Getting started with the MWB demo

The basics

MWB has a full undo/redo stack and file load/save capability. It supports a number of different simulators, which can be selected between using Simulate → Simulator:

• Open Dynamics Engine, developed by Russell Smith.
• Bullet Physics Library
• Ageia PhysX SDK. To use this, you need to have the drivers installed on your system, which can be found here; otherwise it will be disabled.

Modes

The application has two basic modes of operation: Edit and Browse. Switch between these modes using the mode selector in the upper-left-hand corner of the window.

Edit mode is for any sort of initial scene setup: creating simulation objects, moving them around, etc. During the browsing process you can always switch back to edit mode if you need to change initial state, create new objects, etc. While in edit mode, you can additionally preview the simulation without having to enter browsing mode using Simulate → Run Simulation.

Browse mode is where all of the Many-Worlds Browsing happens: new simulation paths are computed and displayed, and you can browse, select, and refine them.

Camera controls

The keyboard and mouse controls were modeled after Autodesk Maya, since this is what I am most familiar with. To move the camera around, hold down the Alt key. The left button rotates, middle button pans, and right button dollies the camera. Note that in this particular setup, the up vector (y) is always fixed, which makes sense here since this is also the direction of gravity.

In the initial setup, you have 4 scene cameras. Switch between the 4-view mode and the 1-view mode by pressing the spacebar. You can change the camera for the currently selected view using the Camera menu. To create a new camera (should this be necessary), use the Create menu.

Edit mode

The first step in the process is to actually set up your scene, including the various objects that will be in it and their initial state. This process is performed in Edit mode, which you will return to later if you need to add aditional objects or other scene modifications during the browsing process.

Objects once created can be manipulated through two major means: firstly, position, rotate, and scale can be manipulated through the various editing tools. Select among the various modes (select, translate, rotate, scale) using the toolbar buttons, or use the shortcut keys q, w, e, and r. The other major mode of interaction is to edit Attributes, which are listed in the attributes panel. When in edit mode, display the attributes panel with Ctrl-A or through Window → Attributes.

Collision objects

The application supports a number of different collision primitives. Not all objects are supported by all simulators, however. Table 1 shows which simulator choices support which collision primitives. Collision objects are created through the Create menu and can be moved, rotated, and scaled into place through the various edit tools.

Notes on specific collision objects:

Cylinder

To change the length of the cylinder, edit the Length ratio attribute.

Capsule

A cylinder with spheres at the ends. Like the cylinder, you can change the length by editing the Length ratio attribute.

.obj geometry

Arbitrary .obj meshes can only be simulated if they can be decomposed into convex regions. Automated convex decomposition is available by selecting the object and using Edit → Fit Convex Hulls. To fit hulls by hand, go into vertex selection mode (using the toolbar button) and select a set of vertices. Edit → Create convex hull from points will then generate a hull from the selected points. Note also that the mass calculation only works for closed objects; if you need to simulate meshes that are not closed you will need to turn off the hasMass attribute on the mesh and fuse if with some mass-containing proxy objects to generate a valid inertia matrix.

.sph file

MWB supports .sph files generated by Gareth Bradshaw's Sphere-Tree Construction Toolkit, which are simply converted to spheres for the purpose of simulation.

Grouping objects

MWB is a fully hierarchical modeling system. All objects (including joints) can be grouped together and scaled/rotated/translated together. One major limitation is that most of the collision objects do not support shearing and in addition many do not support nonuniform scales. This means if such objects are grouped together they can only be scaled uniformly since nonuniform scaling and rotation can be combined to generate an arbitrary shear. This restriction is not enforced very robustly and can be overridden easily by hand-modifying the generated files, but I cannot predict what the results will be.

To group objects, use Edit → Group or press Ctrl-g. To visualize and manipulate the hierarchy, use the hierarchy view (Window → View Hierarchy); the middle mouse button can be used to drag objects around.

Fusing objects

The provided collision primitives are all convex and mostly very simple. It does, however, support the generation of nonconvex objects using a union of convex objects. To generate such a union, select the objects and press Edit → Fuse.

Joints

MWB supports a number of different joint constraints. To create a joint constraint between two objects, select the two objects and select the appropriate joint in the Create menu. A visualization of the joint object is created at the origin, which can then be moved and rotated into place. Parameters such as the joint center and axes are determined by how the joint object is placed, but this should be fairly intuitive. A more complete description of the various joint types can be found in the Open Dynamics Engine documentation.

Randomizing state

In order to vary initial positions, velocities, etc. during the sampling process, use the randomization features present in the demo. Any float-valued attribute can be randomized. To do this, simply click with the right mouse button on any attribute in the attribute panel and select Randomize property in the context menu. It will ask you for a range of values; during sampling, values are chosen uniformly from this range. To disable randomization, select Remove randomization from the same menu.

Browse mode

As soon as you switch to Browse mode, the software begins sampling different simulation worlds. To visualize these paths, simply select an object in the scene. Note that you have to select at the correct level in the hierarchy (e.g., if you are simulating a box, it does not work to select the box's parent). To make selection simpler, you can change the selection mode to Simulated through Edit → Select by → Simulated. There are a few tools available to help sort through the various possibilities.

Selection tools

There are 3 major tools for selecting and modifying object paths.

Positive constraint

Restrict the search to paths that pass through the selected region. Applies only to selected objects. Some limitations of these constraints are described in the paper.

Negative constraint

Restrict the search to paths that do not pass through the selected region. Applies only to selected objects. Some limitations of these constraints are described in the paper.

Refine paths

Refine the paths of selected objects starting at the selected region. Details of this process can be found in the paper.

Simulation sorter

Sort through simulations ordered by various criteria in the Simulation sorter. This window can be accessed through Window → Simulation sorter, and the interface should hopefully be fairly intuitive.

Modifying the scene

To add/remove/change any object in the scene, go back to edit mode. Edited objects will be marked in red to indicate that their motion will need to be modified to get a physically accurate simulation. When you switch back to edit mode, however, it will still be sampling on the old scene; to correct this, choose Simulation → Update sample which will update the motion of all modified objects with newly computed physics.

Timeline

To visualize a particular simulation, you can either scrub across the timeline using the left mouse button or use the Play button. The standard camera controls also work in the timeline to expand/contract the region visualized.

To make sure simulations finish reasonably quickly, we restrict how long they are allowed to run. This is controlled through the active region, a small bar on top of the timeline. Drag the small handles on the ends of the active region to expand the active region. If you have a particular simulation the you like but which terminates too early, you can always select it and refine the tail end of the path, which will correct this.

Exporting the data

Once you are happy with a particular simulation, there are a variety of different methods for rendering/exporting the motion.

.ma file

A Maya Ascii format file for reading into Autodesk Maya.

Image sequence

An OpenGL-rendered image sequence.

Timelapse image

Renders the motion as a long-exposure photograph, like on the first page of the paper. The image is exported in OpenEXR format, which must then be converted to LDR using some external program.