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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.
One very cool feature that has been added to the MassFX toolset in recent updates is the ability of mCloth objects to have tears appear in them when certain forces and/or collisions are applied to them. Not only does this feature work very nicely in mCloth, but it is also very easy to use. The first step of course is to add an mCloth modifier. I am going to do this by holding down the Shift and Alt modifier keys and then right-clicking in the viewport. From the quad menu, I just want to select the Create mCloth command.
This of course applies an mCloth modifier to the geometry, but you will also notice that we are instantly switched into the Modifier tab of the Command panel. This means we can instantly begin to edit our cloth parameters. Before we go any further however, I just want to demonstrate the current scene setup that we're working with here. Let's come down to our animation controls and press Play. As you can see, our two upright poles begin to move away from one another, ultimately ending up at opposite edges of our stand geometry.
The idea of course is to attach our cloth to these uprights and create a tear as the cloth is pulled apart by the moving poles. The first thing we need to do then is attach our cloth to them. To do this we of course need to enter subobject mode on our mCloth modifier and make a selection. In this case I'm just going to marquee-select those two columns of vertices adjacent to our pole. Then we can use the Make Group button, give our selection an appropriate name, and then click OK.
As our poles are animated, we will need to make use of the node constraint here, so let's again click that button and then of course select the upright that we want to constrain our vertex selection to. When that is done, we need to repeat this process for the opposite side. Give our group an appropriate name. We want to use the node constraint, so let's click that option. With the cloth edges constraint we now need to create a tearable area on our cloth, a selection that lets MassFX know where the weak spot on the cloth object is supposed to be.
In this particular instance, I'm just going to select a few rows in the middle of our cloth. We could though place this selection anywhere we like. Now this time instead of clicking the Make Group button to create a regular constraint, we want to use the clearly marked Make Tear button. we will need to give our tear group a meaningful name. More often than not we will add multiple tears to a piece of geometry in order to get a realistic effect. Being able to quickly tell the various tear groups apart can be important. With those steps complete what we have essentially done is broken apart the vertices in our selection group and added a weld constraint back on top, so as to make it appear as if we're dealing with a single piece of cloth or geometry.
If we exit subobject mode now, we can add the last couple of tweaks to the modifier controls themselves. Firstly, in the Physical Fabric Properties rollout, we want to set our Stretchness value to 0.01. We do then of course need to scroll down to the Tearing rollout and tell the system that we want to make this piece of cloth tearable. With Allow Tearing enabled, let's make certain that our Tear Past value is set to 1.5. This tells the cloth how far it can stretch before any tearing occurs.
If we now set our simulation running, once the tear does occur, we can see that we are clearly getting a result that looks a little unnatural. Cloth very rarely will tear in perfect straight-edged strips or chunks as we have seen it do here. We really need to set up our cloth geometry a little differently. To show you what I mean let's first of all reset the simulation and then open up our 3ds Max Layer Manager. In here we want to select our Tearable Cloth Quads layer and then use the tools to make certain that all of the objects on that layer are selected.
Once they are, we can just hit the Delete key to get rid of them. We will probably want to also delete the layer itself. To add a new cloth object into the scene, we just need to unhide our Tearable Cloth GM layer. This layer contains a cloth object that instead of standing out life as a plain primitive, as that of our first cloth object, it started instead as a rectangular shape. This was a deliberate switch that allowed us to use a garment maker modifier to add detail to the cloth, instead of the standard geometry subdivisions that were being used by our plane.
Now before we can do anything with our new cloth object, we will of course need to add our mCloth modifier and re-create our node constraints. Let's add the modified from the MassFX toolbar and then coming across to the Command panel, we can enter subobject vertex mode. From here we want to again create a selection that we can constrain to our upright poles, so we'll just make a marquee selection of a couple of columns at the end of our cloth object, use the Make Group button, and again give our group an appropriate name. And then of course we can apply the note constraint and again select the appropriate upright.
Then of course repeat the process for the opposite side. Select those two columns of vertices, use the Make Group button, give our selection an appropriate name, and then click OK. We will of course need to also re-create our tear, so let's again select a chunk of vertices around about the center of our cloth geometry, click the Make Tear button, and give our group an appropriate name. With that done we are ready to exit subobject mode and then run the simulation again.
This time around, once our cloth does tear, we can see that it has a much more believable ragged edge to it now. As you can see then, creating tearable mCloth is a pretty straightforward process, one that can produce some very nice results if we take the time to make good use of the tools available to us.
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