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Advanced collision-detection

Collision-detection is a two-steps process. First the BroadPhase detects pairs of colliders that are potentially in contact or intersecting. Second, the NarrowPhase processes all these pairs in order to compute contacts points and generate collision events. Based on these points, the constraints solver computes forces that may generate contact force events.

All the pairs detected by the broad-phase are stored into two graph structures:

  • The contact graph stores all the potential contact pairs (between two non-sensor colliders) as well as the contact points generated by the narrow-phase.
  • The intersection graph stores all the potential intersection pairs (between a sensor collider and another collider) as well as the result of the boolean intersection test executed by the narrow-phase.

These two graphs are part of the narrow-phase of the R3World and are automatically updated by r3Step or r3DetectCollisions. Each node of these graph contains a R3ColliderHandle and there is one graph edge per pair detected by the broad-phase.

Collision and contact force events​

The narrow-phase can generate collision events between two colliders. Each collision event is given optional flags:

  • R3_COLLISION_EVENT_SENSOR is set if at least one of the colliders involved in the collision is a sensor.
  • R3_COLLISION_EVENT_REMOVED is set if a collision stopped because at least one of the colliders involved in the collision was removed from the physics scene.

In addition, after forces are computed by the constraints solver, contact force events may be generated between two colliders subject to non-zero contact forces. Generally, the user isn’t interested in contact force events unless the force magnitudes exceed some threshold. In order to skip low-force events, the engine will compute the sum of the magnitude of all the contacts between the two colliders and only trigger a contact force event if that magnitude is larger than the threshold set with the contactForceEventThreshold field of R3ColliderDesc or r3Collider_SetContactForceEventThreshold (defaults to 0) for any of the two colliders with the R3_CONTACT_FORCE_EVENTS flag enabled.

warning

Collision events (resp. contact force events) are only generated between two colliders if at least one of them has the R3_COLLISION_EVENTS flag (resp. R3_CONTACT_FORCE_EVENTS flags) in its active events.

These events are collected by an R3EventCollector, created by r3NewEventCollector and freed by r3FreeEventCollector. It is given to r3Step (or r3DetectCollisions), which adds to it the collision events, the contact force events, and the tear events of the soft-bodies generated during that step. The collision events and contact force events are copied into buffers given by the application with r3EventCollector_CollisionEvents and r3EventCollector_ContactForceEvents: calling them with a NULL buffer and a zero capacity gives the number of events, then a second call with a buffer large enough copies them. The tear events are read one by one with r3EventCollector_TearEvent, which returns a copy that must be freed with r3FreeSoftBodyTearEvent. Note that reading the events doesn't remove them from the collector: they accumulate from one step to the next until r3EventCollector_Clear is called:

// Initialize the event collector.
R2EventCollector *events = r2NewEventCollector();

r2Step(world, NULL, events);

// Get the number of collision events, then copy them.
size_t count = r2EventCollector_CollisionEvents(events, NULL, 0);
R2CollisionEvent *collision_events = malloc(count * sizeof(*collision_events));
count = r2EventCollector_CollisionEvents(events, collision_events, count);
for (size_t i = 0; i < count; i++) {
    // Handle the collision event.
    R2CollisionEvent event = collision_events[i];
    printf("Received collision event: colliders %u and %u, started: %u, flags: %u\n",
           event.collider1.index, event.collider2.index, event.started, event.flags);
}
free(collision_events);

count = r2EventCollector_ContactForceEvents(events, NULL, 0);
R2ContactForceEvent *contact_force_events = malloc(count * sizeof(*contact_force_events));
count = r2EventCollector_ContactForceEvents(events, contact_force_events, count);
for (size_t i = 0; i < count; i++) {
    // Handle the contact force event.
    R2ContactForceEvent event = contact_force_events[i];
    printf("Received contact force event: colliders %u and %u, force magnitude: %f\n",
           event.collider1.index, event.collider2.index, (double)event.total_force_magnitude);
}
free(contact_force_events);

count = r2EventCollector_TearEventCount(events);
for (size_t i = 0; i < count; i++) {
    // Handle the soft-body tear event. It is a copy that must be freed.
    R2SoftBodyTearEvent *tear_event = r2EventCollector_TearEvent(events, i);
    printf("Received soft-body tear event: soft-body %u\n",
           r2SoftBodyTearEvent_SoftBody(tear_event).index);
    r2FreeSoftBodyTearEvent(tear_event);
}

