Doc:Manual/Physics/Soft Bodies/Combination with armatures

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Manual - Blender 2.49

You can use goals to either pin certain parts of a mesh to a location, or restrict the amount of movement away from the defined goal. So you may for example use armatures to animate your object but use a Goal group to define areas that are flexible and that react to Soft Body deformation. Since a simulation can be set up to take into account mass and gravity, you can easily create animated objects with realistic physical behavior using the Soft Body solver.

You can also use one of the following methods to combine Soft Bodies, objects and armatures:

• Soft Body vertices can be made the vertex parent of an empty, this empty can be the target of an IK solver.
• Using Bone stretching you can allow a bone to follow the length change of a Soft Body.
• You can use constraints to copy animated parameters of a bone onto an object.
• You can use a hook to bind a Soft Body to an armature driven object.

[edit] Swing

Image 1a: The actuator of the Swing is a Soft Body.
Animation
Blend file

The Soft Body simulation will drive the free end of a pendulum. This movement is used to drive the seat of a swing. You could also use a similar simulation for a Bungee cord, if you make the cord less stiff.

• Add a plane. Rename this object to “Pendulum”.
• Change to Edit Mode.
• Make sure you are in top view and delete the left half of the plane, so that all that remains are two vertices connected by an edge.
• Select one vertex and make it the sole member of a vertex group (weight 1). This vertex will stay put. It is the “upper” vertex.
• Bring the “upper” vertex and the object center together, either by moving the vertices or the center.
• Tab out of Edit mode and enable Soft Body for the object.
• Use Goal, and select the vertex group of the “upper” vertex.
• Set G Stiff to 0.

This creates a rig where the “upper” vertex is pinned, but the other vertex (and therefore the edge) is fully affected by the simulation.

Image 1b: The Soft Body object.

When you scrub or play (AltA) your animation, the vertex associated with a goal stays put, but the other is free to be affected by the physics simulation. Since the edge is pretty stiff, and there is gravity and mass and thus inertia, the vertex at the other end swings back and forth, and will eventually come to a stop, even if you turn off all damping. If you don’t want it to stop, you could either increase the calculation accuracy, or use the RKCP solver, which is designed specifically for these types of simulations.

Image 1c: IK settings for the first bone.

The free vertex will become the IK target for an armature. Therefore we need to create a second vertex group.

• Change to Edit mode.
• Select the “lower” vertex.
• Press CtrlG → Add selected to new Group and name the vertex group “TargetGroup”.
• Change back to Object mode.
• Insert an Armature with a single bone.
• Ensure that the armature is selected and change to Edit mode.
• Move the tip of the bone to the “lower” vertex.
• Change to Pose mode and ensure that the bone is selected.
• Add an IK Solver to the bone (Editing context, Constraints panel, Add Constraint), use the object “Pendulum” and the vertex group “TargetGroup” as target.
• Set Stretch in the Armature Bones panel to 1.0.
• Lock Y Rot, so the bone doesn’t rotate around itself.

If you now play the animation, the bone swings back and forth. When the length of the object changes, the length of the bone also changes.

Image 1d: The armature is connected with the Soft Body by the IK Solver Target.

• Now extrude a second bone at an angle perpendicular to the first.
• Turn on S (Hinge Scale), so that the second bone isn’t scaled when the the first bone is.

Now you can parent an object, e.g. the seat of the swing, to this second bone.

Manual