Qhull code

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# Qhull code

This section discusses the code for Qhull.

Copyright © 1995-2015 C.B. Barber

## » Qhull code: Table of Contents

- Reentrant Qhull
- How to convert code to reentrant Qhull
- Qhull on 64-bit computers
- Calling Qhull from C++ programs
  - Cpp questions for Qhull
  - CoordinateIterator
  - Qhull
  - QhullError
  - QhullFacet
  - QhullFacetList
  - QhullFacetSet
  - QhullIterator
  - QhullLinkedList
  - QhullPoint
  - QhullQh
  - QhullPointSet
  - QhullRidge
  - QhullRidgeSet
  - QhullSet
  - QhullVertex
  - QhullVertexList
  - QhullVertexSet
  - RboxPoints
- Calling Qhull from C programs
  - How to avoid exit(), fprintf(), stderr, and stdout
  - Constrained Delaunay triangulation
  - Delaunay triangulations and point indices
  - Locate facet with qh_findbestfacet()
  - On-line construction with qh_addpoint()
  - Sets and quick memory allocation
  - Tricoplanar facets and option 'Qt'
  - Vertex neighbors of a vertex
  - Voronoi vertices of a region
  - Voronoi vertices of a ridge
- Performance of Qhull
- Enhancements to Qhull
- Qhull functions, macros, and data structures

## » Reentrant Qhull

Qhull-2015 introduces reentrant Qhull (libqhull_r). Reentrant Qhull uses a qhT* argument instead of global data structures. The qhT* pointer is the first argument to most Qhull routines. It allows multiple instances of Qhull to run at the same time. It simplifies the C++ interface to Qhull.

New code should be written with libqhull_r. Existing users of libqhull should consider converting to libqhull_r. Although libqhull will be supported indefinitely, improvements may not be implemented. Reentrant qhull is 1-2% slower than non-reentrant qhull.

Note: Reentrant Qhull is not thread safe. Do not invoke Qhull routines with the same qhT* pointer from multiple threads.

## » How to convert code to reentrant Qhull

C++ users need to convert to libqhull_r. The new C++ interface does a better, but not perfect, job of hiding Qhull's C data structures. The previous C++ interface was unusual due to Qhull's global data structures.

All other users should consider converting to libqhull_r. The conversion is straight forward. The original conversion of libqhull to libqhull_r required thousands of changes, mostly global search and replace. The first run of Qhull (unix_r.c) produced the same output, and nearly the same log files, as the original (unix.c).

Suggestions to help with conversion.

- Compare qconvex_r.c with qconvex.c. Define a qhT object and a pointer it. The qhT* pointer is the first argument to most Qhull functions. Clear `qh_qh-<NOerrext` before calling qh_initflags(). Invoke QHULL_LIB_CHECK to check for a compatible Qhull library.
- Compare user_eg2_r.c with user_eg2.c
- Compare user_eg_r.c with user_eg.c. If you use qhT before invoking qh_init_A, call qh_zero() to clear the qhT object. user_eg_r.c includes multiple Qhull runs.
- Review user_eg3_r.cpp. As with the other programs, invoke QHULL_LIB_CHECK. Simple C++ programs should compile as is.
- Compare QhullFacet.cpp with the same file in Qhull-2012.1. UsingLibQhull was replaced with the macro QH_TRY_() and '`qh_qh-<NOerrext= true`'.
- For detailed notes on libqhull_r, see "libqhull_r (reentrant Qhull)" and "Source code changes for libqhull_r" in Changes.txt.
- For detailed notes on libqhullcpp, see "C++ interface" and following sections in Changes.txt.
- For regexps and conversion notes, see README_r.txt (unedited).

## » Qhull on 64-bit computers

Qhull compiles for 64-bit hosts. Since the size of a pointer on a 64-bit host is double the size on a 32-bit host, memory consumption increases about 50% for simplicial facets and up-to 100% for non-simplicial facets.

You can check memory consumption with option Ts. It includes the size of each data structure:

- 32-bit -- merge 24 ridge 20 vertex 28 facet 88 normal 24 ridge vertices 16 facet vertices or neighbors 20
- 64-bit -- merge 32 ridge 32 vertex 48 facet 120 normal 32 ridge vertices 40 facet vertices or neighbors 48

For Qhull 2015, the maximum identifier for ridges, vertices, and facets was increased from 24-bits to 32-bits. This allows for larger convex hulls, but may increase the size of the corresponding data structures. The sizes for Qhull 2012.1 were

- 32-bit -- merge 24 ridge 16 vertex 24 facet 88
- 64-bit -- merge 32 ridge 32 vertex 40 facet 120

## » Calling Qhull from C++ programs

Qhull 2015 uses reentrant Qhull for its C++ interface. If you used the C++ interface from qhull 2012.1, you may need to adjust how you initialize and use the Qhull classes. See How to convert code to reentrant Qhull.

