3ds Max 2012 Bible - Kelly L. Murdock [463]
Regardless of the object type and its default values, you can set the actual physical properties for the selected object using the settings in the Physical Material rollout in the Command Panel when the object is selected. You can access these same properties in the Edit panel of the MassFX Tools dialog box, shown in Figure 43.4. You also can select and set the properties for multiple selected objects at the same time.
Tip
Although the Command Panel and the MassFX Tools dialog box have mostly the same settings, the MassFX Tools dialog box lets you set values for multiple selections at once. •
FIGURE 43.4
The Edit panel of the MassFX Tools dialog box includes physical property values for the selected object.
Density, mass, friction, and elasticity
Although many different physical properties are available, the MassFX system is mainly concerned with collisions between rigid body objects, which can be computed using a short list of physical properties including Density, Mass, Static Friction, Dynamic Friction, and Bounciness.
The Density property is the amount of mass per volume, so it's related to mass. The Mass property defines how heavy the object is. For example, a bowling ball has a higher mass value than a Ping-Pong ball, and a Ping-Pong ball is not as dense (or heavy) as a golf ball, even though they are roughly the same size.
Friction is the force of contact between two touching objects. It defines how resistant the object is to rolling or sliding along the floor. For example, when you slide an air-hockey puck over an air-hockey table, the friction is very low because of the jets of air, but moving a piece of sandpaper across a piece of wood has a very high value of friction that resists the movement.
Friction is actually defined by two properties. Static friction is the initial force required to start an object sliding across another, and dynamic friction is the amount of force required to keep the object sliding over the top of the other. Both of these properties are available in the MassFX system.
Note
A rigid body with a Mass value of 0 causes problems for all calculations, so the system automatically sets the Density or Mass value to 0.01 when you try to set it to 0.
Creating presets
If you've done some research and figured out the exact physical properties for a specific object, you can use the Create Preset button in the Physical Material rollout of the MassFX Tools dialog box to save and name the physical values for the selected object. All defined presets are populated in the drop-down list. This provides a great way to reuse values that you know are right. The eyedropper tool lets you quickly pick the properties from another object in the scene.
When a preset is used, all its properties are automatically locked, but you can unlock and change them using the Lock icon in the upper-left corner of the Physical Material Properties rollout of the MassFX Tools dialog box in the Edit panel.
Defining collision boundaries
Another common property that you can set pertains to how the object deals with collision detection. You can select the volume to use to determine when two objects collide with each other. If this sounds a bit funny because any collision volume that doesn't use the actual mesh would be inaccurate, then you need to realize that a complex simulation with lots of collisions of complex objects could take a long time to compute. If MassFX has only to compute collisions based on the object's bounding box instead of the actual mesh object, the simulation runs much more quickly, and the inaccuracies aren't even noticeable.
Before deciding on the collision boundary to use, you need to determine whether an object is convex or concave. A convex object is one that you can penetrate with a ray and cross its mesh boundary only twice. Concave objects require more than two crossings with an imaginary ray. In other words, concave meshes have surface areas that bend inward like a doughnut and convex meshes don't, like a normal sphere. Convex meshes are much easier to