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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.
Once we've refined a simulation to the point where we are happy with the motion of our simulated objects, we would probably want--in fact, I would go as far as to say need--to capture the simulation as standard animation keyframe data. This process is known in MassFX is baking. Of course this is not just an arbitrary step in a production pipeline. There are a couple of very good and interconnected reasons as to why we would want, or even need, to run through the baking process, possibly several times, at this point.
The overriding reason is the need to test the quality, the look and feel of our simulated objects in full motion, as it were. You see, up to this point we've been relying on the speed of our computer and its ability to play back a live simulation in the 3ds Max viewport in order to gauge the motion we're getting from our simulated objects. The simple truth is though that not all computers can play back a MassFX simulation live in the viewport and give it to us at the actual frames-per- second setting that we will be rendering our work at.
In our particular case, we are currently only getting a live simulation playback of about 13-15 frames per second in our viewport. This of course is quite a bit short of the final render setting of 24 frames per second that we're using in this scene. This means that we're not seeing objects in the simulation moving at their actual speed. At this point then, we're really only guessing or estimating just what the final motion could possibly look like. Clearly, it would not be a good idea to commit our simulation to final render, which of course can conceivably take days to complete, with such a level of uncertainty hanging over the quality of our motion.
The second reason for baking our simulation is, well, in reality, one that actually makes our previous point somewhat redundant, in that we currently have no way of rendering out our simulation without actually baking it down to keyframe data first. The history-dependant nature of dynamic simulations would mean that any attempt to render a live simulation could only result in a much longer render time than necessary. For this reason, as we say, such a thing isn't actually possible in MassFX and 3ds Max.
All that having been said then, let's take a very quick at the controls available for baking out our simulation. As with other MassFX tools, these can't be accessed in a number of ways, but we will just focus on the Simulation Tools tab in our MassFX Tools dialog here. The first option available is entitled Bake All. When enabled this will run through the simulation and store transform keyframes for all dynamic objects inside the simulation, and that would include any mCloth objects that we have in there.
When the process is complete, dynamic objects are converted to kinematic ones, and an internal baked flag is applied to them basically telling the system that they can now be unbaked if that is what the user chooses. The next option, Bake Selected, is of course pretty much the same as Bake All, except the baking process is only applied to selected dynamic objects. So if we have ten dynamic rigid bodies in our scene and we select two and apply Bake Selected, then only those two will have keyframe data written for them.
The final two selections, Unbake All and Unbake Selected, do just as the name suggests: they undo or reverse the baking process, again, applying it to all or selected objects inside of the scene. A one very nice aspect of visibility to bake is MassFX is the fact that we can essentially stack up simulation effects in our scene. So, if for example we have a rather large and complex sequence of simulation events, we can break them down into smaller chucks, bake particular portions, and yet still have those newly created kinematic rigid bodies affect dynamic rigid bodies that are still live inside of the simulation.
Then result of course would be the requirements for collision calculations inside of the simulation would be greatly reduced, which can of course significantly speed up our ability to create complex simulation scenarios. Of course, the drawback would be that the newly created kinematic rigid bodies inside of the simulation could no longer themselves be affected by dynamic rigid bodies, and of course depending upon the type of simulation we're working with, that may or may not present a problem. In our case, of course we do most certainly want to create as much interaction as possible in our simulation, so we're just going to click the Bake All function here and create keyframes for every dynamic object in the simulation.
Once we've clicked the button, depending on the complexity of our simulation, 3ds Max may take quite a while to bake our keyframes out. Once it is done, however, if I just select what was one of our dynamic rigid bodies, you can see that it has been converted, or been set now, to a kinematic rigid body. In fact, with the object selected, if you just take a look down on the timeline, you can see that we now have a keyframe set for every frame of our animation. In fact, if I just hit play in our animation controls you can very plainly see that everything in the scene is now being controlled by keyframe or animation data.
So with the baking process complete, we're ready in our next video to move on to, well, really the next phase of producing our simulation, which would be to render out what we have, review the motion, and make note of any changes that may be required in order to just polish up the simulation before we would be ready to present it for final rendering.
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