Introduction

This chapter describes how to use PhysX' collision functionality with individual geometry objects. There are four main kinds of geometry queries:

• raycasts ("raycast queries") test a ray against a geometry object.
• sweeps ("sweep queries") move one geometry object along a line to find the first point of intersection with another geometry object.
• overlaps ("overlap queries") determine whether two geometry objects intersect.
• penetration depth computations ("minimal translational distance queries", abbreviated here to "MTD") test two overlapping geometry objects to find the direction along which they can be separated by the minimum distance.

In addition, PhysX provides helpers to compute the AABB of a geometry object, and to compute the distance between a point and a geometry object.

In all of the following functions, a geometry object is defined by its shape (a PxGeometry structure) and its pose (a PxTransform structure). All transforms and vectors are interpreted as being in the same space, and the results are also returned in that space.

Raycasts

A raycast query traces a point along a line segment until it hits a geometry object. PhysX supports raycasts for all geometry types.

The following code illustrates how to use a raycast query:

PxRaycastHit hitInfo;
PxU32 maxHits = 1;
PxHitFlags hitFlags = PxHitFlag::ePOSITION|PxHitFlag::eNORMAL|PxHitFlag::eDISTANCE|PxHitFlag::eUV;
PxU32 hitCount = PxGeometryQuery::raycast(origin, unitDir,
                                          geom, pose,
                                          maxDist,
                                          hitFlags,
                                          maxHits, &hitInfo);

The arguments are interpreted as follows:

• origin is the start point of the ray.
• unitDir is a unit vector defining the direction of the ray.
• maxDist is the maximum distance to search along the ray. It must be in the [0, inf) range. If the maximum distance is 0, a hit will only be returned if the ray starts inside a shape, as detailed below for each geometry.
• geom is the geometry to test against.
• pose is the pose of the geometry.
• hitFlags specifies the values that should be returned by the query, and options for processing the query.
• maxHits is the maximum number of hits to return.
• hitInfo specifies the PxRaycastHit structure(s) into which the raycast results will be stored.
• The anyHit parameter is deprecated. It is equivalent to PxHitFlag::eMESH_ANY, which should be used instead.

The returned result is the number of intersections found. For each intersection, a PxRaycastHit is populated. The fields of this structure are as follows:

PxRigidActor*   actor;
PxShape*        shape;
PxVec3          position;
PxVec3          normal;
PxF32           distance;
PxHitFlags      flags;
PxU32           faceIndex;
PxF32           u, v;

Some fields are optional, and the flags field indicates which members have been filled with result values. The query will fill fields in the output structure if the corresponding flags were set in the input - for example, if the PxHitFlag::ePOSITION is set in the input hitFlags, the query will fill in the PxRaycastHit::position field, and set the PxHitFlag::ePOSITION flag in PxRaycastHit::flags. If the input flag is not set for a specific member, the result structure may or may not contain valid data for that member. Omitting the eNORMAL and ePOSITION flags in the input can sometimes result in faster queries.

For a raycast which is not initially intersecting the geometry object, the fields are populated as follows (optional fields are listed together with the flag that controls them):

• actor and shape are not filled (these fields are used only in scene-level raycasts, see Scene Queries).
• position (PxHitFlag::ePOSITION) is the position of the intersection.
• normal (PxHitFlag::eNORMAL) is the surface normal at the point of intersection.
• distance (PxHitFlag::eDISTANCE) is the distance along the ray at which the intersection was found.
• flags specifies which fields of the structure are valid.
• faceIndex is the index of the face which the ray hit. For triangle mesh and height field intersections, it is a triangle index. For convex mesh intersections it is a polygon index. For other shapes it is always set to 0xffffffff.
• u and v (PxHitFlag::eUV) are the barycentric coordinates of the intersection. These fields (and the flag) are supported only for meshes and heightfields.

The position field is related to the barycentric coordinates via the following formula, where v0, v1 and v2 are the vertices from the hit triangle:

position = (1 - u - v)*v0 + u*v1 + v*v2;

This mapping is implemented in PxTriangle::pointFromUV().

See Geometry for details of how to retrieve face and vertex data from triangle meshes, convex meshes and height fields using face and vertex indices.

Exceptions to the above behavior may apply if a ray starts inside an object, in which case PhysX may not be able to compute meaningful output values for some fields. In these cases the field will remain unmodified and the corresponding flag will not be set. Specific details vary by geometry type, and are described below.

The exact conditions for raycast intersections are as follows:

Raycasts against Triangle Meshes

Triangle meshes are treated as thin triangle surfaces rather than solid objects. They may be configured to return either an arbitrary hit, the closest hit, or multiple hits.

• if maxHits is 1 and PxHitFlag::eMESH_ANY is not set, the query will return the closest intersection.
• if maxHits is 1 and PxHitFlag::eMESH_ANY is set, the query will return an arbitrary intersection. Use this when it is sufficient to know whether or not the ray hit the mesh, e.g. for line-of-sight queries or shadow rays.
• if maxHits is greater than 1, the query will return multiple intersections, up to maxHits. If more than maxHits intersection points exist, there is no guarantee that the results will include the closest. Use this for e.g. wall-piercing bullets that hit multiple triangles, or where special filtering is required. Note that PxHitFlag::eMESH_MULTIPLE must be used in this case.

