openmesh/Doc/python_tutorial.docu

/** \page python_tutorial Python Tutorial

This tutorial will introduce the basic concepts behind the %OpenMesh Python
Bindings. We will cover the following topics:

\li How to build the Python Bindings
\li How to create an empty mesh
\li How to add vertices and faces to a mesh
\li How to navigate on a mesh using iterators and circulators
\li How to add and remove custom properties
\li How to read and write meshes from files

In addition, we will briefly discuss some of the differences between the Python
Bindings and the original C++ implementation of %OpenMesh.



\section python_build Building the Python Bindings

The Python Bindings depend on the following libraries:

\li Python (2.7 or later)
\li Boost Python (1.54.0 or later)

\note Make sure that your Boost Python and Python versions match, i.e. that
Boost Python was linked against the correct Python version.

The Python Bindings are automatically build with %OpenMesh. For more information
on how to build %OpenMesh see \ref compiling.



\section python_start Getting Started

To use the %OpenMesh Python Bindings we first need to import the openmesh module:

\dontinclude python_tutorial.py
\skipline from

The module provides two mesh classes: One for polygonal meshes (PolyMesh) and
one for triangle meshes (TriMesh). You should use triangle meshes whenever
possible, since they are usually more efficient. In addition, some algorithms
are only implemented for triangle meshes while triangle meshes inherit the full
functionality of polygonal meshes.

The following code creates a new triangle mesh:

\skipline mesh



\section python_add Adding Items to a Mesh

We can add a new vertex to the mesh by calling the add_vertex() member function.
This function gets a coordinate and returns a handle to the newly inserted
vertex.

\skipline vh0
\until vh4

To add a new face to the mesh we have to call add_face(). This function gets the
handles of the vertices that make up the new face and returns a handle to the
newly inserted face:

\skipline fh0
\until fh2

We can also use a Python list to add a face to the mesh:

\skipline vh_list
\until fh3



\section python_iterators Iterators and Circulators

Now that we have added a couple of vertices to the mesh, we can iterate over
them and print out their indices:

\skipline for
\until vh.idx()

We can also iterate over halfedges, edges and faces by calling mesh.halfedges(),
mesh.edges() and mesh.faces() respectively:

\skipline iterate
\until fh.idx()

To iterate over the items adjacent to another item we can use one of the
circulator functions. For example, to iterate over the vertices adjacent to
another vertex we can call mesh.vv() and pass the handle of the center vertex:

\skipline for
\until vh.idx()

We can also iterate over the adjacent halfedges, edges and faces:

\skipline iterate
\until fh.idx()

To iterate over the items adjacent to a face we can use the following functions:

\skipline iterate
\until fh.idx()



\section python_props Properties

%OpenMesh allows us to dynamically add custom properties to a mesh. We can add
properties to vertices, halfedges, edges, faces and the mesh itself. To
add a property to a mesh (and later access its value) we have to use a property
handle of the appropriate type:

\li VPropHandle (for vertex properties)
\li HPropHandle (for halfedge properties)
\li EPropHandle (for edge properties)
\li FPropHandle (for face properties)
\li MPropHandle (for mesh properties)

The following code shows how to add a vertex property to a mesh:

\skipline prop_handle
\until mesh

The second parameter of the function add_property() is optional. The parameter
is used to specify a name for the new property. This name can later be used
to retrieve a handle to the property using the get_property_handle() member
function.

Now that we have added a vertex property to the mesh we can set and get its
value. Here we will use the property to store the center of gravity of each
vertex' neighborhood:

\skipline for
\until mesh.set_property

Properties use Python's type system. This means that we can use the same
property to store values of different types (e.g. store both strings and
integers using the same vertex property). Properties are initialized to the
Python built-in constant None.

To remove a property we have to call remove_property() with the appropriate
property handle:

\skipline mesh.remove_property



\section python_propman Property Managers

Another way to add and remove a property is to use a property manager. A
Property manager encapsulates a property and manages its lifecycle. A Property
manager also provides a number of convenience functions to access the enclosed
property.

There are four different types of property managers. One for each type of mesh
item:

\li VPropertyManager (for vertex properties)
\li HPropertyManager (for halfedge properties)
\li EPropertyManager (for edge properties)
\li FPropertyManager (for face properties)

Property managers automatically add a new property to a mesh when they are
initialized. Thus the following code not only creates a new vertex property
manager, but also adds a new vertex property to the mesh:

\skipline prop_man

Property managers allow us to conveniently set the property value for an entire
range of mesh items: 

\skipline prop_man

They also allow us to use the subscript operator to set and get property values.
Here we will once again use a property to store the center of gravity of each
vertex' neighborhood:

\skipline for
\until prop_man[vh] /= valence

Properties that are encapsulated by a property manager are automatically removed
from the mesh when the property manager goes out of scope (i.e. the property
manager is garbage collected).



\section python_io Read and write meshes from files

You can read and write meshes from files using the read_mesh() and write_mesh()
functions:

\skipline mesh
\until write_mesh(mesh, "bunny.obj")

The file type is automatically deduced from the file extension. %OpenMesh
currently supports four file types: .off, .obj, .stl and .om

The behaviour of the I/O functions can be controlled by passing an instance of
the Options class to either read_mesh() or write_mesh(). The class controls the
behaviour of the I/O functions by means of enabled/disabled bits in a bitset:

\skipline mesh
\until print "something went wrong"

Other available option bits include:

-# mode bits - control binary reading/writing
 - Options.Binary
 - Options.MSB
 - Options.LSB
 - Options.Swap (MSB|LSB)
-# property bits - controls which standard properties to read/write
 - Options.VertexNormal
 - Options.VertexTexCoord
 - Options.VertexColor
 - Options.FaceNormal
 - Options.FaceColor
 - Options.ColorAlpha
 - Options.ColorFloat

\note You have to pass an instance of the Options class to the I/O functions,
i.e. you cannot directly pass one of the option bits. For example, directly
passing Options.Binary to either one of the functions will cause an error.

When reading a file the options are used as hints, i.e. depending on the format
we can help the reader to interpret the data correctly.

\note If you want to read a property from a file the property must have been
requested prior to reading the file.

When writing the mesh the mode bits control whether to use the binary variant of
the respective file format and the desired byte-ordering.



\section python_examples Additional Code Examples

You can use our unit tests to learn more about the %OpenMesh Python Bindings.
They are located in the src/Python/Unittests subdirectory.



\section python_cpp Python and C++

The interface of the Python Bindings is to a large extent identical to the
interface of the original C++ implementation of %OpenMesh. You should therefore
be able to use the C++ documentation as a reference for the Python Bindings. In
particular, the classes KernelT, PolyMeshT and TriMeshT provide a good overview
of the available mesh member functions. That being said, there are a number of
small differences. For example, whenever the C++ implementation returns a
reference to an object that is managed by %OpenMesh, the Python Bindings will
return a copy of that object. This is due to the fact that Python does not have
a language feature that is analogous to C++ references. One example of such a
function is the point() member function of the PolyMesh and TriMesh classes.
Unlike its C++ counterpart, the function does not return a reference to the
requested point. It instead returns a copy of the point. This implies that you
have to use the set_point() member function to change the value of a point. The
same applies to the following functions: normal(), color(), property(),
status(), etc.



<br>The complete source looks like this:

\include python_tutorial.py


**/