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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.
If you've ever spent any time working with a photographic camera--be that film or digital--set in manual mode, you will probably become aware of the exposure triangle-- that is, the shutter speed, ISO, or film speed, and F-stop or F number values. These three settings are all interdependent, and any one of them can be used to alter the exposure, or level of light captured in a photograph. In fact, if you've spent any time lighting and rendering inside a 3D application or render engine that uses a physical camera model, you will also have encountered this triangle.
Now, we mention this because when it comes to creating believable dynamic simulations, we do come across a very similar triangle system, one that affects how objects will behave inside the simulation. These are the volume, mass, and density parameters of an object. These three measurements are interdependent, and so a change to any one of them will affect the makeup of the other two. They are all used by a physics simulation engine to determine how an object ought to behave whilst it is both at rest and in motion.
Now, the volume of an object is already known to your 3D application. It is based upon its size, or the amount of space that it occupies inside the 3D environment. If we want an object's volume to be accurately calculated in a MassFX simulation, we must create or model our geometry at real-world scale. This really is vital if we want realistic behavior from our simulated objects. Mass is a measurement that is often confused with weight, but the two are not, strictly speaking, the same thing, although it is true that here on Earth greater mass does oftentimes equate to a heavier object.
Mass is, in reality, a measurement of the amount of matter making up an object. This of course never changes, regardless of the environment that an object is placed in. So for example, an object with a mass of 1500 kg here on the earth still has a mass of 1500 kg if it is placed on the moon. The amount of matter making up the object never alters. Of course the change in environmental gravity will mean that the object would weigh less on the moon as weight is really a measurement of gravity at work on an object in a specific location.
In order for the simulator to know how much mass our objects have, we generally need to supply that information in a setting of either grams or kilograms. In MassFX, the Mass parameter is set using kilograms. Density is a measurement of an object's mass based on the volume or area of three-dimensional space that it is packed into. The greater the mass packed into a fixed volume, the denser a material is said to be. Now, this could also be expressed as, the smaller the volume a fixed mass is packed into, the denser the material is set to be.
Once our software knows the volume and mass of an object, it can automatically determine the correct density, as this is calculated through the formula of mass divided by volume. Naturally, once you have all of that information correctly piped into our system, our objects are going to stand a much better chance of behaving in a realistic manner when they're simulated. Now, whilst it is vital to understand both the environmental--such as gravity and drag--and object-specific properties-- such as volume, mass, and density that affect our simulations--they are naturally all the elements that we need to have an understanding of, if we're going to be able to fine-tune our simulations and make them thoroughly believable to our audiences.
These come in the form of Newton's three basic laws of motion, which we will be taking a brief look at in our next video.
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