Qhull's C++ interface allows you to explore the results of running Qhull. It provides access to Qhull's data structures. Most of the classes derive from the corresponding qhull data structure. For example, QhullFacet is an instance of Qhull's facetT.

You can retain most of the data in Qhull and use the C++ interface to explore its results. Each object contains a reference the Qhull's data structure (via QhullQh), making the C++ representation less memory efficient.

Besides using the C++ interface, you can also use libqhull_r directly. For example, the FOREACHfacet_(...) macro will visit each facet in turn.

The C++ interface to Qhull is incomplete. You may need to extend the interface. If so, you will need to understand Qhull's data structures and read the code. Example (c.f., `user_eg3 eg-100`). It prints the facets generated by Qhull.

        RboxPoints rbox;
        rbox.appendRandomPoints("100");
        Qhull qhull;
        qhull.runQhull("", rbox);
        QhullFacetList facets(qhull);
        cout<< facets;

The C++ iterface for RboxPoints redefines the fprintf() calls in rboxlib.c. Instead of writing its output to stdout, RboxPoints appends the output to a std::vector. The same technique may be used for calling Qhull from C++.

- Run Qhull with option 'Ta' to annotate the output with qh_fprintf() identifiers.
- Redefine qh_fprintf() for these identifiers.
- See RboxPoints.cpp for an example.

Since the C++ interface uses reentrant Qhull, multiple threads may run Qhull at the same time. Each thread is one run of Qhull.

Do not have two threads accessing the same Qhull instance. Qhull is not thread-safe.

### » CoordinateIterator

A CoordinateIterator or ConstCoordinateIterator [RboxPoints.cpp] is a `std::vector<realT>::iterator` for Rbox and Qhull coordinates. It is the result type of RboxPoints.coordinates().

Qhull does not use CoordinateIterator for its data structures. A point in Qhull is an array of reals instead of a std::vector. See QhullPoint.

### » Qhull

Qhull is the top-level class for running Qhull. It initializes Qhull, runs the computation, and records errors. It provides access to the global data structure QhullQh, Qhull's facets, and vertices.

### » QhullError

QhullError is derived from `std::exception`. It reports errors from Qhull and captures the output to stderr.

If error handling is not set up, Qhull exits with a code from 1 to 5. The codes are defined by qh_ERR* in libqhull_r.h. The exit is via qh_exit() in usermem_r.c. The C++ interface does not report the captured output in QhullError. Call Qhull::setErrorStream to send output to cerr instead.

### » QhullFacet

A QhullFacet is a facet of the convex hull, a region of the Delaunay triangulation, a vertex of a Voronoi diagram, or an intersection of the halfspace intersection about a point. A QhullFacet has a set of QhullVertex, a set of QhullRidge, and a set of neighboring QhullFacets.

### » QhullFacetList

A QhullFacetList is a linked list of QhullFacet. The result of `Qhull.runQhull` is a QhullFacetList stored in QhullQh.

### » QhullFacetSet

A QhullFacetSet is a QhullSet of QhullFacet. QhullFacetSet may be ordered or unordered. The neighboring facets of a QhullFacet is a QhullFacetSet. The neighbors of a QhullFacet is a QhullFacetSet. The neighbors are ordered for simplicial facets, matching the opposite vertex of the facet.

### » QhullIterator

QhullIterator contains macros for defining Java-style iterator templates from a STL-style iterator template.

### » QhullLinkedList

A QhullLinkedLIst is a template for linked lists with next and previous pointers. QhullFacetList and QhullVertexList are QhullLinkedLists.

### » QhullPoint

A QhullPoint is an array of point coordinates, typically doubles. The length of the array is QhullQh.hull_dim. The identifier of a QhullPoint is its 0-based index from QhullQh.first_point followed by QhullQh.other_points.

### » QhullPointSet

A QhullPointSet is a QhullSet of QhullPoint. The QhullPointSet of a QhullFacet is its coplanar points.

### » QhullQh

QhullQh is the root of Qhull's data structure. It contains initialized constants, sets, buffers, and variables. It contains an array and a set of QhullPoint, a list of QhullFacet, and a list of QhullVertex. The points are the input to Qhull. The facets and vertices are the result of running Qhull.

Qhull's functions access QhullQh through the global variable, `qh_qh`. The global data structures, qh_stat and qh_mem, record statistics and manage memory respectively.

### » QhullRidge

A QhullRidge represents the edge between two QhullFacet's. It is always simplicial with qh.hull_dim-1 QhullVertex)'s.

### » QhullRidgeSet

A QhullRidgeSet is a QhullSet of QhullRidge. Each QhullFacet contains a QhullRidgeSet.

### » QhullSet

A QhullSet is a set of pointers to objects. QhullSets may be ordered or unordered. They are the core data structure for Qhull.