// The events accumulate until the collector is cleared.
r2EventCollector_Clear(events);

Each R3CollisionEvent indicates whether the collision started or stopped, and its flags combine the R3_COLLISION_EVENT_SENSOR and R3_COLLISION_EVENT_REMOVED bits described above. In addition to the force-related fields, the started field of an R3ContactForceEvent indicates whether this is the first step during which the contact force exceeds the threshold (it is 0 during the next steps as long as the force remains above that threshold).

These events only identify the colliders involved. The contact geometry can be read from the contact graph afterwards, but there are some cases when the contact information is no longer available at the end of the timestep (e.g. when running multi-step CCD and the contact start during one substep and stops at a substep right after). If you need to access the contact information at the exact time a contact event happens, you may give an R3EventCallbacks to the event collector with r3EventCollector_SetCallbacks (taking effect from the next step). Its callbacks are called during the step with each event, right after it was added to the collector: they complement (and don't replace) the collection of the events. The collision event callback is also given the geometric contacts of the pair at that time (in the collider order of the event, and none for a sensor), which are only valid during that call. Just like the physics hooks, they only have a read-only access to the world, and must be thread-safe if the library was built with parallelism enabled.

info

Collision events identify the involved colliders by their handle. It is possible to retrieve the handle of the rigid-body a collider is attached to: r3Collider_Parent(collider_handle).

The contact graph​

The contact graph can be read in order to determine whether two specific non-sensor colliders are in contact, or to determine all the non-sensor colliders in contact with one particular non-sensor collider. Contact points and contact normals will also be provided when a contact exists.

The contact geometry (contact points, contact normal, penetration depth, etc.) can be read from the contact manifolds stored in a contact pair:

  1. Each contact pair may contain multiple contact manifolds. Each contact manifold represents a set of contacts sharing the same contact normal.
  2. Each contact manifold contains the list of geometric contacts detected by the narrow-phase.
  3. Each contact manifold also contains a list of contacts that were processed by the constraints solver for force calculation (aka. the solver contacts). These solver contacts are a subset of the contacts detected by the narrow-phase, expressed in a way that is more efficient for the constraints solver to process. These solver contacts can be modified or deleted by the user using contact modification.

All the geometric contact data are expressed in the local-space of the colliders. The solver contacts hold one anchor per body surface, expressed in the local-space of the body that surface belongs to (so they ride rigidly with it); r3SolverContacts resolves them back to world-space through the bodies' current poses. Inside a contact-modification hook they are world-space instead, since the hook runs before they are localized.

info

Because the solver contacts can be modified by the user, they are transients by nature: they are recomputed at each frame from the geometric contacts. Because of their transient nature, the constraint solver will store the forces it computes inside of the geometric contacts (the impulse field of R3ContactPoint) instead of the solver contacts themselves.

Keep in mind that the contact graph contains one graph edge per pair detected by the broad-phase. So the fact that a contact pair can be found in the graph doesn't mean that the corresponding colliders are actually in contact (they may just be very close to one another, without touching). It is necessary to check either:

  • the has_any_active_contact field of the R3ContactPair if you need to know if there exist at least one solver contact between the colliders.
  • the length of the geometric contacts (num_points) for each manifold in the result of r3ContactManifolds to determine if the colliders are really geometrically touching (independently from contact-modification).
info

There will always be only up to one contact manifold between two colliders with convex primitive shapes. If one collider has a shape composed of several pieces (trimesh, polyline, heightfield, or compound shape) then there will be multiple contact manifolds, one for each piece that may result in an actual contact.

