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Node Params > Node > Dynamics > Soft body

Soft bodies are deformable and elastic objects, for example cloth, rubber or jelly. This solver provides a wide variety of features, great stability and accurate results. Users of previous versions will quickly discover that there are absolutely new parameters available. Another new feature is that soft bodies can now keep the deformation they experience (“Plasticity”). With this approach it is possible to simulate bending metal, as can be observed after car crashes, for example. These sophisticated possibilities open up a whole new world of soft body dynamics and, of course, the new solver can interact with particles, rigid bodies and waves. Soft bodies also support "MultiJoints" – RealFlow’s “invisible” connection between dynamic objects.

Mass

A dynamic object always needs a certain amount of mass to become influenced by forces. In RealFlow, mass is always given in kilograms and the initial value is calculated automatically. Mass is one of the most important parameters with dynamic bodies, because it strongly influences the object’s entire movement and behaviour. To move an object, a force is needed. Inside RealFlow forces can be introduced with daemons or other objects (also particles) hitting the body. For each object, whether it is native or imported from external sources, RealFlow automatically calculates its mass. Please bear in mind that a body's acceleration does not depend on its mass. In other words: in a vacuum, all objects will have exactly the same fall velocity regardless of the adjusted “Mass” value. If you want to create differences, please consider changing “Air friction”.

Resolution

Higher settings create a more detailed simulation of the soft body, but also need longer calculation time. The idea behind this parameter is to make the body react much more accurately with more deformations. All in all, “Resolution” strongly enhances a simulation’s realism. The default value is 64, but can be raised to a (virtually) infinite level.

Length stiffness

Formally, this is the length recovery constant relative to the object. A high value means that soft body offers a high resistance against changes in its longitudinal magnitudes. This parameter accepts values between 0.0 and 1,000.0. The principle behind stiffness parameters is not so easy to explain: through its mass and gravitational acceleration, a body has a certain self-weight. This weight always causes deformation to a soft body and the amount is measured when it rests on a horizontal plane. Stiffness affects this kind of self-loaded deformation and a value of 1.0 means that a body nearly maintains its original corresponding magnitudes under these conditions. One could say that a soft body becomes more rigid with higher stiffness settings (length or volume) and, thus, higher settings lead to more stability.

Volume stiffness

This is the volume recovery constant relative to the object. A high value means that the observed soft body offers a high resistance to changes of its original volume. Again, your input can be between 0.0 and 1,000.0.

Elasticity

To describe a soft body’s internal motion, “Elasticity” and “Internal Damping” are the critical factors. “Elasticity” can be seen as the amount of energy that is kept by the body when it collides or experiences the previously mentioned internal motions. It appears as the magnitude of bounces when the body collides and also as a visible “wobbling”, decreasing after a certain time. You can apply any value between 0.0 and 1.0. With 0.0, the body quickly loses all its energy and stops shivering/bouncing. A value of 1.0 results in a much longer tremble and stronger bounces.

Friction

“Friction” occurs between objects with rough surfaces. In nature, even the most even surfaces have a certain amount of roughness, causing friction. It decelerates moving objects and can even stop them completely. The value ranges between 0.0 and 1.0. When bodies collide, RealFlow takes the average friction of all involved nodes into account.

Air friction

“Air friction” might appear rather weak in daily life, but it is a very important parameter. It counteracts the soft body’s motion and high values can even stop it completely. The range goes from 0.0 to infinity. A slight amount of “Air friction” should always be added.

Internal Damping

A ductile body always shows a certain amount of internal motion, controlled by “Elasticity” and “Internal Damping”. With higher values a body loses its internal motion rather fast and stops “wobbling” after a short time. It will also experience smaller bounces. “Internal Damping” accepts settings between 0.0 and infinity: a value of 0.5 will stop the entire internal movement after 2 seconds, a value of 1, after 1 second, for example.

Autocollision

Since soft bodies can show a very high level of deformation, it is very likely that some of its parts collide among each other. Without this option enabled, these areas would interpenetrate and lead to more or less fuzzy results – “Autocollision” helps to avoid this behaviour. Please note that “Autocollision” can take much longer to simulate, especially with higher “Resolution” settings.

Plasticity

By default, this option is set to “No”, but when enabled it unlocks four related parameters to control the node’s ability to become permanently deformed. “Plasticity” means that the deformations of the body will not relax or recover, and the object remains in a distorted state.

@ threshold

This value depends on a body’s change of its initial length to produce a permanent deformation. The range goes from 0.0 to 1.0. So, a value of 0.5 means that permanent deformation will only happen when the body’s length change is at least 50% of its initial size.

@ acquired

Like “@ threshold”, this parameter is also between 0.0 and 1.0, representing a percentage value. A setting of 0.5 will keep 50% of the node’s deformation as permanent. The other half is able to relax and turn back to its initial state. Please keep in mind that these values are only approximations.

@ compression limit

To prevent a soft body from very high permanent compression, it is recommended to specify a certain limit. Permanent means that the body rests in this compressed state and the deformation is not reversible. Without such a limit, objects might become totally flat and that is not always wanted. The range lies between 0.0 and 1.0. For example, a value of 0.5 means that permanent deformations in one direction can compress the body approximately until the half of its original length in this direction.

@ expansion limit

This value works similar to “@ compression limit”, but is related to a body’s permanent expansion after its deformation. To parameter avoids unnatural changes in size. The range goes from 1.0 to 100.0. For example, a value of 2 means that permanent deformations in one direction can expand the body approximately until it has reached twice its original length in this direction.

@ Velocity

Sometimes you do not want a simulation to start from zero, but the objects should have some initial velocity. With this parameter you can assign such a behaviour by simply entering positive or negative values. Please keep in mind that “@ Velocity” directly determines the body’s trajectory. A value of [ 2,0,0 ], for example, creates a linear motion along the positive X axis. The unit is metres per second [m/s].

@ Rotation W

This parameter actually works the same way as “@ Velocity”. Instead of an initial velocity, you can add an initial rotation and it is mostly used to avoid a uniform look. You also have three values and each one is measured in degrees per second [deg/s]. Negative angles are also accepted.

Hires object

If you performed a simulation with a low resolution mesh, you can keep the results and transfer them to a high resolution object with this feature. To establish such a “projection”, you simply specify the desired node and everything will be carried out automatically. Of course, both objects should share the same shape and size to get reasonable results.

@ Update at frame

When set to “Yes”, this option updates the soft bodies with each frame in the viewport.

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