### » QhullVertex

A QhullVertex is a vertex of the convex hull. A simplicial QhullFacet has qh.hull_dim-1 vertices. A QhullVertex contains a QhullPoint. It may list its neighboring QhullFacet's.

### » QhullVertexList

A QhullVertexList is a QhullLinkedList of QhullVertex. The global data structure, QhullQh contains a QhullVertexList of all the vertices.

### » QhullVertexSet

A QhullVertexSet is a QhullSet of QhullVertex. The QhullVertexSet of a QhullFacet is the vertices of the facet. It is ordered for simplicial facets and unordered for non-simplicial facets.

### » RboxPoints

RboxPoints is a std::vector of point coordinates (QhullPoint). It's iterator is CoordinateIterator.

`RboxPoints.appendRandomPoints()` appends points from a variety of distributions such as uniformly distributed within a cube and random points on a sphere. It can also append a cube's vertices or specific points.

### » Cpp questions for Qhull

Developing C++ code requires many conventions, idioms, and technical details. The following questions have either mystified the author or do not have a clear answer. See also C++ and Perl Guidelines. and FIXUP notes in the code. Please add notes to Qhull Wiki.

- FIXUP QH11028 Should return reference, but get reference to temporary

      iterator Coordinates::operator++() { return iterator(++i); }

- size() as size_t, size_type, or int
- Should all containers have a reserve()?
- Qhull.feasiblePoint interface
- How to avoid copy constructor while logging, maybeThrowQhullMessage()
- How to configure Qhull output. Trace and results should go to stdout/stderr
- Qhull and RboxPoints messaging. e.g., ~Qhull, hasQhullMessage(). Rename them as QhullErrorMessage?
- How to add additional output to an error message, e.g., qh_setprint
- Is idx the best name for an index? It's rather cryptic, but BSD strings.h defines index().
- Qhull::feasiblePoint Qhull::useOutputStream as field or getter?
- Define virtual functions for user customization of Qhull (e.g., qh_fprintf, qh_memfree,etc.)
- Figure out RoadError::global_log. clearQhullMessage currently clearGlobalLog
- Should the false QhullFacet be NULL or empty? e.g., QhullFacet::tricoplanarOwner() and QhullFacetSet::end()
- Should output format for floats be predefined (qh_REAL_1, 2.2g, 10.7g) or as currently set for stream
- Should cout << !point.defined() be blank or 'undefined'
- Infinite point as !defined()
- qlist and qlinkedlist define pointer, reference, size_type, difference_type, const_pointer, const_reference for the class but not for iterator and const_iterator vector.h --

      reference operator[](difference_type _Off) const

- When forwarding an implementation is base() an approriate name (e.g., Coordinates::iterator::base() as std::vector::iterator).
- When forwarding an implementation, does not work "returning address of temporary"
- Also --, +=, and -=

      iterator       &operator++() { return iterator(i++); }

- if vector inheritance is bad, is QhullVertexSet OK?
- Should QhullPointSet define pointer and reference data types?

## » Calling Qhull from C programs

Warning: Qhull was not designed for calling from C programs. You may find the C++ interface easier to use. You will need to understand the data structures and read the code. Most users will find it easier to call Qhull as an external command.

For examples of calling Qhull, see GNU Octave's computational geometry code, and Qhull's user_eg_r.c, user_eg2_r.c, and user_r.c. To see how Qhull calls its library, read unix_r.c, qconvex.c, qdelaun.c, qhalf.c, and qvoronoi.c. The '*_r.c' files are reentrant, otherwise they are non-reentrant. Either version may be used. New code should use reentrant Qhull.

See Reentrant Qhull functions, macros, and data structures for internal documentation of Qhull. The documentation provides an overview and index. To use the library you will need to read and understand the code. For most users, it is better to write data to a file, call the qhull program, and read the results from the output file.

If you use non-reentrant Qhull, be aware of the macros "qh" and "qhstat", e.g., "qh hull_dim". They are defined in `libqhull.h`. They allow the global data structures to be pre-allocated (faster access) or dynamically allocated (allows multiple copies).

Qhull's `Makefile` produces a library, `libqhull_r.a`, for inclusion in your programs. First review `libqhull_r.h`. This defines the data structures used by Qhull and provides prototypes for the top-level functions. Most users will only need libqhull_r.h in their programs. For example, the Qhull program is defined with `libqhull_r.h` and `unix_r.c`. To access all functions, use `qhull_ra.h`. Include the file with "`#include <libqhull_r/qhull_ra.h>`". This avoids potential name conflicts.

If you use the Qhull library, you are on your own as far as bugs go. Start with small examples for which you know the output. If you get a bug, try to duplicate it with the Qhull program. The 'Tc' option will catch many problems as they occur. When an error occurs, use 'T4 TPn' to trace from the last point added to the hull. Compare your trace with the trace output from the Qhull program.

