diff options
Diffstat (limited to 'src/Alpha_complex/doc/Intro_alpha_complex.h')
| -rw-r--r-- | src/Alpha_complex/doc/Intro_alpha_complex.h | 31 |
1 files changed, 21 insertions, 10 deletions
diff --git a/src/Alpha_complex/doc/Intro_alpha_complex.h b/src/Alpha_complex/doc/Intro_alpha_complex.h index b075d1fc..a8b1a106 100644 --- a/src/Alpha_complex/doc/Intro_alpha_complex.h +++ b/src/Alpha_complex/doc/Intro_alpha_complex.h @@ -31,26 +31,37 @@ namespace alpha_complex { * circumsphere is empty (the simplex is then said to be Gabriel), and as the minimum of the filtration * values of the codimension 1 cofaces that make it not Gabriel otherwise. * - * All simplices that have a filtration value strictly greater than a given alpha squared value are not inserted into - * the complex. + * All simplices that have a filtration value \f$ > \alpha^2 \f$ are removed from the Delaunay complex + * when creating the simplicial complex if it is specified. * * \image html "alpha_complex_representation.png" "Alpha-complex representation" * * Alpha_complex is constructing a <a target="_blank" * href="http://doc.cgal.org/latest/Triangulation/index.html#Chapter_Triangulations">Delaunay Triangulation</a> - * \cite cgal:hdj-t-15b from <a target="_blank" href="http://www.cgal.org/">CGAL</a> (the Computational Geometry - * Algorithms Library \cite cgal:eb-15b) and is able to create a `SimplicialComplexForAlpha`. + * \cite cgal:hdj-t-19b from <a target="_blank" href="http://www.cgal.org/">CGAL</a> (the Computational Geometry + * Algorithms Library \cite cgal:eb-19b) and is able to create a `SimplicialComplexForAlpha`. * * The complex is a template class requiring an Epick_d <a target="_blank" * href="http://doc.cgal.org/latest/Kernel_d/index.html#Chapter_dD_Geometry_Kernel">dD Geometry Kernel</a> - * \cite cgal:s-gkd-15b from CGAL as template parameter. + * \cite cgal:s-gkd-19b from CGAL as template parameter. * * \remark - * - When the simplicial complex is constructed with an infinite value of alpha, the complex is a Delaunay - * complex. + * - When an \f$\alpha\f$-complex is constructed with an infinite value of \f$ \alpha^2 \f$, the complex is a Delaunay + * complex (with special filtration values). * - For people only interested in the topology of the \ref alpha_complex (for instance persistence), * \ref alpha_complex is equivalent to the \ref cech_complex and much smaller if you do not bound the radii. * \ref cech_complex can still make sense in higher dimension precisely because you can bound the radii. + * - Using the default `CGAL::Epeck_d` makes the construction safe. If you pass exact=true to create_complex, the + * filtration values are the exact ones converted to the filtration value type of the simplicial complex. This can be + * very slow. If you pass exact=false (the default), the filtration values are only guaranteed to have a small + * multiplicative error compared to the exact value, see <code><a class="el" target="_blank" + * href="https://doc.cgal.org/latest/Number_types/classCGAL_1_1Lazy__exact__nt.html"> + * CGAL::Lazy_exact_nt<NT>::set_relative_precision_of_to_double</a></code> for details. A drawback, when computing + * persistence, is that an empty exact interval [10^12,10^12] may become a non-empty approximate interval + * [10^12,10^12+10^6]. Using `CGAL::Epick_d` makes the computations slightly faster, and the combinatorics are still + * exact, but the computation of filtration values can exceptionally be arbitrarily bad. In all cases, we still + * guarantee that the output is a valid filtration (faces have a filtration value no larger than their cofaces). + * - For performances reasons, it is advised to use `Alpha_complex` with \ref cgal ≥ 5.0.0. * * \section pointsexample Example from points * @@ -124,13 +135,13 @@ namespace alpha_complex { * * \subsubsection nondecreasing Non decreasing filtration values * - * As the squared radii computed by CGAL are an approximation, it might happen that these alpha squared values do not - * quite define a proper filtration (i.e. non-decreasing with respect to inclusion). + * As the squared radii computed by CGAL are an approximation, it might happen that these \f$ \alpha^2 \f$ values do + * not quite define a proper filtration (i.e. non-decreasing with respect to inclusion). * We fix that up by calling `SimplicialComplexForAlpha::make_filtration_non_decreasing()`. * * \subsubsection pruneabove Prune above given filtration value * - * The simplex tree is pruned from the given maximum alpha squared value (cf. + * The simplex tree is pruned from the given maximum \f$ \alpha^2 \f$ value (cf. * `SimplicialComplexForAlpha::prune_above_filtration()`). * In the following example, the value is given by the user as argument of the program. * |