- How to get and set the value of a custom property
In the last example we computed the barycenter of each vertex'
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@@ -11,44 +11,69 @@ let %OpenMesh manage the data.
It would be even more helpful if we could attach such properties
dynamically to the mesh.
%OpenMesh provides dynamic properties, which can be attached to each
mesh entity (vertex, face, edge, halfedge, and the mesh itself). We
distinguish between custom and standard properties. A custom property
is any user-defined property and is accessed via the member function
\c property(..) via a handle and an entity handle
(e.g. VertexHandle). Whereas the standard properties are accessed via
special member functions, e.g. the vertex position is accessed with \c
point(..) and a vertex handle.
Custom properties can be conveniently created and attached to meshes with the following functions:
- makeTemporaryProperty() creates a property that is temporary to the current scope.
- getOrMakeProperty() is used for creating and accessing permanent named properties on a mesh.
- getProperty() is used for accessing an existing permanent named property on a mesh.
In this example we will store the \c cog-value (see previous example)
in an additional vertex property instead of keeping it in a separate
array. To do so we define first a so-called property handle with the desired
type (\c MyMesh::Point) and register the handle at the mesh:
All three functions take two template arguments:
- First, the type of the mesh element that the property is attached to (i.e. OpenMesh::VertexHandle, OpenMesh::HalfedgeHandle, OpenMesh::EdgeHandle, or OpenMesh::FaceHandle).
- Second, the type of the property value that is attached to each element (e.g., \p int, \p double, etc.).
All three functions return a handle object (of type OpenMesh::PropertyManager) that manages the lifetime of the property and provides read / write access to its values.
In this example, we will store the \c cog value (see previous example) in a vertex property instead of keeping it in a separate array.
To do so, we first add a (temporary) property of the desired element type (OpenMesh::VertexHandle) and value type (\c %MyMesh::Point) to the mesh:
\dontinclude 03-properties/smooth.cc
\skipline vertex property stores
\until mesh.add
\skipline makeTemporaryProperty
<br>The \c mesh allocates enough memory to hold as many elements of type
\c MyMesh::Point as number of vertices exist, and of course the mesh
synchronizes all insert and delete operations on the vertices with the
vertex properties.
Enough memory is allocated to hold as many values of \c %MyMesh::Point as there are vertices.
All insert and delete operations on the mesh are synchronized with the attached properties.
Once the wanted property is registered we can use the property to
calculate the barycenter of the neighborhood of each vertex \c v_it
Once the property is created, we can use it to compute the centers of the neighborhood of each vertex:
\dontinclude 03-properties/smooth.cc
\skipline vv_it=
\skipline mesh.vertices
\until cog[vv] /= valence
\until }
\until mesh.prop
<br>and finally set the new position for each vertex \c v_it
Finally, we set the new position for each vertex:
\dontinclude 03-properties/smooth.cc
\skipline mesh.set_point
\skipline mesh.vertices
\until mesh.point
\until }
---
Since we chose to use makeTemporaryProperty(), the created property is automatically removed from the mesh as soon as we leave the scope of the associated handle variable \c cog.
If, instead, a property is desired to survive its local scope, it should be created with using getOrMakeProperty(). In that case, the property must be given a name that can later be used to retrieve the property. For example:
\code
auto face_area = OpenMesh::makeTemporaryProperty<OpenMesh::FaceHandle, double>(mesh, 'face_area');
\endcode
At a later time, we can use the getProperty() function to obtain a handle to a property that was previously created by refering to its name:
\code
try {
auto face_area = OpenMesh::getProperty<OpenMesh::FaceHandle, double>(mesh, 'face_area');
// Use the face_area property.
}
catch (const std::runtime_error& e) {
// Property not found. Handle the error here.
}
\endcode
---
The functions makeTemporaryProperty(), getOrMakeProperty(), and getProperty() are the convenient high-level interface for creating and accessing mesh properties.
Beneath these convenience functions, there is also a low-level property interface where handle and property lifetime must be managed manually. This interface is accessed through a mesh's add_property(), remove_property(), and property() functions and several property handle classes (OpenMesh::VPropHandleT, OpenMesh::HPropHandleT, OpenMesh::EPropHandleT, OpenMesh::FPropHandleT, OpenMesh::MPropHandleT).
