Understanding Physics MotorsSunday, June 25, 2017 3:15 AM
This page explains the important concepts behind using bone and region mesh motors dealing with Physics (Bend Physics, Rope Physics etc.) You will gain an understanding of what those parameters on the motors actually mean to aid you in tuning your physics based animations.
Physics, Springs, Animation!
There are a quite a number of bone and region mesh motors that employ the power of procedural physics to aid you in generating high quality animation. There are a number of important points to keep in mind when you first start to use such motors:
The Base Motion is the motion of the underlying parent bone driving the list of bones/regions that actually employ the physics motor. Because we are basing the animation of physics calculations, the actual animation of the parent bone will result in physically based reactions from your physics motor bones/regions. The faster the animation fo the parent bone, the more reaction/motion you should expect from the underlying physics motor bones/regions.
In other words, if you do not want the physics to become unstable, it is crucial to ensure that the Base Motion is relatively smooth and contains no popping.
Relationship between Stiffness vs Damping
The 2 very important concepts to understand when using such motors are Stiffness and Damping.
Stiffness: The material property which determines the frequency of the oscillations. It also functions like its name Stiffness. The stiffer the material, the less likely it is to bend. The lower the Stiffness, the looser the material appears to be.
Damping: The material property which determines how quickly the object loses energy. The higher the Damping, the faster energy is bled from the system. Systems with low damping will have oscillations that persist for a much longer time.
This brings forth a very important point: You should always consider tweaking Damping as well if you make a large change in Stiffness. Why is this the case? The reason is due to the concept of Simulation Stability. In general, really stiff systems are unstable unless you put in an appropriately higher value of damping.
In other words, the higher the stiffness, the higher the damping and vice versa. Keep this tip in mind to get stable, high quality physics-based animations.
Making a Floppy Tail
You can very easily make a floppy tail that bounces automatically over the course of your animation using 2 components: Bend Physics Motor and Rotate Cycle Motor.
In the example above, we install a Rotate Cycle Motor at the base of the tail. This is the motor that drives the overall tail motion. You should set the StartAngle and EndAngle to appropriate values for your animation. You can also play around with the Speed value to adjust how rapid this base motion will be.
Next, we install a Bend Physics Motor along the remaining bones chain of the tail. Press play on the animation to see your tail flop around using procedural physics!
If you desire even more motion, you can switch the physics mode to Motion on the Bend Physics Motor. Do remember to set appropriate Stiffness and Damping parameters in this mode. Higher values are recommended since there is less simulation stability during the Motion physics mode.
Simulating Flesh and Muscle Responses
The ability to key both stiffness and damping values over time is extremely powerful. One such application involves the simulation of flesh/muscle type responses.
When flesh/muscle tenses up, the material becomes more rigid and hence has a higher stiffness value. When it relaxes, the stiffness value drops as it becomes a lot looser. Hence, by simply varying the stifness and damping values over time, you can get very interesting muscle/flesh type behaviour in your animation.
For the Deform Physics Motor (associated with the region mesh), the typical thing to do is to first paint the interior region of the mesh as glue. This simulated the rigid bone of the mesh. The other points are free to move about under physics. Here is an example of how the region is painted:
Notice how the interior "bone" regions are painted white as glue. The amount to paint as glue is dependent on the type of material you are trying to simulate. If you think the object is composed mostly of flesh, then you will have a very narrow bone region and vice versa.
My physics animation moves very erratically, all over the place!
This is a classic case of the simulation blowing up, or the simulation becoming unstable. You can try increasing the amount of damping to improve simulation stability. The other option is to reduce your stiffness value.
I want less of an arc in my physics-based bends.
You can reduce the gravity value in the physics motor to make the arc less prominent. The other option is to increase the stiffness but this might affect the desired motion.
I switched from "stability" to "motion" for my bend physics and now things are going crazy!
Motion mode is biased towards more motion and hence more unstable is simulations. You will have to increase the damping value to obtain a more stable result.