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This course introduces basic physics simulation principles in Autodesk 3ds Max using MassFX, a system that makes it cost effective to animate rigid body objects, cloth, and particle systems. Author Brian Bradley introduces basic concepts such as gravity, drag, volume, and density, and how Newton's Laws of Motion can help you understand the interaction of objects with these unseen forces. Using the purpose built scene, Brian walks through the tools and features of the MassFX (PhysX) system, applying the principles discussed as he goes. Along the way, discover how to combine rigid bodies and constraints, mCloth fabrics, and mParticles geometry to create fairground-style effects.
As a technical artist, when it comes to simulation work, we really do need to understand the workings of the world around us. This will naturally help us accurately re-create a dynamic motion and behavior of objects in a very believable way. For this reason, over the next few videos, we want to briefly touch on a number of laws or principles that govern objects in motion, laws and principles that really anyone wanting to work on dynamic simulations would need to have an understanding of. In fact, figuring out how things work in the real world would be an ongoing field of study for any serious simulation artist.
So, in this video let's give consideration to a couple of extremely important global environmental effects, these being gravity and drag. As we will tackle gravity first, we just need to make a quick disclaimer as neither Sir Isaac Newton nor Professor Albert Einstein were able to fully answer the questions of just what gravity is and how it actually works. We will be focusing in this instance on a discussion of the effects of gravity as we see them in a general sense here on earth. This, after all, is the premise upon which tools like MassFX work.
Gravity as a force determines that any object having mass, if suspended in mid- air and then released, will fall to the ground; hence the old adage, what goes up must come down. Objects come down at a fixed--that is, unchanging--rate of 9.8 meters per second squared. This is a constant value and it is essentially irrespective of mass. It is very true that when affected only by gravity, any two objects, no matter what their respective size and mass, should fall to earth at exactly the same constant speed, as we have mentioned, of 9.8 meters per second.
So, unless we are creating very unique or stylized situations, animations, then really all the objects in our dynamic simulation will need to be subject to the law of gravity. Getting this setting right is going to be critical to the finished quality of our simulations. If, then, all objects irrespective of mass should fall at the same speed under the influence of gravity, we may well ask why two very different objects, such as a feather and a brick, appear to fall at very different speeds.
Well, the answer is found in the second effect we want to consider here, which is drag, or air resistance. Air, or the atmosphere around us, can be thought of as an upward force of friction that acts against gravity; it slows down the rate at which an object will fall. If the structure or form of an object creates a lot of friction, as it would in our feather's case, then air resistance will slow it down very noticeably indeed. If an object's fall doesn't really create friction, as in the case of a brick, then air will have very little, if any, effect on it and so it will appear to fall faster.
However, if the feather and brick were dropped together in a vacuum--that is, an environment from which all air or cause of friction had been removed, but that was still subject to the law of gravity-- they would in fact fall at the same rate and hit the ground at the same time. Now, as the 3D environments in which our simulations will take place are, in reality, virtual vacuums with gravity enabled, we will need to use the tools available to re-create this atmospheric resistance, this drag. Again, this is going to be critical to the quality of our finished simulation.
Gravity and drag are two naturally occurring environmental effects that play a huge role in determining how objects move and react in the real world. We may, though, wonder about the objects themselves: Does the way that they are made or constructed affect their motion. Well, in our next video we will get an idea regarding the answer to that question as we take a look at volume, mass, and density.
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