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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.
Up to this point in our chapter, we have looked at how we apply an mCloth modifier to geometry; we have worked a little with constraints, specifically pinning; and we have taken an overview of the physical fabric properties that are so important when it comes to ensuring that our cloth behaves appropriately. What we need to look at now are mCloth's interaction controls. These determine how mCloth interacts with both itself and any rigid body objects it comes into contact with during the simulation.
First though, let's just take a quick look at what we have here in terms of scene setup. We have our cloth object, which is an editable poly with an mCloth modifier applied to it. No mCloth parameters have been altered as of yet, so what we see here are all default settings. We also have a number of static rigid body objects with which our cloth can collide. These have all had their physical shape set to Original. Now, if I just run the simulation without animation, we can see how the setup is working.
To get an idea then of how the interaction options available tools will work, let's, with our Cloth plane selected, go over to the Command panel and then open up our mCloth Interaction rollout. The first option we come across, the Self Collisions checkbox, is again hopefully fairly self-explanatory. With it on, our mCloth object will attempt to prevent any kind of self- intersection or self-penetration. Now despite what we may initially think, it may not always be necessary to have this option enabled.
When we are dealing with simple cloth motions, we may be able to save some processing power by leaving this option disabled. In fact, if we just run our simulation with Self Collisions enabled and then reset disable Self Collisions and run again, you can see we have no discernible difference in terms of the cloth motion we are getting. It is a good idea to lighten the calculation requirements for a simulation whenever we can, so maybe some quick tests with this option both on and off might be appropriate.
Just below the checkbox, we have a Self Thickness value. This determines, or sets, the thickness of our mCloth object in connection with self-collisions. If we find our cloth is intersecting with itself, we may want to try increasing this value just a little. We do need to be careful though, that things don't start to look unrealistic by pushing this value too high. Folded or crumpled cloth looks very strange indeed when the folded surfaces refuse to come into contact with one another, which is what can happen if we push this value too much.
We next come to a range of controls that will all affect how our mCloth object interacts with rigid bodies in the simulation. The first option again is a simple on/off checkbox, Collide to Rigid Objects. With this on, our mCloth object can interact with rigid bodies; with it off of course, it cannot. Let's just disable this and run the simulation to show you the effect this has. As you would expect, our cloth now completely ignores the rigid bodies inside the simulation.
Just below the checkbox, we again have a Thickness value. This really determines how far a cloth vertex needs to be from a rigid body object before a collision is counted. Again, we need to be careful with this value, as we can make things look unrealistic. In fact, if we just set this to a very high value of 20 and then run the simulation, you can see how a collision is detected long before our cloth object gets visually anywhere near the rigid body objects. Before we move on of course, I do want to undo that thickness change.
Our next two options determine the extent to which our mCloth objects can affect rigid bodies inside the simulation. The Push Rigid Objects checkbox enables or disables our mCloth objects' ability to not only collide with, but also effect or push rigid bodies inside the simulation. If we want our rigid bodies to collide with, but not be affected by, the mCloth object then we can just simply turn this option off. The Push value determines the strength or force that our mCloth object can apply to the rigid bodies that it collides with.
The Attach to Colliders option gives us the ability to create a very interesting effect with our mCloth objects. Essentially, we can attach or stick our mCloth to any rigid bodies it comes into contact with inside the simulation. If I just use the P key to switch my viewport over to a perspective view and then just reposition our cloth object using the Move tool and using the middle-mouse button, reposition our view, if we now run the simulation, you can see once our cloth object comes into contact with the stand, after a moment or two, it simply slides off.
However, if we reset the simulation and then enable Attach to Colliders, this time, rather than sliding off, our mCloth object now sticks or attaches to the rigid body. The Influence value will determine how much of an effect our mCloth object has on any rigid bodies that it attaches to, whilst the Detach Past option will determine how far the cloth can stretch before it actually detaches from the rigid body. Clearly, the settings in our interaction controls will play a big part in determining how mCloth objects in the simulation behave.
Some of them of course are and probably should be subtle in their effect. All this, such as the Attach to Colliders option, can dramatically and very visually alter the behavior of our mCloth objects. In our next video, we're going to see how we can tackle the common production requirement of attaching or fixing simulated cloth to an animated object. This means we will be going back and revisiting mCloth constraint options with this particular need in mind.
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