ITK  5.4.0 Insight Toolkit
Examples/DataRepresentation/Mesh/Mesh2.cxx
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// Software Guide : BeginLatex
//
// A \doxygen{Mesh} can contain a variety of cell types. Typical cells are
// the \doxygen{LineCell}, \doxygen{TriangleCell},
// \doxygen{PolygonCell}. Additional flexibility is provided for managing
// cells at the price of a bit more of complexity than in the case of point
// management.
//
// The following code creates a polygonal line in order to illustrate the
// simplest case of cell management in a mesh. The only cell type used here
// is the \code{LineCell}. The header file of this class must be included.
//
//
// Software Guide : EndLatex
#include "itkMesh.h"
// Software Guide : BeginCodeSnippet
#include "itkLineCell.h"
// Software Guide : EndCodeSnippet
int
main(int, char *[])
{
using PixelType = float;
using MeshType = itk::Mesh<PixelType, 3>;
// Software Guide : BeginLatex
//
// For consistency with \code{Mesh}, cell types have to be configured
// with a number of custom types taken from the mesh traits. The set of
// traits relevant to cells are packaged by the Mesh class into the
// \code{CellType} trait. This trait needs to be passed to the actual cell
// types at the moment of their instantiation. The following line shows how
// to extract the Cell traits from the Mesh type.
//
// \index{itk::Mesh!CellType}
// \index{itk::Mesh!traits}
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
using CellType = MeshType::CellType;
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// The \code{LineCell} type can now be instantiated using the traits
// taken from the Mesh.
//
// \index{itk::LineCell!Instantiation}
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
using LineType = itk::LineCell<CellType>;
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// The main difference in the way cells and points are managed by
// the Mesh is that points are stored by copy on the
// \code{PointsContainer} while cells are stored as pointers in the
// \code{CellsContainer}. The reason for using pointers is that cells
// use C++ polymorphism on the mesh. This means that the mesh is only
// aware of having pointers to a generic cell which is the base
// class of all the specific cell types. This architecture makes it
// possible to combine different cell types in the same
// mesh. Points, on the other hand, are of a single type and have a
// small memory footprint, which makes it efficient to copy them
// directly into the container.
//
// \index{itk::Cell!CellAutoPointer}
// \index{itk::Mesh!CellAutoPointer}
// \index{CellAutoPointer}
// \index{itk::AutoPointer}
//
// Managing cells by pointers adds another level of complexity to the Mesh
// since it is now necessary to establish a protocol to make clear who is
// responsible for allocating and releasing the cells' memory. This
// protocol is implemented in the form of a specific type of pointer called
// the \code{CellAutoPointer}. This pointer, based on the
// \doxygen{AutoPointer}, differs in many respects from the
// \code{SmartPointer}. The \code{CellAutoPointer} has an internal pointer
// to the actual object and a boolean flag that indicates whether the
// \code{CellAutoPointer} is responsible for releasing the cell memory when
// the time comes for its own destruction. It is said that a
// \code{CellAutoPointer} \emph{owns} the cell when it is responsible for
// its destruction. At any given time many \code{CellAutoPointer}s can
// point to the same cell, but only \textbf{one} \code{CellAutoPointer} can
// own the cell.
//
// The \code{CellAutoPointer} trait is defined in the \code{MeshType} and
// can be extracted as follows.
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
using CellAutoPointer = CellType::CellAutoPointer;
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// Note that the \code{CellAutoPointer} points to a generic cell type. It
// is not aware of the actual type of the cell, which could be (for
// example) a \code{LineCell}, \code{TriangleCell} or
// \code{TetrahedronCell}. This fact will influence the way in which we
// access cells later on.
//
// At this point we can actually create a mesh and insert some points on
// it.
//
// \index{itk::Mesh!New()}
// \index{itk::Mesh!SetPoint()}
// \index{itk::Mesh!PointType}
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
auto mesh = MeshType::New();
p0[0] = -1.0;
p0[1] = 0.0;
p0[2] = 0.0;
p1[0] = 1.0;
p1[1] = 0.0;
p1[2] = 0.0;
p2[0] = 1.0;
p2[1] = 1.0;
p2[2] = 0.0;
mesh->SetPoint(0, p0);
mesh->SetPoint(1, p1);
mesh->SetPoint(2, p2);
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// The following code creates two \code{CellAutoPointers} and initializes
// them with newly created cell objects. The actual cell type
// created in this case is \code{LineType}. Note that cells are
// created with the normal \code{new} C++ operator. The
// CellAutoPointer takes ownership of the received pointer by using
// the method \code{TakeOwnership()}. Even though this may seem
// verbose, it is necessary in order to make it explicit
// that the responsibility of memory release is assumed by the
// \code{AutoPointer}.