Errors in the Qhull library are more likely than errors in the Qhull program. These are usually due to feature interactions that do not occur in the Qhull program. Please report all errors that you find in the Qhull library. Please include suggestions for improvement.

### » How to avoid exit(), fprintf(), stderr, and stdout

Qhull sends output to qh.fout and errors, log messages, and summaries to qh.ferr. qh.fout is normally stdout and qh.ferr is stderr. qh.fout may be redefined by option 'TO' or the caller. qh.ferr may be redirected to qh.fout by option 'Tz'.

Qhull does not use stderr, stdout, fprintf(), or exit() directly.

Qhull reports errors via qh_errexit() by writting a message to qh.ferr and invoking longjmp(). This returns the caller to the corresponding setjmp() (c.f., QH_TRY_ in QhullQh.h). If qh_errexit() is not available, Qhull functions call qh_exit(). qh_exit() normally calls exit(), but may be redefined by the user. An example is libqhullcpp/usermem_r-cpp.cpp. It redefines qh_exit() as a 'throw'.

If qh_meminit() or qh_new_qhull() is called with ferr==NULL, then they set ferr to stderr. Otherwise the Qhull libraries use qh->ferr and qh->qhmem.ferr for error output.

If an error occurs before qh->ferr is initialized, Qhull invokes qh_fprintf_stderr(). The user may redefine this function along with qh_exit(), qh_malloc(), and qh_free().

The Qhull libraries write output via qh_fprintf() [userprintf_r.c]. Otherwise, the Qhull libraries do not use stdout, fprintf(), or printf(). Like qh_exit(), the user may redefine qh_fprintf().

### » sets and quick memory allocation

You can use `mem_r.c` and `qset_r.c` individually. `Mem_r.c` implements quick-fit memory allocation. It is faster than malloc/free in applications that allocate and deallocate lots of memory.

`Qset_r.c` implements sets and related collections. It's the inner loop of Qhull, so speed is more important than abstraction. Set iteration is particularly fast. `qset_r.c` just includes the functions needed for Qhull.

### » Delaunay triangulations and point indices

Here some unchecked code to print the point indices of each Delaunay triangle. Use option 'QJ' if you want to avoid non-simplicial facets. Note that upper Delaunay regions are skipped. These facets correspond to the furthest-site Delaunay triangulation.

>       facetT *facet;
>       vertexT *vertex, **vertexp;

>       FORALLfacets {
>         if (!facet->upperdelaunay) {
>           printf ("%d", qh_setsize (facet->vertices);
>           FOREACHvertex_(facet->vertices)
>             printf (" %d", qh_pointid (vertex->point));
>           printf ("\n");
>         }
>       }

### » locate a facet with qh_findbestfacet()

The routine qh_findbestfacet in `poly2_r.c` is particularly useful. It uses a directed search to locate the facet that is furthest below a point. For Delaunay triangulations, this facet is the Delaunay triangle that contains the lifted point. For convex hulls, the distance of a point to the convex hull is either the distance to this facet or the distance to a subface of the facet.

> Warning: If triangulated output ('Qt') and the best facet is triangulated, qh_findbestfacet() returns one of the corresponding 'tricoplanar' facets. The actual best facet may be a different tricoplanar facet.

> See qh_nearvertex() in poly2.c for sample code to visit each tricoplanar facet. To identify the correct tricoplanar facet, see Devillers, et. al., ['01] and Mucke, et al ['96]. If you implement this test in general dimension, please notify qhull@qhull.org.

qh_findbestfacet performs an exhaustive search if its directed search returns a facet that is above the point. This occurs when the point is inside the hull or if the curvature of the convex hull is less than the curvature of a sphere centered at the point (e.g., a point near a lens-shaped convex hull). When the later occurs, the distance function is bimodal and a directed search may return a facet on the far side of the convex hull.

Algorithms that retain the previously constructed hulls usually avoid an exhaustive search for the best facet. You may use a hierarchical decomposition of the convex hull [Dobkin and Kirkpatrick '90].

To use qh_findbestfacet with Delaunay triangulations, lift the point to a paraboloid by summing the squares of its coordinates (see qh_setdelaunay in geom2_r.c). Do not scale the input with options 'Qbk', 'QBk', 'QbB' or 'Qbb'. See Mucke, et al ['96] for a good point location algorithm.

The intersection of a ray with the convex hull may be found by locating the facet closest to a distant point on the ray. Intersecting the ray with the facet's hyperplane gives a new point to test.

### » on-line construction with qh_addpoint()

The Qhull library may be used for the on-line construction of convex hulls, Delaunay triangulations, and halfspace intersections about a point. It may be slower than implementations that retain intermediate convex hulls (e.g., Clarkson's hull program). These implementations always use a directed search. For the on-line construction of convex hulls and halfspace intersections, Qhull may use an exhaustive search (qh_findbestfacet).

