Attributes

In this guide, you will learn fundamental elements to understand and manipulate attributes in OpenGeode.

What are attributes?

Attributes stand for properties you can add to OpenGeode objects, typically meshes. These properties are not directly stored on objects or meshes, so you can add Attributes on other objects.

Attributes are managed by an AttributeManager. Meshes have managers defined on their vertices, edges, facets, and so on. Updating the mesh (e.g. by adding or removing vertices) automatically updates attached AttributeManager.

You can also create an AttributeManager that is not attached to a mesh.

Create an Attribute

First of all, let's get an AttributeManager by the dedicated mesh methods:

TriangulatedSurface3D mesh; auto manager = mesh.polygon_attribute_manager(); ...

or by creating it:

AttributeManager manager; manager.resize( 10 ); auto attribute_ptr = manager.find_or_create_attribute< VariableAttribute, double >( "first_attribute", 12.3 ); manager.resize( 6 );

The first line of the above code creates a default AttributeManager.

The second line resizes the created AttributeManager. It means that all Attributes associated to this AttributeManager will have exactly 10 values.

Note that this method can be called before and after the creation of Attributes associated to the AttributeManager (as done at the fourth line of the above code).

As in the standard library, there is also a method to reserve Attributes capacities.

Now, take a look at the third line of the above code. This line creates an attribute using the method find_or_create_attribute. Four elements are needed to define the Attribute to create:

  • The type of Attribute storage (here a VariableAttribute): the first template parameter of the method (more details below);
  • The type of Attribute values (here double): the second template parameter of the method;
  • An attribute name (here first_attribute): the first parameter of the method find_or_create_attribute;
  • The default value (here 12.3): the second parameter of the method.

If the attribute already exists and has the same name, same storage and same value type, a pointer to the pre-existing attribute is returned without creating a new attribute. If the attribute name already exist but with a different storage or a different value type, an exception is thrown.

Attribute Storage

Each attribute may be stored using a storage adapted to its use and its values. In OpenGeode, there are three types of attribute storage, but you can easily define your own. These storages are:

  • VariableAttribute: one value is stored per attribute element. The number of stored values is equal to the attribute size;
  • ConstantAttribute: all the attribute elements have the given default value as value. Only one value is stored (the default value);
  • SparseAttribute: all the attribute elements have the given default value as value, excepted the ones that have been explicitly defined. The number of stored values is between one (only the default value) and the attribute size plus one (one value for each attribute element and the default value). This storage is useful if the number of values different from the default value is small with regards to the attribute size.

Remove an Attribute

AttributeManager provides a method to remove an attribute given its name.

manager.delete_attribute( "first_attribute" );

Get an attribute with a read-only access

AttributeManager provides a method to get a read-only access pointer to an attribute given its name and its value type:

const auto read_only_attr = manager.find_attribute< double >( "first_attribute" );

If you do not know the type of the attribute, an other way is to get a non-type Attribute called a generic attribute:

const auto generic_attr = manager.find_generic_attribute( "first_attribute" ); const auto value = generic_attr->generic_value( 2 ); // get a float

The pointed object is an AttributeBase which is a non-typed attribute from which derive ConstantAttribute, SparseAttribute, and VariableAttribute.

The type of generic values you can get by this way is float.

For not arythmetic types (e.g. std::vector< double >) or custom types (e.g. MyType), you should define how to convert this type into float. For instance:

template <> struct GenericAttributeConversion< std::vector< double > > { static float converted_value( const std::vector< double >& value ) { return std::accumulate(value.begin(), value.end(), 0); } };

or

template <> struct GenericAttributeConversion< MyType > { static float converted_value( const MyType& value ) { return value.get_a_float(); } };

Modify attribute size

To modify the attribute size, you should use the AttributeManager. The first solution is to use the method resize as shown above. The second solution is to select the element to remove:

AttributeManager manager; manager.resize( 6 ); auto attribute_ptr = manager.find_or_create_attribute< VariableAttribute, double >( "first_attribute", 12.3 ); std::vector< bool > to_delete( 6, false ); to_delete[0] = true; to_delete[3] = true;

Here the elements 0 and 3 are removed from all the attributes associated to the AttributeManager.

Modify attribute values

To modify the value of attribute elements, you may use either the attribute itself or its associated AttributeManager. In the code below, we present four ways to modify attribute values:

AttributeManager manager; manager.resize( 6 ); auto attribute_ptr = manager.find_or_create_attribute< VariableAttribute, double >( "first_attribute", 12.3 ); // values: 12.3 12.3 12.3 12.3 12.3 12.3 attribute_ptr->set_value( 1, 5.0 ); // values: 12.3 5.0 12.3 12.3 12.3 12.3 const auto factor = 2; attribute_ptr->modify_value( 4, [&factor]( double& value ){ value *= factor; } ); // values: 12.3 5.0 12.3 12.3 24.6 12.3 manager.assign_attribute_value( 1, 2 ); // values: 12.3 5.0 5.0 12.3 24.6 12.3 const AttributeLinearInterpolation interpolator{ {2, 4}, {0.5, 0.5} }; manager.interpolate_attribute_value( 0, interpolator ); // values: 14.8 5.0 5.0 12.3 24.6 12.3

If you use AttributeManager methods, the modification will be operated on all associated attributes.