The contact pair between two colliders is given by r3TryContactPair, which sets the found field of its result to 0 if the pair doesn't exist (whereas r3ContactPair reports it as the R3_NOT_FOUND error). The contact pairs involving one particular collider are given by r3Collider_ContactPairs, and all the contact pairs of the world by r3ContactPairs. In addition to has_any_active_contact, an R3ContactPair gives a summary of the contact impulses applied between the two colliders during the last step (total_impulse, max_impulse, etc.) The contact manifolds of a pair are given by r3ContactManifolds, their geometric contacts by r3ContactPoints (each tagged with the manifold_index of its manifold), and the solver contacts of one manifold by r3SolverContacts:

/* Find the contact pair, if it exists, between two colliders. */
R2OptionalContactPair contact_pair = r2TryContactPair(collider_handle1, collider_handle2);
if (contact_pair.found) {
    // The contact pair exists meaning that the broad-phase identified a potential contact.
    if (contact_pair.pair.has_any_active_contact) {
        // The contact pair has active contacts, meaning that it
        // contains contacts for which contact forces were computed.
    }

    // We may also read the contact manifolds to access the contact geometry.
    size_t num_manifolds = r2ContactManifolds(collider_handle1, collider_handle2, NULL, 0);
    R2ContactManifold *manifolds = malloc(num_manifolds * sizeof(*manifolds));
    num_manifolds = r2ContactManifolds(collider_handle1, collider_handle2, manifolds, num_manifolds);
    // The geometric contacts of all the manifolds, each with the index of its manifold.
    size_t num_points = r2ContactPoints(collider_handle1, collider_handle2, NULL, 0);
    R2ContactPoint *points = malloc(num_points * sizeof(*points));
    num_points = r2ContactPoints(collider_handle1, collider_handle2, points, num_points);

    for (size_t i = 0; i < num_manifolds; i++) {
        R2ContactManifold manifold = manifolds[i];
        printf("Local-space contact normal: (%f, %f)\n", (double)manifold.local_n1.x, (double)manifold.local_n1.y);
        printf("Local-space contact normal: (%f, %f)\n", (double)manifold.local_n2.x, (double)manifold.local_n2.y);
        printf("World-space contact normal: (%f, %f)\n", (double)manifold.normal.x, (double)manifold.normal.y);

        // Read the geometric contacts.
        for (size_t j = 0; j < num_points; j++) {
            if (points[j].manifold_index != i) {
                continue;
            }
            // Keep in mind that all the geometric contact data are expressed in the local-space of the colliders.
            R2ContactPoint contact_point = points[j];
            printf("Found local contact point 1: (%f, %f)\n", (double)contact_point.local_p1.x,
                   (double)contact_point.local_p1.y);
            printf("Found contact distance: %f\n", (double)contact_point.distance); // Negative if there is a penetration.
            printf("Found contact impulse: %f\n", (double)contact_point.impulse);
            printf("Found friction impulse: %f\n", (double)contact_point.tangent_impulse[0]);
        }