You may use qh_findbestfacet and qh_addpoint (`libqhull.c`) to add a point to a convex hull. Do not modify the point's coordinates since qh_addpoint does not make a copy of the coordinates. For Delaunay triangulations, you need to lift the point to a paraboloid by summing the squares of the coordinates (see qh_setdelaunay in geom2.c). Do not scale the input with options 'Qbk', 'QBk', 'QbB' or 'Qbb'. Do not deallocate the point's coordinates. You need to provide a facet that is below the point (qh_findbestfacet).

You can not delete points. Another limitation is that Qhull uses the initial set of points to determine the maximum roundoff error (via the upper and lower bounds for each coordinate).

For many applications, it is better to rebuild the hull from scratch for each new point. This is especially true if the point set is small or if many points are added at a time.

Calling qh_addpoint from your program may be slower than recomputing the convex hull with qh_qhull. This is especially true if the added points are not appended to the qh first_point array. In this case, Qhull must search a set to determine a point's ID. [R. Weber]

See user_eg.c for examples of the on-line construction of convex hulls, Delaunay triangulations, and halfspace intersections. The outline is:

>     initialize qhull with an initial set of points
>     qh_qhull();

>     for each additional point p
>        append p to the end of the point array or allocate p separately
>        lift p to the paraboloid by calling qh_setdelaunay
>        facet= qh_findbestfacet (p, !qh_ALL, &bestdist, &isoutside);
>        if (isoutside)
>           if (!qh_addpoint (point, facet, False))
>              break;  /* user requested an early exit with 'TVn' or 'TCn' */

>     call qh_check_maxout() to compute outer planes
>     terminate qhull

### » Constrained Delaunay triangulation

With a fair amount of work, Qhull is suitable for constrained Delaunay triangulation. See Shewchuk, ACM Symposium on Computational Geometry, Minneapolis 1998.

Here's a quick way to add a constraint to a Delaunay triangulation: subdivide the constraint into pieces shorter than the minimum feature separation. You will need an independent check of the constraint in the output since the minimum feature separation may be incorrect. [H. Geron]

### » Tricoplanar facets and option 'Qt'

Option 'Qt' triangulates non-simplicial facets (e.g., a square facet in 3-d or a cubical facet in 4-d). All facets share the same apex (i.e., the first vertex in facet->vertices). For each triangulated facet, Qhull sets facet->tricoplanar true and copies facet->center, facet->normal, facet->offset, and facet->maxoutside. One of the facets owns facet->normal; its facet->keepcentrum is true. If facet->isarea is false, facet->triowner points to the owning facet.

Qhull sets facet->degenerate if the facet's vertices belong to the same ridge of the non-simplicial facet.

To visit each tricoplanar facet of a non-simplicial facet, either visit all neighbors of the apex or recursively visit all neighbors of a tricoplanar facet. The tricoplanar facets will have the same facet->center.

See qh_detvridge for an example of ignoring tricoplanar facets.

### » Voronoi vertices of a region

The following code iterates over all Voronoi vertices for each Voronoi region. Qhull computes Voronoi vertices from the convex hull that corresponds to a Delaunay triangulation. An input site corresponds to a vertex of the convex hull and a Voronoi vertex corresponds to an adjacent facet. A facet is "upperdelaunay" if it corresponds to a Voronoi vertex "at-infinity". Qhull uses qh_printvoronoi in `io.c` for 'qvoronoi o'

>     /* please review this code for correctness */
>     qh_setvoronoi_all();
>     FORALLvertices {
>        site_id = qh_pointid (vertex->point);
>        if (qh hull_dim == 3)
>           qh_order_vertexneighbors(vertex);
>        infinity_seen = 0;
>        FOREACHneighbor_(vertex) {
>           if (neighbor->upperdelaunay) {
>             if (!infinity_seen) {
>               infinity_seen = 1;
>               ... process a Voronoi vertex "at infinity" ...
>             }
>           }else {
>             voronoi_vertex = neighbor->center;
>             ... your code goes here ...
>           }
>        }
>     }

### » Voronoi vertices of a ridge

Qhull uses qh_printvdiagram() in io.c to print the ridges of a Voronoi diagram for option 'Fv'. The helper function qh_eachvoronoi() does the real work. It calls the callback 'printvridge' for each ridge of the Voronoi diagram.

You may call qh_printvdiagram2(), qh_eachvoronoi(), or qh_eachvoronoi_all() with your own function. If you do not need the total number of ridges, you can skip the first call to qh_printvdiagram2(). See qh_printvridge() and qh_printvnorm() in io.c for examples.

### » vertex neighbors of a vertex

To visit all of the vertices that share an edge with a vertex:

- Generate neighbors for each vertex with qh_vertexneighbors in `poly2.c`.
- For simplicial facets, visit the vertices of each neighbor
- For non-simplicial facets,
  - Generate ridges for neighbors with qh_makeridges in `merge.c`.
  - Generate ridges for a vertex with qh_vertexridges in `merge.c`.
  - Visit the vertices of these ridges.