//
// \index{itk::AutoPointer!TakeOwnership()}
// \index{CellAutoPointer!TakeOwnership()}
// \index{CellType!creation}
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
CellAutoPointer line0;
CellAutoPointer line1;
line0.TakeOwnership(new LineType);
line1.TakeOwnership(new LineType);
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// The LineCells should now be associated with points in the mesh. This is
// done using the identifiers assigned to points when they were inserted
// in the mesh. Every cell type has a specific number of points that must
// be associated with it.\footnote{Some cell types like polygons have a
// variable number of points associated with them.} For example, a
// \code{LineCell} requires two points, a \code{TriangleCell}
// requires three, and a \code{TetrahedronCell} requires four. Cells use
// an internal numbering system for points. It is simply an index in the
// range $\{0,NumberOfPoints-1\}$. The association of points and cells is
// done by the \code{SetPointId()} method, which requires the user to
// provide the internal index of the point in the cell and the
// corresponding \code{PointIdentifier} in the \code{Mesh}. The internal
// cell index is the first parameter of \code{SetPointId()} while the mesh
// point-identifier is the second.
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
line0->SetPointId(0, 0); // line between points 0 and 1
line0->SetPointId(1, 1);
line1->SetPointId(0, 1); // line between points 1 and 2
line1->SetPointId(1, 2);
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// Cells are inserted in the mesh using the \code{SetCell()} method. It
// requires an identifier and the AutoPointer to the cell. The Mesh will
// take ownership of the cell to which the \code{CellAutoPointer} is
// pointing. This is done internally by the \code{SetCell()} method. In
// this way, the destruction of the \code{CellAutoPointer} will not
// induce the destruction of the associated cell.
//
// \index{itk::Mesh!SetCell()}
// \index{SetCell()!itk::Mesh}
// \index{itk::Mesh!Inserting cells}
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
mesh->SetCell(0, line0);
mesh->SetCell(1, line1);
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// After serving as an argument of the \code{SetCell()} method, a
// \code{CellAutoPointer} no longer holds ownership of the cell. It is
// important not to use this same \code{CellAutoPointer} again as
// argument to \code{SetCell()} without first securing ownership of
// another cell.
//
// Software Guide : EndLatex
std::cout << "Points = " << mesh->GetNumberOfPoints() << std::endl;
// Software Guide : BeginLatex
//
// The number of Cells currently inserted in the mesh can be queried with
// the \code{GetNumberOfCells()} method.
//
// \index{itk::Mesh!GetNumberOfCells()}
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
std::cout << "Cells = " << mesh->GetNumberOfCells() << std::endl;
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// In a way analogous to points, cells can be accessed using Iterators to
// the \code{CellsContainer} in the mesh. The trait for the cell iterator
// can be extracted from the mesh and used to define a local type.
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
using CellIterator = MeshType::CellsContainer::Iterator;
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// Then the iterators to the first and past-end cell in the mesh can be
// obtained respectively with the \code{Begin()} and \code{End()}
// methods of the \code{CellsContainer}. The \code{CellsContainer} of
// the mesh is returned by the \code{GetCells()} method.
//
// \index{itk::Mesh!Iterating cells}
// \index{itk::Mesh!GetCells()}
// \index{CellsContainer!Begin()}
// \index{CellsContainer!End()}
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
CellIterator cellIterator = mesh->GetCells()->Begin();
CellIterator end = mesh->GetCells()->End();
// Software Guide : EndCodeSnippet
// Software Guide : BeginLatex
//
// Finally, a standard loop is used to iterate over all the cells. Note the
// use of the \code{Value()} method used to get the actual pointer to the
// cell from the CellIterator. Note also that the value returned is
// a pointer to the generic CellType. This pointer must be downcast
// in order to be used as actual LineCell types. Safe down-casting is
// performed with the \code{dynamic\_cast} operator, which will throw an
// exception if the conversion cannot be safely performed.
//
// \index{down casting}
// \index{CellIterator!Value()}
// \index{CellIterator!increment}
// \index{itk::Mesh!CellType casting}
// \index{Print()}
// \index{CellType!Print()}
// \index{CellType!GetNumberOfPoints()}
// \index{LineCell!Print()}
// \index{LineCell!GetNumberOfPoints()}
//
// Software Guide : EndLatex
// Software Guide : BeginCodeSnippet
while (cellIterator != end)
{
MeshType::CellType * cellptr = cellIterator.Value();
auto * line = dynamic_cast<LineType *>(cellptr);
if (line == nullptr)
{
continue;
}
std::cout << line->GetNumberOfPoints() << std::endl;
++cellIterator;
}
// Software Guide : EndCodeSnippet
return EXIT_SUCCESS;
}
itk::GTest::TypedefsAndConstructors::Dimension2::PointType
ImageBaseType::PointType PointType
Definition: itkGTestTypedefsAndConstructors.h:51
itk::FixedArray::Begin
Iterator Begin()
itkLineCell.h
itkMesh.h
itk::LineCell
Represents a line segment for a Mesh.
Definition: itkLineCell.h:45
itk::Mesh
Implements the N-dimensional mesh structure.
Definition: itkMesh.h:126
New
static Pointer New()