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soft_body_particles

The state of a soft-body is the state of its particles, which are identified by their index in the body. Their positions and their velocities can be read (particle_position, particle_positions, particle_velocity, particle_velocities) and modified (set_particle_position, set_particle_velocity) at any time, one by one or all at once. The elements built from them (edges, cells, and boundary) can be read as well, e.g., in order to render the body with your own mesh.

A particle can also be pinned (set_particle_pinned). A pinned particle is kinematic: it is no longer affected by the forces nor by the contacts, and it will simply hold its position, or follow the kinematic target (set_particle_kinematic_target) or the velocity it is given. This is, e.g., how a piece of cloth is hung on a wall, or how a rope is dragged by the player. Releasing the particle gives it back its nominal mass and lets it keep its current velocity:

let soft_body = &mut world.soft_bodies[sheet_handle];
// Read the particles.
let position = soft_body.particle_position(0);
let velocity = soft_body.particle_velocity(0);
let positions: Vec<Vector> = soft_body.particle_positions().collect();
assert_eq!(positions.len(), soft_body.num_particles());
// Move a particle.
soft_body.set_particle_position(1, position + Vector::new(0.0, 0.1));
soft_body.set_particle_velocity(1, velocity);
// Pin (or release) a particle; a pinned particle can be driven like a kinematic body.
soft_body.set_particle_pinned(2, true);
soft_body.set_particle_kinematic_target(2, Vector::new(-3.5, 3.5));
// The elements: edges, cells and the boundary segments.
let num_edges = soft_body.edges().len();
let num_cells = soft_body.cells().len();
let boundary: &[[u32; 2]] = soft_body.boundary();
assert!(num_edges > 0 && num_cells > 0 && !boundary.is_empty());
warning

Setting the position of a particle explicitly teleports it: no contact is taken into account along the way, so a particle can be moved inside of another object this way. Whenever the motion must be seen by the contacts and by the friction (to drag a piece of cloth, for example), it is recommended to pin the particle and to give it a kinematic target instead.

Controlling a region kinematically​

A whole region of the body is controlled at once through a cluster covering it. Pinning the cluster (set_cluster_pinned) pins all of its particles, and its kinematic target (set_cluster_kinematic_target) moves them rigidly: each pinned particle is sent where the rest shape of the cluster places it at the target pose, with the matching velocity. The rest of the body is then simulated as usual, and drags behind the controlled region, e.g., the hand of a soft character carrying something:

// Pin every particle of the cluster, then move it along a path: the cluster behaves like a
// kinematic rigid part dragging the rest of the body.
let jelly = &mut world.soft_bodies[jelly_handle];
jelly.set_cluster_pinned(cluster, true);
jelly.set_cluster_kinematic_target(cluster, Pose::from_translation(Vector::new(3.0, 2.5)));
// Release it: the cluster is simulated again.
jelly.set_cluster_pinned(cluster, false);