In general "any hit" queries are faster than "closest hit" queries, and "closest hit" queries are faster than "multiple hits" queries.

By default, back face hits (where the triangle's outward-facing normal has a positive dot product with the ray direction) are culled, and so for any triangle hit the reported normal will have a negative dot product with the ray direction. This behavior may be modified by the mesh instance's PxMeshGeometryFlag::eDOUBLE_SIDED flag and the query's PxHitFlag::eMESH_BOTH_SIDES flag:

• if either PxMeshGeometryFlag::eDOUBLE_SIDED or PxHitFlag::eMESH_BOTH_SIDES is set, culling is disabled.
• if PxMeshGeometryFlag::eDOUBLE_SIDED is set, the reported normal is reversed for a back face hit.

For example a transparent glass window could be modeled as a double-sided mesh, so that a ray would hit either side with the reported normal facing opposite to the ray direction. A raycast tracing the path of a bullet that may penetrate the front side of a mesh and emerge from the back could use eMESH_BOTH_SIDES to find both front and back facing triangles even when the mesh is single-sided.

The following diagram shows what happens with different flags, for a single raycast intersecting a mesh in several places.

To use PxHitFlag::eMESH_BOTH_SIDES for selected meshes rather than all, set the flag inside the PxQueryFilterCallback.

If the start or end point of a ray is very close to the surface of a triangle, it may be treated as being on either side of the triangle.

If the start or end point of a ray is very close to the surface of a triangle, it may be treated as being on the either side of the triangle.

Overlaps

Overlap queries simply check whether two geometry objects overlap. One of the geometries must be a box, sphere, capsule or convex, and the other may be of any type.

The following code illustrates how to use an overlap query:

bool isOverlapping = overlap(geom0, pose0, geom1, pose1);

Overlaps do not support hit flags and return only a boolean result.

• A plane is treated as a solid half-space: that is, everything behind the plane is considered part of the volume.
• Triangle meshes are treated as thin triangle surfaces rather than solid objects.
• Heightfields are treated as triangle surface extruded by their thickness. Overlap geometries that do not intersect with the heightfield surface but are within the extruded space will report a hit.

If more than a boolean result is needed for meshes and heightfields, use the PxMeshQuery API instead (see PxMeshQuery).

Penetration Depth

When two objects are intersecting, PhysX can compute the minimal distance and direction by which the objects must be translated to separate them (this quantity is sometimes referred to as MTD, for minimum translational distance, as it is the vector of minimal length by which translation will separate the shapes). One geometry object must be a box, sphere, capsule or convex mesh, and the other may be of any type.

The following code illustrates how to use a penetration depth query:

bool isPenetrating = PxGeometryQuery::computePenetration(direction, depth,
                                                         geom0, pose0,
                                                         geom1, pose1);

The arguments are interpreted as follows:

• direction is set to the direction in which the first object should be translated in order to depenetrate from the second.
• distance is set to the distance by which the first object should be translated in order to depenetrate from the second.
• geom0 is the first geometry.
• pose0 is the transform of the first geometry.
• geom1 is the second geometry.
• pose2 is the transform of the second geometry.

The function returns true if the objects are penetrating, in which case it sets the direction and depth fields. Translating the first object by the depenetration vector D = direction * depth will separate the two objects. If the function returns true, the returned depth will always be positive or zero. If objects do not overlap, the function returns false, and the values of the direction and distance fields are undefined.

For simple (convex) shapes, returned results are accurate.

For meshes and heightfields, an iterative algorithm is used and dedicated functions are exposed in PxExtensions:

PxVec3 direction = PxComputeMeshPenetration(direction, depth,
                                            geom, geomPose,
                                            meshGeom, meshPose,
                                            maxIter, nb);

PxVec3 direction = PxComputeHeightFieldPenetration(direction, depth
                                                   geom, geomPose,
                                                   heightFieldGeom, heightFieldPose,
                                                   maxIter, nb);

Here, maxIter is the maximum number of iterations for the algorithm, and nb is an optional output argument which will be set to the number of iterations performed. If no overlap is detected, nb is set to zero. The code will attempt at most maxIter iterations but may exit earlier if a depenetration vector is found. Usually maxIter = 4 gives good results.

These functions only compute an approximate depenetration vector, and work best when the amount of overlap between the geometry object and the mesh/heightfield is small. In particular, an intersection with a triangle will be ignored when the object's center is behind the triangle, and if this holds for all intersecting triangles then no overlap is detected, and the functions do not compute an MTD vector.

Sweeps

A sweep query traces one geometry object through space to find the impact point on a second geometry object, and reports information concerning the impact point if one is found. PhysX only supports sweep queries where the first geometry object (the one that is traced through space) is a sphere, box, capsule or convex geometry. The second geometry object may be of any type.