        // Read the solver contacts.
        size_t num_solver_contacts = r2SolverContacts(collider_handle1, collider_handle2, i, NULL, 0);
        R2SolverContact *solver_contacts = malloc(num_solver_contacts * sizeof(*solver_contacts));
        num_solver_contacts =
            r2SolverContacts(collider_handle1, collider_handle2, i, solver_contacts, num_solver_contacts);
        for (size_t j = 0; j < num_solver_contacts; j++) {
            // Solver contacts are anchored in the local-space of the body they touch, so
            // they ride rigidly with it. `r2SolverContacts` resolves them through the bodies'
            // current poses to give the world-space contact point on each body's surface.
            R2SolverContact solver_contact = solver_contacts[j];
            printf("Found solver contact points: (%f, %f), (%f, %f)\n", (double)solver_contact.point1.x,
                   (double)solver_contact.point1.y, (double)solver_contact.point2.x,
                   (double)solver_contact.point2.y);
            // The solver contact distance is negative if there is a penetration.
            printf("Found solver contact distance: %f\n", (double)solver_contact.distance);
        }
        free(solver_contacts);
    }
    free(points);
    free(manifolds);
}
/* Iterate through all the contact pairs involving a specific collider. */
size_t num_pairs = r2Collider_ContactPairs(collider_handle1, NULL, 0);
R2ContactPair *pairs = malloc(num_pairs * sizeof(*pairs));
num_pairs = r2Collider_ContactPairs(collider_handle1, pairs, num_pairs);
for (size_t i = 0; i < num_pairs; i++) {
    R2ColliderHandle other_collider =
        same_collider(pairs[i].collider1, collider_handle1) ? pairs[i].collider2 : pairs[i].collider1;

    // Process the contact pair in a way similar to what we did in
    // the previous example.
    (void)other_collider;
}
free(pairs);

Finally, keep in mind that the contacts and contact manifolds field names frequently end with a digit 1 or 2. For example the local_n1 and local_n2 fields of an R3ContactManifold. Fields ending with the digit 1 relate to the collider identified by contact_pair.collider1. Fields ending with the digit 2 relate to the collider identified by contact_pair.collider2.

In other words local_n1 is the contact normal expressed in the local space of the collider collider_pair.collider1, it points towards the exterior of the shape of collider_pair.collider1. On the other hand, local_n2 is expressed in the local space of the collider collider_pair.collider2 and points towards the exterior of the shape of collider_pair.collider2.

warning

The contact pair returned by r3TryContactPair(handle1, handle2) does not necessarily have contact_pair.collider1 == handle1 && contact_pair.collider2 == handle2. It could be swapped: contact_pair.collider1 == handle2 && contact_pair.collider2 == handle1.

So keep that in mind when reading the contact information because it's contact_pair.collider1 and contact_pair.collider2 that determine to what collider the digits 1 and 2 relate in the contacts and contact manifolds fields. The same applies to the contacts given by r3ContactManifolds(handle1, handle2), r3ContactPoints(handle1, handle2), and r3SolverContacts(handle1, handle2): compare contact_pair.collider1 with your handles (i.e. their index and generation fields) to know which one is the first collider.

The intersection graph​

The intersection graph can be read in order to determine whether two specific colliders (assuming at least one of them is a sensor) are intersecting, or to determine all the colliders intersecting one particular collider (assuming at least one collider of each pair is a sensor). The intersection graph contains one graph edge for each pair of colliders such that:

  1. At least one of the collider is a sensor.
  2. And they are close enough so the broad-phase considers they have a chance to be intersecting.

Each such edge contains one boolean indicating if the colliders are actually intersecting or not:

The intersection pair between two colliders is given by r3TryIntersectionPair (r3IntersectionPair reports the R3_NOT_FOUND error if that pair doesn't exist), the intersection pairs involving one particular collider by r3Collider_IntersectionPairs, and all the intersection pairs of the world by r3IntersectionPairs. The boolean of each edge is the intersecting field of the R3IntersectionPair. Unlike the contact pairs, the pair given by r3TryIntersectionPair and r3IntersectionPair keeps the order of their arguments:

