DGtal  0.9.3
Sets of points in digital spaces

Table of Contents

Author(s) of this documentation:
Jacques-Olivier Lachaud


Digital sets are sets of digital points within some domain of a digital space. They are thus subsets of a given domain (e.g. see Domains and HyperRectDomains). As such, their elements can be enumerated with iterators through the range [begin(),end()). Furthermore, digital sets can be complemented within their domain. Last, a digital set is also a predicate on point, returning true whenever the points lies inside the set.

The concept concepts::CDigitalSet and models of concepts::CDigitalSet

There are several ways for defining concretely a digital set, this is why digital sets are specified through the concept concepts::CDigitalSet. All types and required methods are specified in the documentation of CDigitalSet. It is worthy to note that a digital set is also associated to a digital domain, i.e. any model of concepts::CDomain.

There exists several models for concepts::CDigitalSet; their difference lies in the way elements are stored (let n be the number of points in the set).

You may choose yourself your representation of digital set, or let DGtal chooses for you the best suited representation with the class DigitalSetSelector. This is done by choosing among the following properties:

By default, Z2i::DigitalSet and Z3i::DigitalSet in StdDefs.h refer to the associative container with hash functions (fastest on large sets).

The following lines selects a rather generic representation for digital sets, since the set may be big, will be iterated many times and points will be tested many times:

typedef SpaceND<2> Z2;
typedef HyperRectDomain<Z2> Domain;
// here SpecificSet is DigitalSetByAssociativeContainer<(boost or std)::unordered_set<Point>, Domain>.

Using digital sets

The following snippet shows how to define a digital set in space Z2, within a rectangular domain, then how to insert points, how to visit them and how to test if a point belongs to the set.

typedef SpaceND<2> Z2;
typedef HyperRectDomain<Z2> Domain;
typedef Z2::Point Point;
// instantiating rectangular domain
Point p1( -10, -10 );
Point p2( 10, 10 );
Domain domain( p1, p2 );
// instanciating set within this domain.
SpecificSet mySet( domain );
Point c( 0, 0 );
mySet.insert( c ); // inserting point (0,0)
Point d( 5, 2 );
mySet.insert( d ); // inserting point (5,2)
Point e( 1, -3 );
mySet.insert( e ); // inserting point (1,-3)
// Iterating through the set
for ( ConstIterator it = mySet.begin(), itEnd = mySet.end();
it != itEnd; ++it )
std::cout << *it << std::endl;
// Checking points within set
bool ok_c = mySet( c ); // should be true
bool ok_d = mySet( d ); // should be true
bool ok_e = mySet( e ); // should be true
bool not_ok = mySet( Point( 1,1) ); // should be false

Other methods may be found in the concept definition of digital sets (CDigitalSet).

Digital sets and point predicates

Since 0.6, any digital set is also a model of concepts::CPointPredicate. You may thus use a digital set directly in functions and methods requiring a predicate on points. However, the class deprecated::SetPredicate, which builds a predicate on points around a digital set, is deprecated since 0.6. You may still used it by referecing it with DGtal::deprecated::SetPredicate, but it will no longer be maintained.

A digital set may be transformed into a domain

Sometimes it is useful to see a digital set as a new domain for further computation. This is possible with the facade class DigitalSetDomain, which wraps around some given digital set all necessary methods and types so that it satisfies the concept concepts::CDomain.

typedef DigitalSetDomain< SpecificSet > RestrictedDomain;
RestrictedDomain myDomain( mySet );
// this domain is limited to three points c, d and e.
The DigitalSetDomain class is only a wrapper around the digital set. The lifetime of the digital set must thus exceed the lifetime of the DigitalSetDomain.