For non-simplicial facets, the ridges form a simplicial decomposition of the (d-2)-faces between each pair of facets -- if you need 1-faces, you probably need to generate the full face graph of the convex hull.

## » Performance of Qhull

Empirically, Qhull's performance is balanced in the sense that the average case happens on average. This may always be true if the precision of the input is limited to at most O(log n) bits. Empirically, the maximum number of vertices occurs at the end of constructing the hull.

Let n be the number of input points, v be the number of output vertices, and f_v be the maximum number of facets for a convex hull of v vertices. If both conditions hold, Qhull runs in O(n log v) in 2-d and 3-d and O(n f_v/v) otherwise. The function f_v increases rapidly with dimension. It is O(v^floor(d/2) / floor(d/2)!).

The time complexity for merging is unknown. Options 'C-0' and 'Qx' (defaults) handle precision problems due to floating-point arithmetic. They are optimized for simplicial outputs.

When running large data sets, you should monitor Qhull's performance with the 'TFn' option. The time per facet is approximately constant. In high-d with many merged facets, the size of the ridge sets grows rapidly. For example the product of 8-d simplices contains 18 facets and 500,000 ridges. This will increase the time needed per facet.

As dimension increases, the number of facets and ridges in a convex hull grows rapidly for the same number of vertices. For example, the convex hull of 300 cospherical points in 6-d has 30,000 facets.

If Qhull appears to stop processing facets, check the memory usage of Qhull. If more than 5-10% of Qhull is in virtual memory, its performance will degrade rapidly.

When building hulls in 20-d and higher, you can follow the progress of Qhull with option 'T1'. It reports each major event in processing a point.

To reduce memory requirements, recompile Qhull for single-precision reals (REALfloat in `user.h`). Single-precision does not work with joggle ('QJ'). Check qh_MEMalign in `user.h` and the match between free list sizes and data structure sizes (see the end of the statistics report from 'Ts'). If free list sizes do not match, you may be able to use a smaller qh_MEMalign. Setting qh_COMPUTEfurthest saves a small amount of memory, as does clearing qh_MAXoutside (both in `user.h`).

Shewchuk is working on a 3-d version of his triangle program. It is optimized for 3-d simplicial Delaunay triangulation and uses less memory than Qhull.

To reduce the size of the Qhull executable, consider qh_NOtrace and qh_KEEPstatistics 0 in `user.h`. By changing `user.c` you can also remove the input/output code in `io.c`. If you don't need facet merging, then version 1.01 of Qhull is much smaller. It contains some bugs that prevent Qhull from initializing in simple test cases. It is slower in high dimensions.

The precision options, 'Vn', 'Wn', 'Un'. 'A-n', 'C-n', 'An', 'Cn', and 'Qx', may have large effects on Qhull performance. You will need to experiment to find the best combination for your application.

The verify option ('Tv') checks every point after the hull is complete. If facet merging is used, it checks that every point is inside every facet. This can take a very long time if there are many points and many facets. You can interrupt the verify without losing your output. If facet merging is not used and there are many points and facets, Qhull uses a directed search instead of an exhaustive search. This should be fast enough for most point sets. Directed search is not used for facet merging because directed search was already used for updating the facets' outer planes.

The check-frequently option ('Tc') becomes expensive as the dimension increases. The verify option ('Tv') performs many of the same checks before outputting the results.

Options 'Q0' (no pre-merging), 'Q3' (no checks for redundant vertices), 'Q5' (no updates for outer planes), and 'Q8' (no near-interior points) increase Qhull's speed. The corresponding operations may not be needed in your application.

In 2-d and 3-d, a partial hull may be faster to produce. Option 'QgGn' only builds facets visible to point n. Option 'QgVn' only builds facets that contain point n. In higher-dimensions, this does not reduce the number of facets.

`User.h` includes a number of performance-related constants. Changes may improve Qhull performance on your data sets. To understand their effect on performance, you will need to read the corresponding code.

GNU `gprof` reports that the dominate cost for 3-d convex hull of cosperical points is qh_distplane(), mainly called from qh_findbestnew(). The dominate cost for 3-d Delaunay triangulation is creating new facets in qh_addpoint(), while qh_distplane() remains the most expensive function.

## » Enhancements to Qhull

There are many ways in which Qhull can be improved.