/* Find the intersection pair, if it exists, between two colliders. */
R2OptionalIntersectionPair intersection_pair = r2TryIntersectionPair(collider_handle1, collider_handle2);
if (intersection_pair.found && intersection_pair.pair.intersecting) {
    printf("The colliders %u and %u are intersecting!\n", collider_handle1.index, collider_handle2.index);
}
/* Iterate through all the intersection pairs involving a specific collider. */
size_t num_intersections = r2Collider_IntersectionPairs(collider_handle1, NULL, 0);
R2IntersectionPair *intersections = malloc(num_intersections * sizeof(*intersections));
num_intersections = r2Collider_IntersectionPairs(collider_handle1, intersections, num_intersections);
for (size_t i = 0; i < num_intersections; i++) {
    if (intersections[i].intersecting) {
        printf("The colliders %u and %u are intersecting!\n", intersections[i].collider1.index,
               intersections[i].collider2.index);
    }
}
free(intersections);
warning

Keep in mind that intersection tests are performed between two colliders only if at least one of the colliders is a sensor. If they are both non-sensor colliders then they will be involved in the contact graph instead of the intersection graph.

Physics hooks​

Physics hooks are user-defined callbacks used to change the behavior of the physics simulation. In particular, they can be used to filter contacts (in a more flexible way than collision groups and solver groups) and to modify contacts before they are processed by the constraints solver.

Physics hooks are given as an argument of r3Step and r3DetectCollisions. They are grouped into the R3PhysicsHooks structure, which contains one callback per kind of hook, as well as a user_data pointer given as the first argument of each of these callbacks. A NULL callback keeps the default behavior of Rapier, so a zero-initialized R3PhysicsHooks doesn't change anything.

The callbacks are called during the step, possibly from several threads at once if the library was built with parallelism enabled (in which case the callbacks and their user_data must be thread-safe). They are given an R3ReadContext which gives a read-only access to the rigid-bodies and colliders (through the r3ReadRigidBody_* and r3ReadCollider_* functions): any other access to the world being stepped reports the R3_WORLD_BUSY error, so any modification of the world must be performed after the step. Finally, a callback must not keep its arguments after it returns.

info

If no physics hooks are needed by your simulation, it is possible to use NULL as the physics hooks argument of r3Step.

Contact and intersection filtering​

Sometimes, collision groups and solver groups are not flexible enough to achieve the desired behavior. In that case, the contact filtering hooks let you apply custom rules to filter contact pairs and intersection pairs:

  • For each potential contact pair (between two non-sensor colliders) detected by the broad-phase, if at least one of the colliders involved in the pair has the bit R3_FILTER_CONTACT_PAIRS enabled in its active hooks, then the filter_contact_pair callback will be called. If it returns -1 then no contact computation will happen for this pair of colliders. Otherwise the narrow-phase will compute contact points.
  • For each potential intersection pair (between a sensor colliders and another collider) detected by the broad-phase, if at least one of the colliders involved in the pair has the bit R3_FILTER_INTERSECTION_PAIR enabled in its active hooks, then the filter_intersection_pair callback will be called. If it returns 0 then no intersection computation will happen for this pair of colliders. If it returns a positive value then the narrow-phase will test whether or not they are intersecting.

When filter_contact_pair doesn't return -1, its return value also indicates what happen with the contacts of this contact pair afterwards:

  • If it returns 1, then the constraints solver will compute forces for these contacts.
  • If it returns 0, then the contact points are computed, but no contact force will be computed for this pair of colliders.
// This is a silly example of contact pair filter that:
// - Enables contact and force computation if both colliders have even user-data.
// - Enables contact computation but not force computation if both colliders have equal user-data.
// - Disables contact computation otherwise.
static int32_t RAPIER_CALL filter_contact_pair(void *user_data, const R2ReadContext *read,
                                               R2ColliderHandle collider1, R2ColliderHandle collider2,
                                               R2RigidBodyHandle body1, R2RigidBodyHandle body2) {
    (void)user_data;
    (void)body1;
    (void)body2;
    uint64_t user_data1 = r2ReadCollider_UserData(read, collider1).low;
    uint64_t user_data2 = r2ReadCollider_UserData(read, collider2).low;

    if (user_data1 % 2 == 0 && user_data2 % 2 == 0) {
        return 1; // Compute the contacts and the contact forces.
    } else if (user_data1 == user_data2) {
        return 0; // Compute the contacts, but not the contact forces.
    } else {
        return -1; // Don't compute any contact.
    }
}

