50 lines
2.4 KiB
Plaintext
50 lines
2.4 KiB
Plaintext
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/** \page optional_component
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Optional Component
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===
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There are many cases where some vertex or face attributes (like for example the Face-Face adjacency) are not necessary all the time.
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VCG Lib gives you a way to specify optional components, i.e. attributes that are not statically stored within the simplex but can be
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dynamically allocated only when you need them. Their use is totally transparent with respect to the standard, statically allocated components.
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To 'define' optional component you need to do two things:
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- to use a special type of container (derived from std::vector) in the mesh definition
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- to specify the right type of component in the simplex definition
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An optional component can be used only when it is 'enabled', e.g. when the memory for it has been allocated.
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An optional component can be enable by calling the function Enable.
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VCG Lib handles optional components with two alternative mechanisms: the first is called 'Ocf' (for Optional Component Fast)
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which uses one pointer for each simplex but it makes accessing optional attribute almost as fast as non-optional ones;
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the second is called 'Occ' (for Optional Component Compact) which only requires a little extra space for each mesh
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(unrelated to its size in terms of number of simplicies) but which can result in a slower access.
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In the following, only their use is discussed. The implementation detail can be found here.
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Optional Component Fast
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---
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The following definition of MyMesh specifies that a few components of the vertex and face types are optional.
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Note that the special attribute vcg::vertex::InfoOcf and vcg::face::InfoOcf are the first attribute of vertex and face types and that we
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use the vcg::vertex::vector_ocf and vcg::face::vector_ocf as containers;
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\dontinclude trimesh_optional.cpp
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\skip MyVertexOcf
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\until MyMeshOcf
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Before accessing the data contained in the optional component you must enable that attribute by calling EnableXXX() corresponding function;
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afterwards the space for the component will be allocated and accessible until you call again DisableXXXX().
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\dontinclude trimesh_optional.cpp
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\skip cmof
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\until cmof
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\skip HasFFAdjacency
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\until true
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Trying to access the value of a unallocated component before enabling or after disabling the corresponding component
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will throw an assertion.
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\dontinclude trimesh_optional.cpp
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\skip EnableNormal
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\until UpdateNormal
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\sa trimesh_optional.cpp
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*/
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