Join Joel Bradley for an in-depth discussion in this video Understanding physics for better cloth sims, part of Blender: Cloth Simulation.
Although a complete understanding of real world physics is in no way a requirement for CG artists wanting to add convincing simulations to their work. It is true to say that a solid understanding of the basics, that is the influence that physics has on the motion of objects in the real world around us, can and probably will go a long way towards helping improve the quality of any Blender simulations that we produce. Gravity for instance is a force that in the real world, is constantly affecting the way in which cloth objects behave.
This force will cause any object to having mass to fall downward, notice toward the ground whenever it is suspended and then released in mid-air. Thinking of gravity in terms of cloth objects, this downward force, which operates at a fixed, unchanging rate of 9.81 meters per second squared is what creates the hangar draping of cloth, once it's in its pinned to or draped over another object. It also contributes to any motion seen in cloth object. Again, whether they be attached to another object, or are free floating.
This, of course, means that gravity, unless we are creating a very unique or stylized simulation, should really effect all cloth objects in our seams at all times. The good news for Blender users is that the setup of this is taken care of automatically whenever it is that we add any of the simulation modifiers to an object including of course the cloth modifier. The settings for Blender's gravity can be checked and altered from inside the same tab, down here in the gravity section.
As you can see the world zed or z-axis is currently set to use a real world value of negative 9.81 meters per second squared. As noted though this can be changed should we want to produce a non-physical result in our simulation. Gravity, though, is not the only real world force at work when it comes to affecting both cloth motion and behavior. Drag, or air resistance, will most certainly need to be factored into the equation if, that is, we are wanting to create anything like realistic looking cloth.
The atmosphere or air in which we live can actually be thought of as a force that in a small way acts against gravity and slowing down or restricting the rate at which an object is allowed to move or fall. If the structure or form of an object creates a lot of air friction as is the case with most cloth objects then air resistance will slow it down in a very noticeable manner. The classic example of a parachute showing just how strong this force can be.
Again, Blender helps us mimic this natural phenomenon, by this time, adding an air parameter to the cloth modifier's general controls. Volume, mass and density are three physical aspects of a real world object that will also affect how it behaves when acted upon by external forces, such as those mentioned. Again, each of these physical parameters will need to be taken into account in some way on cloth objects, in order for them to behave in an expected manner when simulated. The volume of any piece of geometry inside Blender is, of course, already known to the 3D application as it is based upon the object's size or the amount of space that it occupies inside the 3D environment.
Obviously then, no specific volume control is required for a cloth object. But, if we want the object's volume to be accurately calculated inside of Blender simulation, then we will need to model it at real world scale and give it real world thickness. This is a vitally important point to keep in mind whenever we want realistic behavior from simulated objects in Blender. That having been said, however, cloth objects are often times created as infinitely thin planes and so obviously have no real volume to them.
When using this type of geometry then, we will need to factor in the difference this could make with regard to the cloth motion being generated. Mass is a measurements that is oftentimes confused and used interchangeably with weight and even though the two are not, strictly speaking, the same thing. Mass is a measurement of the amount of matter present in the makeup of an object. This measurement never changes regardless of the environment in which an object is placed. Weight, on the other hand, is a measurement of an object's mass in a very specific environment.
In order for the blend of physics engine to know how much mass our scene objects have we need to supply the relevant information inside the cloth modifiers general controls using the mass parameter. Density is a measurement of an object mass based on the volume or area of 3 dimensional space into which that mass is packed. The greater the mass packed into a fixed volume, the denser a material is set to be. This, of course, could also be expressed as, the smaller the volume a fixed mass is packed into, the denser the material is set to be.
Once our software knows the volume and mass of an object, it can automatically determined the correct density. As this is calculated through the formula of mass divided by volume. And so again, no specific density control is required by Blender. Naturally and once we have all of this information correctly piped into our system, our objects are going to stand a much better chance of behaving or simulating in a realistic manner. In conclusion, then, once we have a clear understanding of how and why cloth in the real world looks and behaves as it does, we find ourselves in the position of being able to use real world reference in a very meaningful and easy to translate way.
Indeed, taking the time to study the behavior of real world cloth objects is something that I would highly recommend you do each and every time you come to work with the cloth simulation tools in Blender. With the importance of real world physics nicely taking care of then, let's move onto another pre-simulation choice that can affect the behavior and motion of cloth inside a simulation. This being the subdivision method that we apply to our cloth geometry.
- Understanding physics for better cloth simulations
- Simulating cloth
- Baking simulations
- Using Cloth presets
- Adjusting Quality and Material controls
- Creating and animating vertex groups
- Working with collisions