Surface mesh point queries: C API

Quest provides the in/out and distance field queries to test a point against a surface mesh. These queries take a mesh composed of triangles in 3D and a query point. The in/out query tests whether the point is contained within the surface mesh. The distance query calculates the signed distance from the query point to the mesh.

Fig. 27 Types of point-vs-surface-mesh queries provided by Quest. Left: In/out query, characterizing points as inside or outside the mesh. Points that lie on the boundary might or might not be categorized as inside the mesh. Right: Distance query, calculating the signed distance from each query point to its closest point on the mesh. A negative result indicates an interior query point.

The following examples show Quest’s C interface to these queries. The general pattern is to set some query parameters, then pass a mint::Mesh * or file name string to an initialization function, call an evaluate function for each query point, then clean up with a finalize function.

In/out C API

The in/out query operates on a 3D surface mesh, that is, triangles forming a watertight surface enclosing a 3D volume. The in/out API utilizes integer-valued return codes with values quest::QUEST_INOUT_SUCCESS and quest::QUEST_INOUT_FAILED to indicate the success of each operation. These examples are excerpted from <axom>/src/axom/quest/examples/quest_inout_interface.cpp.

To get started, we first include some header files.

#include "axom/quest/interface/inout.hpp"

#ifdef AXOM_USE_MPI
  #include <mpi.h>
#endif

Before initializing the query, we can set some parameters, for example, to control the logging verbosity and to set a threshold for welding vertices of the triangle mesh while generating the spatial index.

  // -- Set quest_inout parameters
  rc = quest::inout_set_verbose(isVerbose);
  if(rc != quest::QUEST_INOUT_SUCCESS)
  {
    cleanAbort();
  }

  rc = quest::inout_set_vertex_weld_threshold(weldThresh);
  if(rc != quest::QUEST_INOUT_SUCCESS)
  {
    cleanAbort();
  }

By default, the verbosity is set to false and the welding threshold is set to 1E-9.

We are now ready to initialize the query.

#ifdef AXOM_USE_MPI
    rc = quest::inout_init(fileName, MPI_COMM_WORLD);
#else
    rc = quest::inout_init(fileName);
#endif

The variable fileName is a std::string that indicates a triangle mesh file. Another overload of quest::inout_init() lets a user code pass a reference to a mint::Mesh* to query meshes that were previously read in or built up. If initialization succeeded (returned quest::QUEST_INOUT_SUCCESS), the code can

• Query the mesh bounds with quest::inout_mesh_min_bounds(double[3]) and quest::inout_mesh_max_bounds(double[3]).
• Find mesh center of mass with quest::inout_mesh_center_of_mass(double[3]).
• Test if a query point is inside the mesh surface. In this example pt is a double[3] and numInside is a running total.

    const double x = pt[0];
    const double y = pt[1];
    const double z = pt[2];

    const bool ins = quest::inout_evaluate(x, y, z);
    numInside += ins ? 1 : 0;

Once we are done, we clean up with the following command:

  quest::inout_finalize();

Signed Distance query C API

Excerpted from <axom>/src/axom/quest/examples/quest_signed_distance_interface.cpp.

Quest header:

#include "axom/quest.hpp"

Before initialization, a code can set some parameters for the distance query.

• Passing true to quest::signed_distance_set_closed_surface(bool) lets the user read in a non-closed “terrain mesh” that divides its bounding box into “above” (positive distance to the surface mesh) and “below” (negative distance).
• quest::signed_distance_set_max_levels() and quest::signed_distance_set_max_occupancy() control options for the BVH tree spatial index used to accelerate signed distance queries.
• If the SLIC logging environment is in use, passing true to quest::signed_distance_set_verbose() will turn on verbose logging.
• Use of MPI-3 shared memory can be enabled by passing true to quest::signed_distance_use_shared_memory().

The distance query must be initialized before use. In this example, Arguments is a POD struct containing parameters to the executable. As with the in/out query, a user code may pass either a file name or a reference to a mint::Mesh * to quest::signed_distance_init().

  int rc = quest::signed_distance_init(args.fileName, global_comm);

Once the query is initialized, the user code may retrieve mesh bounds with quest::signed_distance_get_mesh_bounds(double* lo, double* hi). Test query points against the mesh with quest::signed_distance_evaluate(). Here, pt is a double[3], phi is a double array, and inode is an integer.

      phi[inode] = quest::signed_distance_evaluate(pt[0], pt[1], pt[2]);

Finally, clean up.

  quest::signed_distance_finalize();