简介

../../_images/physics_soft-body_introduction_hidden-text.jpg

A soft body cloth uncovering a text.

A soft body in general, is a simulation of a soft or rigid deformable object. It is useful for everything that tends to bend, deform, in reaction to forces like gravity or wind, or when colliding with other objects.

In Blender, this system is best for simple cloth objects and closed meshes e.g. for skin or rubber. There is dedicated Cloth Simulation physics that use a different solver, and is better for cloth.

This simulation is done by applying forces to the vertices or control points of the object. There are exterior forces like gravity or force fields and interior forces that hold the vertices together. This way you can simulate the shapes that an object would take on in reality if it had volume, was filled with something, and was acted on by real forces.

Soft bodies can interact with other objects through Collision. They can interact with themselves through Self-Collision.

The result of the soft body simulation can be converted to a static object. You can also bake edit the simulation, i.e. edit intermediate results and run the simulation from there.

Typical Scenarios for using Soft Bodies

../../_images/physics_soft-body_introduction_windcone.jpg

A wind cone. The cone is a soft body, as the suspension.

Soft bodies are well suited for:

  • Elastic objects with or without collision.
  • Flags, fabric reacting to forces.
  • Certain modeling tasks, like a cushion or a table cloth over an object.
  • Blender has another simulation system for clothing (see Clothes). But you can sometimes use soft bodies for certain parts of clothing, like wide sleeves.
  • Hair (as long as you minimize collision).
  • Animation of swinging ropes, chains and the like.

The following videos may give you some more ideas:

Creating a Soft Body

Soft body simulation works for all objects that have vertices or control points:

  • Meshes.
  • Curves.
  • Surfaces.
  • Lattices.

To activate the soft body simulation for an object:

  • In the Properties editor, go to the Physics tab (it is all the way on the right, and looks like a bouncing ball).
  • Activate the Soft Body button.

A lot of options appear. For a reference of all the settings see this page.

  • You start a soft body simulation with Alt-A.
  • You pause the simulation with Spacebar, continue with Alt-A.
  • You stop the simulation with Esc.

Soft Body Solver

Reference

Panel:Physics ‣ Soft Body Solver

The settings in the Soft Body Solver panel determine the accuracy of the simulation.

Step Size

Min Step
Minimum simulation steps per frame. Increase this value, if the soft body misses fast-moving collision objects.
Max Step
Maximum simulation steps per frame. Normally the number of simulation steps is set dynamically (with the Error Limit) but you have probably a good reason to change it.
Auto-Step
Use Velocities for automatic step sizes.

Error Limit

Rules the overall quality of the solution delivered. Default 0.1. The most critical setting that says how precise the solver should check for collisions. Start with a value that is 1/2 the average edge length. If there are visible errors, jitter, or over-exaggerated responses, decrease the value. The solver keeps track of how "bad" it is doing and the Error Limit causes the solver to do some "adaptive step sizing".

Helpers

Choke
Calms down (reduces the exit velocity of) a vertex or edge once it penetrates a collision mesh.
Fuzzy
Simulation is faster, but less accurate.

Diagnostics

Print Performance to Console
Prints on the console how the solver is doing.
Estimate Matrix
Estimate matrix. Split to COM, ROT, SCALE

Cache

Reference

Panel:Physics ‣ Soft Body Cache

Soft bodies and other physic simulations use a unified system for caching and baking. See Particle Cache for reference.

The results of the simulation are automatically cached to disk when the animation is played, so that the next time it runs, it can play again quickly by reading in the results from the disk. If you Bake the simulation the cache is protected and you will be asked when you are trying to change a setting that will make a recalculating necessary.

Tip

Beware of the Start and End settings

The simulation is only calculated for the frames in between the Start and End frames (Bake panel), even if you do not actually bake the simulation! So if you want a simulation longer than the default setting of 250 frames you have the change the End frame.

Caching

  • As animation is played, each physics system writes each frame to disk, between the simulation start and end frames. These files are stored in folders with prefix blendcache, next to the blend-file.
  • The cache is cleared automatically on changes. But not on all changes, so it may be necessary to free it manually, e.g. if you change a force field. Note that for the cache to fill up, one has to start playback before or on the frame that the simulation starts.
  • If you are not allowed to write to the required subdirectory caching will not take place.
  • The cache can be freed per physics system with a button in the panels, or with the Ctrl-B shortcut key to free it for all selected objects.
  • You may run into trouble if your blend-file path is very long and your operating system has a limit on the path length that is supported.

Baking

  • The system is protected against changes after baking.
  • The Bake result is cleared also with Ctrl-B for all selected objects or click on Free Bake for the current soft body system.
  • If the mesh changes the simulation is not calculated anew.

For render farms, it is best to bake all the physics systems, and then copy the blendcache to the render farm as well.

Interaction in Real-Time

To work with a soft body simulation, you will find it handy to use the Timeline editor. You can change between frames and the simulation will always be shown in the actual state. The option Continue Physics in the Playback menu of the Timeline editor lets you interact in real-time with the simulation, e.g. by moving collision objects or shaking a soft body object.

Tip

Continue Physics does not work while playing the animation with Alt-A. It will work only if you start the animation with the Play button of the Timeline editor.

You can then select the soft body object while running the simulation and Apply the modifier in the Modifiers tab of the Properties editor. This makes the deformation permanent.

Tips

  • Soft bodies work especially well if the objects have an even vertex distribution. You need enough vertices for good collisions. You change the deformation (the stiffness) if you add more vertices in a certain region (see the animation of Fig. A wind cone. The cone is a soft body, as the suspension.).
  • The calculation of collisions may take a long time. If something is not visible, why calculate it?
  • To speed up the collision calculation it is often useful to collide with an additional, simpler, invisible, somewhat larger object (see the example to Fig. A soft body cloth uncovering a text.).
  • Use soft bodies only where it makes sense. If you try to cover a body mesh with a tight piece of cloth and animate solely with soft body, you will have no success. Self-collision of soft body hair may be activated, but that is a path that you have to wander alone. We will deal with Collisions in detail later.
  • Try and use a Lattice or a Curve Guide soft body instead of the object itself. This may be magnitudes faster.