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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.
Having run through a refining pass on our launchers and ironed out a number of simulation problems there, it's time now to step through the rest of our rigid body simulation and take care of any further issues that we maybe running into. The first thing we would probably want to do of course is run our simulation once again, just to get a clear idea of the issues that may exist. If we just go into MassFX toolbar, we can hit the Start Simulation button and as we examine the simulation, you can see, well, that there does appear to be quite a number of issues that we will need to deal with.
Our stacks of cans for instance are dropping straight through the stand, which as a static rigid body ought to be colliding with, or more accurately, catching them. It is also clear that our launch spheres are not in actual fact getting anywhere near they cans themselves. In fact, it looks as though they are running into some kind of invisible force field. Finally, if we look carefully at the cans upon the shelves, we can see that they too appear to be behaving in a very odd manner indeed.
In truth, these noted behaviors are in fact symptoms of the same basic problem. If we just select our standard geometry and come over to the Modifier options, you can see, if we come down to the Physical Shapes rollout, that we have mistakenly left this option set to Convex when we're clearly dealing with a concave object. If you remember, with static rigid bodies that have a concave graphical mesh we need to actually switch our collision shape over to using the Original option, so let's do that.
Our shelves, too, if I just select one, although having had a physical mesh type of box automatically set, which would seem entirely appropriate, do also need switching over to a different physical shape type. In this instance Convex will work nicely for us. Indeed, if we just run the simulation once more, we can see that things are definitely improved. Whenever we run into misbehaving dynamic rigid bodies in a simulation it is always a good idea to check our Physical Shape options, maybe even experiment with them a little bit.
We still of course have a number of other issues that we need to tackle before we can say that we're happy with this simulation. In fact, what we really do need at this point to take a close look of what is going on with our stacks of cans. To move in a little closer, let's hit C key on the keyboard and switch over to our Target CloseUp Camera. If we now run the simulation from this particular viewpoint, we can clearly see that our stacks of cans are simply collapsing of their own accord. Now, this unwanted behavior is caused by the initial drop that the objects are making once the simulation starts and the rigid bodies come under the influence of gravity.
To solve this problem we can again make use of some MassFX global controls. Before we do that, however, we may want to just correctly set the physical properties for our cans, as this is certainly going to help them behave in a more realistic manner. So let's select one of our cans and come over to the Command panel. In the Physical Material Properties rollout you'll notice that the Mass for this object has been automatically set somewhat higher than an empty tin can ought to be. Really, we should have a mass of about 75-90 gm or so applied here.
Now in MassFX, the mass values for rigid bodies are measured in kilograms, so for 75 grams we should have a setting here of 0.075. You'll notice that as I apply the setting our object's Density value is automatically calculated and updated for us. We do also need to set the physical Ss down to Shapes rollout, you can see we are currently using the Capsule option, which given the shape of our geometry, is not nearly accurate and not for what we want to accomplish.
So let's switch this over to Convex. If we run the simulation now, we can see our stacks of cans do still collapse, but they do so much less violently. For the final part of the solution to our collapsing stacks, we needs to work a little bit in the MassFX Tools dialog. In the World Parameters tab, let's first of all copy the existing earth gravity value to the clipboard, so let's just select it and use Ctrl+C. Then we want to reset this value to -0.1, so a much reduced gravity effect.
Now we need to select all of the cans in our stacks, so let's open up 3ds Max's Layer Manager and using the tools in here, we can make certain that all of those objects are indeed selected. What we want to do now is run the simulation, but this time without animation, and we just want to do this until our cans touch down on the stand and on one another, and when they do, we want to quickly stop the simulation. Making certain that the cans remain selected in the Simulation Tools tab of the Tools dialog, we want to click the Capture Transform button and set the current location of our cans as the starting position in the simulation.
Again, with the object still selected, let's go into the Multi-Object Editor and set our cans to Start Simulation in Sleep mode. This tells our dynamic rigid bodies to ignore everything in the simulation, such as gravity, until another rigid body collides with them. Do remember of course we need to paste the correct value for gravity back in before we run the simulation again, so let's do that. Now we can switch over to our main camera view and then again run the simulation. And as you can see, things appear to be working exactly as we would like them.
So with the refinement phases having seemingly yielded good results, we need to now check just what this simulation will look like when it is played back at its final render speed. Before we can do that, however, we are going to have to create animation keyframe data for all of our currently simulated objects; this means taking a look at MassFX's Simulation Baking options.
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