    [Jan 2015] Suggestions
    ------------
    To do for a future verson of Qhull
     - Add a merge pass before cone creation to remove duplicate subridges between horizon facets
     - Option to add a bounding box for Delaunay triangulations, e,g., nearly coincident points
     - Report error when rbox Cn,r,m does not produce points (e.g., 'r' distributions)
     - Rescale output to match 'QbB' on input [J. Metz, 1/30/2014 12:21p]
     - Run through valgrind
     - Notes to compgeom on conformant triangulation and Voronoi volume
     - Git: Create signed tags for Qhull versions
     - Implement weighted Delaunay triangulation and weighted Voronoi diagram [A. Liebscher]
       e.g., Sugihara, "Three-dimensional convex hull as a fruitful source of diagrams," Theoretical Computer Science, 2000, 235:325-337
     - testqset: test qh_setdelnth and move-to-front
     - Makefile: Re-review gcc/g++ warnings.  OK in 2011.
     - Break up -Wextra into its components or figure out how to override -Wunused-but-set-variable
       unused-but-set-variable is reporting incorrectly.  All instances are annotated.
     - CMakelists.txt:  Why are files duplicated for cmake build
     - CMakeLists.txt: configure the 'ctest' target
     - The size of maxpoints in qh_maxsimplex should be d+3 unique points to help avoid QH6154

     - Can countT be defined as 'int', 'unsigned int', or 64-bit int?
       countT is currently defined as 'int' in qset_r.h
       Vertex ID and ridge ID perhaps should be countT, They are currently 'unsigned'
       Check use of 'int' vs. countT in all cpp code
       Check use of 'int' vs. countT in all c code
       qset_r.h defines countT -- duplicates code in user_r.h -- need to add to qset.h/user.h
       countT -1 used as a flag in Coordinates.mid(), QhullFacet->id()
       Also QhullPoints indexOf and lastIndexOf
       Also QhullPointSet indexOf and lastIndexOf
       Coordinates.indexOf assumes countT is signed (from end)
       Coordinates.lastIndexOf assumes countT is signed (from end)
       All error messages with countT are wrong, convert to int?
       RboxPoints.qh_fprintf_rbox, etc. message 9393 assumes countT but may be int, va_arg(args, countT);  Need to split

    ------------
    To do for a furture version of the C++ interface
     - Fix C++ memory leak in user_eg3 [M. Sandim]
     - Document C++ using Doxygen conventions (//! and //!<)
     - Should Qhull manage the output formats for doubles?  QH11010 user_r.h defines qh_REAL_1 as %6.8g
     - Allocate memory for QhullSet using Qhull.qhmem.  Create default constructors for QhullVertexSet etc.  Also mid() etc.
     - Add interior point for automatic translation?
     - Add hasNext() to all next() iterators (e.g., QhullVertex)
     - Add defineAs() to each object
     - Write a program with concurrent Qhull
     - Write QhullStat and QhullStat_test
     - Add QList and vector instance of facetT*, etc.
     - Generalize QhullPointSetIterator
     - qh-code.htm: Document changes to C++ interface.
          Organize C++ documentation into collection classes, etc.
     - Review all C++ classes and C++ tests
     - QhullVertexSet uses QhullSetBase::referenceSetT() to free it's memory.   Probably needed elsewhere
     - The Boost Graph Library provides C++ classes for graph data structures. It may help
       enhance Qhull's C++ interface [Dr. Dobb's 9/00 p. 29-38; OOPSLA '99 p. 399-414].

    [Jan 2010] Suggestions
     - Generate vcproj from qtpro files
       cd qtpro && qmake -spec win32-msvc2005 -tp vc -recursive
       sed -i 's/C\:\/bash\/local\/qhull\/qtpro\///' qhull-all.sln
       Change qhullcpp to libqhull.dll
       Allow both builds on same host (keep /tmp separate)
     - Make distribution -- remove tmp, news, .git, leftovers from project, change CRLF
         search for 2010.1, Dates
         qhulltest --all added to output
         Add md5sum
         Add test of user_eg3, etc.
     - C++ class for access to statistics, accumulate vs. add
     - Add dialog box to RoadError-- a virtual function?
     - Option 'Gt' does not make visible all facets of the mesh example, rbox 32 M1,0,1 | qhull d Gt
     - Option to select bounded Voronoi regions [A. Uzunovic]
     - Merge small volume boundary cells into unbounded regions [Dominik Szczerba]
     - Postmerge with merge options
     - Add const to C code
     - Add modify operators and MutablePointCoordinateIterator to PointCoordinates
     - Add Qtest::toString() functions for QhullPoint and others.  QByteArray and qstrdup()
     - Fix option Qt for conformant triangulations of merged facets
     - Investigate flipped facet -- rbox 100 s D3 t1263080158 | qhull R1e-3 Tcv Qc
     - Add doc comments to c++ code
     - Measure performance of Qhull, seconds per point by dimension
     - Report potential wraparound of 64-bit ints -- e.g., a large set or points