// This is a silly example of intersection pair filter that
// enables the intersection test if both colliders have odd
// user-data.
static int32_t RAPIER_CALL filter_intersection_pair(void *user_data, const R2ReadContext *read,
                                                    R2ColliderHandle collider1, R2ColliderHandle collider2,
                                                    R2RigidBodyHandle body1, R2RigidBodyHandle body2) {
    (void)user_data;
    (void)body1;
    (void)body2;
    uint64_t user_data1 = r2ReadCollider_UserData(read, collider1).low;
    uint64_t user_data2 = r2ReadCollider_UserData(read, collider2).low;

    return user_data1 % 2 == 1 && user_data2 % 2 == 1;
}

static void step_with_pair_filters(R2World *world) {
    // NULL callbacks keep the default behavior.
    R2PhysicsHooks hooks = {0};
    hooks.user_data = NULL; // Given to every callback as its first argument.
    hooks.filter_contact_pair = filter_contact_pair;
    hooks.filter_intersection_pair = filter_intersection_pair;

    r2Step(world, &hooks, NULL);
}

Keep in mind that these filters don't replace the built-in filtering of Rapier: they are only called for the pairs that passed it. The pairs of colliders attached to the same rigid-body, or to rigid-bodies linked by a joint with contacts disabled, are discarded first, then the pairs rejected by the active collision types of both colliders (e.g. between two non-dynamic rigid-bodies by default), then the pairs rejected by their collision groups. The solver groups are applied to the result of filter_contact_pair afterwards.

Contact modification​

It is possible to modify contacts after they have been computed by the narrow-phase. Contact-modification can have multiple advanced usages, for example:

  • The simulation of conveyor belts by modifying the tangent_velocity of solver contacts.
  • The simulation of one-way-platforms by deleting some contacts depending on the contact normal.
  • The simulation of colliders whose friction or restitution depends on where they are touched, by setting the coefficients from the contact points' location.

The modify_solver_contacts and modify_solver_contacts_context callbacks are called on each contact manifold between two colliders where at least one of them has the R3_MODIFY_SOLVER_CONTACTS flag enabled in its active hooks.

warning

Contact modification can be used to remove some (or all) solver contacts from a contact manifold. However, it cannot be used to add new contacts manually. If this is something that could useful to you, please consider opening an issue to let us know about your use-case so we can see if this is worth adding.

Contact-modification lets you change most characteristics of a contact: the contact normal, the contact points and their penetration depth, and the tangent velocity. The friction and restitution coefficients are combined once per manifold, so they are set for the whole manifold (the friction / restitution fields of the R3ContactModification) rather than per contact. None of these modifications are persistent (they are overwritten during the next timestep). There is one exception though: you can modify a user_data associated to each ContactManifold. This user_data will persist throughout timesteps as long as the ContactManifold remains alive (i.e. as long as some contacts exist between the touching parts of the colliders shapes). This can be useful to apply modification rules that depend on previous states of the contact (like whether or not this contact manifold existed during previous timesteps).

The contact modification is split into two callbacks of the R3PhysicsHooks, both called for each contact manifold:

  • modify_solver_contacts is given an R3ContactModification holding the properties of the whole manifold: its world-space contact normal, its friction and restitution coefficients, and its persistent user_data. Setting its enabled field to 0 removes all the solver contacts of the manifold.
  • modify_solver_contacts_context, called right after, is given an R3ContactModificationContext for modifying the solver contacts themselves. r3ContactModificationContext_SetTangentVelocity sets the tangent velocity of every solver contact of the manifold (e.g. for conveyor belts), and r3ContactModificationContext_UpdateAsOnewayPlatform implements the removal of the contacts required for one-way-platforms (it relies on the user_data of the manifold).

