MassFX provides tools for creating rigid bodies as well as adjusting their settings and their physical meshes: objects' representations within the simulation.
You create a rigid body by applying a MassFX Rigid Body modifier to an object. You can do this from the Modify panel, with the Rigid Body flyout on the , or via the commands on the Animation menu Simulation - MassFX Rigid Bodies submenu.
The different toolbar buttons and menu commands all create the same modifier while setting the value of the Rigid Body Type parameter based on your choice. You are free to change the type of the rigid body after creation. For more information on the different rigid body types, see " ".
If you tried the preceding procedure, you saw that the MassFX simulation moves dynamic rigid bodies in the scene. Clicking (Reset Simulation) moves all dynamic rigid bodies back to their starting positions. This starting spot is referred to as the "initial transform" of the dynamic rigid body.
How does MassFX know where this transform is? By default, any change you make to the position or rotation of a dynamic rigid body when the time slider is at the first frame automatically updates the initial transform. Any changes you make to the transform of a dynamic rigid body when the Time Slider is not on the first frame will be lost when you click (Reset Simulation).
The Density and Mass parameters for a physical material are tied together through the volume of the object. For example, a tiny steel pin and a large steel bollard both have a density around 7.85 g/cm3, but (of course) the pin has a mass that is far less. Heavier objects carry more inertia and can more easily push light objects out of their way.
To make a dynamic rigid body fall faster, you might try increasing its density (or mass) in the Physical Material rollout for the modifier, but that won't do anything. In the idealized world (ignoring air resistance), objects of different sizes and weights all fall at the same speed. (Perhaps you remember something apocryphal about Galileo dropping things from the Leaning Tower of Pisa?) The only ways that you can make an object fall faster are to either:
In a graphics-only world, a brick tower 50 feet tall seen by a person six feet tall standing 10 feet away appears identical to a brick tower 500 feet feet tall seen by a giant 60 feet tall standing 100 feet away. However, a (gigantic) brick falling from that (enormous) 500 feet tower is going to take a lot longer to hit the ground than a (normal) brick falling from just 50 feet up. If you are looking through the giant's eyes while thinking you are a human, you are going to wonder why the brick is falling so slowly.
So if your simulation feels like it is on the moon, before cranking up the gravity, first check how big everything is. You can almost hear your high-school teacher saying, "When it comes to physics, units matter!".
The amount of friction you apply to an object determines how easily it slides along other surfaces. For example, a teflon cube on a teflon table with friction values of 0.05 (almost no friction) will slide almost indefinitely before coming to a stop. Conversely, two pieces of sandpaper-covered-blocks with friction values of 0.95 (almost full friction) will almost never slide on each other.
If you've every tried to push a car or a couch, you may have noticed that it's hard to get it moving, and easier once you're underway. For realistic results, the static friction value should usually be higher than the dynamic friction value.
The Select a Mesh to Modify section of the Physical Meshes rollout lets you create additional physical meshes. For more information on this (uncommon) situation, see the reference section for the . Typically, you'll need multiple physical meshes only if the simulation requires a concave surface on a dynamic rigid body; the easiest way to accomplish this is with the Composite mesh type (see following).
The Mesh Type setting lets you change the type of (selected) physical mesh used for your rigid body. When you choose a mesh type, its parameters appear on the on the Modify panel and MassFX Tools dialog > Edit panel. Following are high-level descriptions of the available choices:
These are simple, "infinitely smooth" MassFX primitives. Although, by default, these primitives roughly bounding the graphical mesh when created, you can use the parameters (Radius, Length, Width, Height) to control the size of the primitive after creation.
The default parameters are 32 vertices and no inflation. For increased performance at the expense of a rougher approximation, lower the number of vertices. To create a hull larger than the graphical mesh, increase the Inflation value.
This option provides the easiest method for creating a concave physical mesh for use as a dynamic rigid body. It automatically creates and combines multiple convex hulls to create the physical mesh based on the vertices from your graphical mesh. If your graphical mesh is concave, this option will generate the most precise composition of convex hulls to support the concave shape. However, using this mesh type consumes a lot of resources; use it only if you need a precise concave rigid body.
This option uses the vertices from your graphical mesh to create the physical mesh. If your graphical mesh is concave, using the Original mesh type doesn't magically make dynamic or kinematic rigid bodies support the concave shape. However, Original gives the best performance to support a concave static rigid body. If you require a precise concave rigid body, use Composite instead.
For example, create a Plane primitive of 32x32 segments and add a Ripple modifier to it with sufficient amplitude and wavelength to make it bumpy. Set it to be a static rigid body and set the physical mesh type to Original. Dropping dynamic rigid bodies onto this will show them bouncing off the ridges and settling into the valleys.
To use a custom object, set Mesh Type to Custom, click the button next to the "Custom" label, click the Pick Source Object button on the Physical Mesh Parameters rollout, and then select the object in the scene you wish to use for your physical mesh. The name of the associated object is then displayed on the button.
The geometry of the custom object is copied to the MassFX Rigid Body modifier. If you like, you can then delete the custom object without problem. However, if you don't delete the custom object, you can change its geometry and then use the Update From Source Object function to re-copy the latest information to the modifier.
What if you want to use a custom physical mesh, but do not have one ready-made? For example, the default convex hull is close to the desired physical mesh, but you want to tweak the placement of some of the vertices. Just click Convert To Custom Mesh on the Physical Meshes rollout of the Rigid Body modifier (unavailable on the MassFX Tools dialog) for a different mesh type to create a new Editable Mesh in the scene that is associated as a Custom object. All you then have to do is tweak the mesh object, then use the Update From Source Object function.