The following code illustrates how to use a sweep query:

PxSweepHit hitInfo;
PxHitFlags hitFlags = PxHitFlag::ePOSITION|PxHitFlag::eNORMAL|PxHitFlag::eDISTANCE;
PxReal inflation = 0.0f;
PxU32 hitCount = PxGeometryQuery::sweep(unitDir, maxDist,
                                        geomToSweep, poseToSweep,
                                        geomSweptAgainst, poseSweptAgainst,
                                        hitInfo,
                                        hitFlags,
                                        inflation);

The arguments are interpreted as follows:

• unitDir is a unit vector defining the direction of the sweep.
• maxDist is the maximum distance to search along the sweep. It must be in the [0, inf) range, and is clamped by SDK code to at most PX_MAX_SWEEP_DISTANCE. A sweep of length 0 is equivalent to an overlap check.
• geomToSweep is the geometry to sweep. Supported geometries are: box, sphere, capsule or convex mesh.
• poseToSweep is the initial pose of the geometry to sweep.
• geomSweptAgainst is the geometry to sweep against (any geometry type can be used here).
• poseSweptAgainst is the pose of the geometry to sweep against.
• hitInfo is the returned result. A sweep will return at most one hit.
• hitFlags determines how the sweep is processed, and which data is returned if an impact is found.
• inflation inflates the first geometry with a shell extending outward from the object surface, making any corners rounded. It can be used to ensure a minimum margin of space is kept around the geometry when using sweeps to test whether movement is possible.

As with raycasts, fields will be filled in the output structure if the corresponding flags were set in the input hitFlags. The fields of PxSweepHit are as follows:

PxRigidActor*   actor;
PxShape*        shape;
PxVec3          position;
PxVec3          normal;
PxF32           distance;
PxHitFlags      flags;
PxU32           faceIndex;

• actor and shape are not filled (these fields are used only in scene-level sweeps, see Scene Queries).
• position (PxHitFlag::ePOSITION) is the position of the intersection. When there are multiple impact points, such as two boxes meeting face-to-face, PhysX will select one point arbitrarily. More detailed information for meshes or height fields may be obtained using the functions in PxMeshQuery.
• normal (PxHitFlag::eNORMAL) is the surface normal at the point of impact. It is a unit vector, pointing outwards from the hit object and backwards along the sweep direction (in the sense that the dot product between the sweep direction and the impact normal is negative).
• distance (PxHitFlag::eDISTANCE) is the distance along the ray at which the intersection was found.
• flags specifies which fields of the structure are valid.
• faceIndex is the index of the face hit by the sweep. This is a face from the hit object, not from the swept object. For triangle mesh and height field intersections, it is a triangle index. For convex mesh intersections it is a polygon index. For other shapes it is always set to 0xffffffff. For convex meshes the face index computation is rather expensive. The face index computation can be disabled by not providing the scene query hit flag PxHitFlag::eFACE_INDEX. If needed the face index can also be computed externally using the function PxFindFaceIndex which is part of the PhysX extensions library.

Unlike raycasts, u,v coordinates are not supported for sweeps.

For the geometry object swept against:

• A plane is treated as a solid half-space: that is, everything behind the plane is considered part of the volume to sweep against.
• The same backface-culling rules as for raycasts apply for sweeps, with the notable difference that eMESH_MULTIPLE is not supported.

Additional PxGeometryQuery functions

The following function computes the distance between a point and a geometry object. Only solid objects (box, sphere, capsule, convex) are supported:

PxReal dist = PxGeometryQuery::pointDistance(point, geom, pose, closestPoint);

closestPoint is an optional output argument which returns the closest point.

The following function computes the axis-aligned bounding box (AABB) enclosing a geometry object, given its pose:

PxBounds3 bounds = PxGeometryQuery::getWorldBounds(geom, pose, inflation);

The bounding box is scaled by the inflation value, which defaults to 1.01f if not explicitly specified.

PxMeshQuery

PhysX provides additional functionality for obtaining multiple results for triangle mesh and height field overlaps, and for sweeping against arrays of triangles. Only boxes, spheres and capsules may be tested against meshes or heightfields using these functions.

PxU32 triangleIndexBuffer[bufferSize];
PxU32 startIndex = 0;
bool bufferOverflowOccured = false;
PxU32 nbTriangles = PxMeshQuery::findOverlapTriangleMesh(sphereGeom, spherePose,
                                                         meshGeom, meshPose,
                                                         triangleIndexBuffer, bufferSize,
                                                         startIndex, bufferOverflowOccured);

for(PxU32 i=0; i < nbTriangles; i++)
{
    PxTriangle tri;
    PxU32 vertexIndices[3];
    PxMeshQuery::getTriangle(meshGeom, meshPose, triangleIndexBuffer[i], tri, vertexIndices);

     ...  // process triangle info
}

bool sweep(const PxVec3& unitDir,
           const PxReal distance,
           const PxGeometry& geom,
           const PxTransform& pose,
           PxU32 triangleCount,
           const PxTriangle* triangles,
           PxSweepHit& sweepHit,
           PxHitFlags hitFlags = PxHitFlag::eDEFAULT,
           const PxU32* cachedIndex = NULL,
           const PxReal inflation = 0.0f,
           bool doubleSided = false);