows the grasping of objects, when activated it will attach a point on the rigid to a point on the deformable object and will attempt to keep the two in the same place.

Devices

OpenHapticsDevice

This device is only available with a custom build of iMSTK. It enable the use of the 3DSystems haptic device.

VrpnDevice

This device is only available with a custom build if iMSTK. It enables interactions with devices run by a VRPN server.

Releases

TBD

Apache License

Version

2.0

Date

January 2004

URL

http://www.apache.org/licenses/

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Other Resources

• iMSTK Discourse Forum

• iMSTK-Unity Gitlab

• iMSTK Documentation

• iMSTK Gitlab