The contacts between two soft surfaces are contact candidates rather than a contact manifold: in that case, modify_solver_contacts isn't called and the functions of the R3ContactModificationContext above do nothing.

// This is a silly example of contact modifier that does silly things
// for illustration purpose:
// - Flip all the contact normals.
// - Set the friction coefficient to 0.3
// - Set the restitution coefficient to 0.4
// - Set the tangent velocities to X * 10.0
// The contacts of two soft surfaces are candidates rather than a manifold:
// only the manifolds of rigid pairs are given to this callback.
static void RAPIER_CALL modify_manifold(void *user_data, const R2ReadContext *read,
                                        R2ColliderHandle collider1, R2ColliderHandle collider2,
                                        R2ContactModification *manifold) {
    (void)user_data;
    (void)read;
    (void)collider1;
    (void)collider2;
    manifold->normal = r2VectorScale(manifold->normal, -1.0);

    // Friction and restitution are combined once per manifold, so they are set
    // for the whole manifold rather than per solver contact.
    manifold->friction = 0.3;
    manifold->restitution = 0.4;

    // Use the persistent user-data to count the number of times
    // contact modification was called for this contact manifold
    // since its creation.
    manifold->user_data += 1;
    printf("Contact manifold has been modified %u times since its creation.\n", manifold->user_data);
}

// Called right after `modify_manifold`, for the same manifold.
static void RAPIER_CALL modify_solver_contacts(void *user_data, const R2ReadContext *read,
                                               R2ColliderHandle collider1, R2ColliderHandle collider2,
                                               R2ContactModificationContext *context) {
    (void)user_data;
    (void)read;
    (void)collider1;
    (void)collider2;
    r2ContactModificationContext_SetTangentVelocity(context, r2Vector(10.0, 0.0));
}

static void step_with_contact_modification(R2World *world) {
    R2PhysicsHooks hooks = {0};
    hooks.modify_solver_contacts = modify_manifold;
    hooks.modify_solver_contacts_context = modify_solver_contacts;

    r2Step(world, &hooks, NULL);
}

The solver contacts can also be modified one by one: r3ContactModificationContext_SolverContactCount gives their number, r3ContactModificationContext_SolverContact and r3ContactModificationContext_SetSolverContact read and write one of them, and r3ContactModificationContext_RemoveSolverContact removes one of them (the last solver contact taking its place). Inside the hook, the contact points of an R3SolverContact (point1 and point2) are expressed in world-space. Finally, r3ContactModificationContext_IsSoft indicates whether the context holds the contact candidates of two soft surfaces rather than a contact manifold:

// Modifies the solver contacts one by one:
// - Delete the first contact.
// - Set the tangent velocities to X * 10.0
static void RAPIER_CALL modify_each_solver_contact(void *user_data, const R2ReadContext *read,
                                                   R2ColliderHandle collider1, R2ColliderHandle collider2,
                                                   R2ContactModificationContext *context) {
    (void)user_data;
    (void)read;
    (void)collider1;
    (void)collider2;
    // The contacts of two soft surfaces are candidates rather than a manifold.
    if (r2ContactModificationContext_IsSoft(context)) {
        return;
    }

    // The last solver contact takes the place of the removed one.
    if (r2ContactModificationContext_SolverContactCount(context) > 0) {
        r2ContactModificationContext_RemoveSolverContact(context, 0);
    }

    size_t count = r2ContactModificationContext_SolverContactCount(context);
    for (size_t i = 0; i < count; i++) {
        R2SolverContact solver_contact = r2ContactModificationContext_SolverContact(context, i);
        solver_contact.tangent_velocity.x = 10.0;
        r2ContactModificationContext_SetSolverContact(context, i, &solver_contact);
    }
}

Continuous Collision Detection​

Continuous Collision Detection (CCD) is used to make sure that fast-moving objects don't miss any contacts (a problem usually called tunneling). See the rigid-body CCD section for details.