    Documentation
    - Qhull::addPoint().  Problems with qh_findbestfacet and otherpoints see
       qh-code.htm#inc on-line construction with qh_addpoint()
    - How to handle 64-bit possible loss of data.  WARN64, ptr_intT, size_t/int
    - Show custom of qh_fprintf
    - grep 'qh_mem ' x | sort | awk '{ print $2; }' | uniq -c | grep -vE ' (2|4|6|8|10|12|14|16|20|64|162)[^0-9]'
    - qtpro/qhulltest contains .pro and Makefile.  Remove Makefiles by setting shadow directory to ../../tmp/projectname
    - Rules for use of qh_qh and multi processes
        UsingQhull
        errorIfAnotherUser
        ~QhullPoints() needs ownership of qh_qh
        Does !qh_pointer work?
        When is qh_qh required?  Minimize the time.
       qhmem, qhstat.ferr
       qhull_inuse==1 when qhull globals active [not useful?]
       rbox_inuse==1 when rbox globals active
       - Multithreaded -- call largest dimension for infinityPoint() and origin()
     - Better documentation for qhmem totshort, freesize, etc.
     - how to change .h, .c, and .cpp to text/html.  OK in Opera
     - QhullVertex.dimension() is not quite correct, epensive
     - Check globalAngleEpsilon
     - Deprecate save_qhull()

    [Dec 2003] Here is a partial list:
     - fix finddelaunay() in user_eg.c for tricoplanar facets
     - write a BGL, C++ interface to Qhull
         http://www.boost.org/libs/graph/doc/table_of_contents.html
     - change qh_save_qhull to swap the qhT structure instead of using pointers
     - change error handling and tracing to be independent of 'qh ferr'
     - determine the maximum width for a given set of parameters
     - prove that directed search locates all coplanar facets
     - in high-d merging, can a loop of facets become disconnected?
     - find a way to improve inner hulls in 5-d and higher
     - determine the best policy for facet visibility ('Vn')
     - determine the limitations of 'Qg'

    Precision improvements:
     - For 'Qt', resolve cross-linked, butterfly ridges.
         May allow retriangulation in qh_addpoint().
     - for Delaunay triangulations ('d' or 'v') under joggled input ('QJ'),
         remove vertical facets whose lowest vertex may be coplanar with convex hull
     - review use of 'Qbb' with 'd QJ'.  Is MAXabs_coord better than MAXwidth?
     - check Sugihara and Iri's better in-sphere test [Canadian
         Conf. on Comp. Geo., 1989; Univ. of Tokyo RMI 89-05]
     - replace centrum with center of mass and facet area
     - handle numeric overflow in qh_normalize and elsewhere
     - merge flipped facets into non-flipped neighbors.
         currently they merge into best neighbor (appears ok)
     - determine min norm for Cramer's rule (qh_sethyperplane_det).  It looks high.
     - improve facet width for very narrow distributions

    New features:
     - implement Matlab's tsearch() using Qhull
     - compute volume of Voronoi regions.  You need to determine the dual face
       graph in all dimensions [see Clarkson's hull program]
     - compute alpha shapes [see Clarkson's hull program]
     - implement deletion of Delaunay vertices
          see Devillers, ACM Symposium on Computational Geometry, Minneapolis 1999.
     - compute largest empty circle [see O'Rourke, chapter 5.5.3] [Hase]
     - list redundant (i.e., coincident) vertices [Spitz]
     - implement Mucke, et al, ['96] for point location in Delaunay triangulations
     - implement convex hull of moving points
     - implement constrained Delaunay diagrams
          see Shewchuk, ACM Symposium on Computational Geometry, Minneapolis 1998.
     - estimate outer volume of hull
     - automatically determine lower dimensional hulls
     - allow "color" data for input points
          need to insert a coordinate for Delaunay triangulations

    Input/output improvements:
     - Support the VTK Visualization Toolkit, http://www.kitware.com/vtk.html
     - generate output data array for Qhull library [Gautier]
     - need improved DOS window with screen fonts, scrollbar, cut/paste
     - generate Geomview output for Voronoi ridges and unbounded rays
     - generate Geomview output for halfspace intersection
     - generate Geomview display of furthest-site Voronoi diagram
     - use 'GDn' to view 5-d facets in 4-d
     - convert Geomview output for other 3-d viewers
     - add interactive output option to avoid recomputing a hull
     - orient vertex neighbors for 'Fv' in 3-d and 2-d
     - track total number of ridges for summary and logging

    Performance improvements:
     - optimize Qhull for 2-d Delaunay triangulations
     -   use O'Rourke's '94 vertex->duplicate_edge
     -   add bucketing
     -   better to specialize all of the code (ca. 2-3x faster w/o merging)
     - use updated LU decomposition to speed up hyperplane construction
     -        [Gill et al. 1974, Math. Comp. 28:505-35]
     - construct hyperplanes from the corresponding horizon/visible facets
     - for merging in high d, do not use vertex->neighbors

Please let us know about your applications and improvements.

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Created: Sept. 25, 1995 --- Last modified: see changes.txt