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Whenever we add a dynamic rigid body modifier to an object or objects in our scene, they become subject to the law of gravity. This should mean of course it is enable in our global options. However having all free-floating objects in the scene automatically fall to the floor may not be the desired end result. We may, for instance, find ourselves needing to fix certain objects in places, it were, locking them to a specific point in 3D space. Alternatively, we may need to create complex relationships between two objects in the scene, such as having them dynamically slide against one another within a fixed set of boundaries or revolve around one another on a fixed axis.
In these and other such situations, we will want to make use of a set of tools called constraints. As you can probably surmise from the name, a constraint inside a dynamic simulation restricts the movement of particular objects that are part of the rigid body simulation. The idea of a constraint is that it creates hierarchical parent-child relationship between two entities. Some examples of constraints in the real world would include hinges, nails, curtain rails, and axles, to name just a few.
Now in MassFX in 3ds Max, all of the constraint types that we will mention here are available as preset options, although it is probably worth noting that they're all just variations of the universal constraint but that have particular settings already applied to them. Of course, all good dynamic simulation tools will have these or similar constraint options available. The simplest constraint type we can use in a rigid body simulation is often referred to as a rigid or fixed constraint, and really, its name tells us exactly what it is designed to do.
By default, a rigid constraint will have all of its transform options locked. This means there can be no movement, rotation, or twist action at all once a simulation starts. It really does lock an object in position. It can also be used to lock two objects together so that they move and behave as one inside the simulation. Another typical constraint type we may come across could be the Slide constraint--very similar in construct to a rigid constraint, except in this case we do have a limited single-axis translation enabled.
In MassFX this is the Y axis by default, but of course this can be altered. We might also find we have a Hinge constraint available. This usually has a single swing axis enabled that has a limited swing range set in degrees. We might also find a twist constraint, which will probably have a single axis twist value that has been set to unlimited or unconstraint. This would allow an object to twist, or as I prefer to think of it, spin freely. In MassFX we also get two other preset constraint types that we can work with, these being the universal constraint that has two swing axes set to a limited value, such as 45 degrees, and a ball-and-socket constraint that uses two separate swing axes limited this time to 80 degrees, but also with a twist amount set to unlimited.
In a rigid body simulation any objects that have been set as dynamic rigid bodies will be subject to the laws of gravity, motion, and collision, meaning they will start to react in a fairly random manner as soon as a simulation is enabled-- unless, that is, we introduce constraints into the mix. With our constraint tools, we can start to craft very specific behaviors from dynamic objects, creating complex motions and interactions that would be otherwise impossible to achieve.
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