root/branches/Dirk_CPtr/Doc/to_port/DrawablesGeometry.dox

#include <OSGConfig.h>

using namespace OSG;

// the used groups

/*! \defgroup GrpSystemNodeCoresDrawablesGeometry Geometry
    \ingroup GrpSystemNodeCoresDrawables
    
    Geometry is the primary leaf node of the tree. It wraps polygonal
    primitives as defined by OpenGLs glBegin()/glEnd() loops. See \ref
    PageSystemGeometry for a description. 
*/

/*! \defgroup GrpSystemDrawablesGeometryIterators Geometry Iterators
    \ingroup GrpSystemNodeCoresDrawablesGeometry
    
    Helpers to access Geometry in a generic way. See \ref PageSystemGeometryIterators 
    for a description.
*/

/*! \defgroup GrpSystemDrawablesGeometryFunctions Geometry Functions
    \ingroup GrpSystemNodeCoresDrawablesGeometry

    The GeoFunctions group contains different different functions to
    manipulate geometry. See \ref PageSystemGeoFunctions for a description.
*/

/*! \defgroup GrpSystemDrawablesGeometryProperties Geometry Properties
    \ingroup GrpSystemNodeCoresDrawablesGeometry

    osg::GeoProperty is the wrapper class for the geometry's attributes. They
    encapsulate and abstract the specific types of the attributes and offer an
    interface that is targeted at glVertexArrays. To simplify usage they also
    have a generic interface, which is easy to use, but not as efficient as
    the specific one.   

    The class hierarchy for the Properties looks very complicated, but
    isn't really, templates and traits are just not easy to handle in doxygen.
    See \ref PageSystemGeoPropertyClasses for a description.
    
*/

/*! \defgroup GrpSystemDrawablesGeometrySimpleGeometry Simple Geometry
    \ingroup GrpSystemNodeCoresDrawablesGeometry  

    SimpleGeometry combines a number of functions to create some specialized 
    geometry very easily. See \ref PageSystemSimpleGeometry for a description.
*/

// the page(s)

/*! \page PageSystemGeometry Geometry

Geometries in most cases define what's being rendered. Note that Geometries
don't necessarily have to be leaves of the tree, as due the Node/Core divison
every Node keeping a osg::Geometry Corehas children anyway, which are used
just as all other osg::Node's children. Geometry has to be flexible, to
accommodate the needs of the application. Different data types for the data
that defines the geometry are useful, as well as different indexing
capabilities to reuse data as much as possible. On the other hand, it also has
to be efficient to render. Flexibility and performance don't always go well
together, thus, there are some simplifications to make.

\section PageSystemGeoProperties Properties

OpenSG geometry is modeled closely following OpenGL. The data that make up the
geometry are stored in separate arrays. Positions, Colors, Normals and Texture
Coordinates all have their own arrays, (or osg::MField, to stay in OpenSG
terminology). As OpenGL can handle a lot of different formats for the data,
some of which might be more appropriate due to speed and memory consumption
than others, depending on the application, OpenSG features different versions
of this data, allowing pretty much all the variants that OpenGL can handle. To
allow that with type safety and without having a separate geometry class for
every possible combination the data fields are stored in separate field
containers, a so called osg::GeoProperty. There are separate osg::GeoProperty for
different attributes, and variants for different data types for each kind of
osg::GeoProperty. The most prominent types are probably osg::GeoPositions3f for osg::Pnt3f
positions, osg::GeoNormals3f for osg::Vec3f normals, osg::GeoColors3f for osg::Color3f colors and
osg::GeoTexCoords2f for osg::Vec2f texture coordinates, but other variants are
possible. 

As properties only have a single field they can mimic that field by exposing
parts of the standard osg::MField interface for their contents, so you can use
a GeoProperty pretty much just like an osg::MField. One problem with the type
variety is that writing functions that work on every type of property can
become tedious, as you have to have a big switch for every kind of data that
could arrive. To make that easier for every property there is defined generic
format, e.g. for Positions the format is osg::Pnt3f. A property has a
getValue()/setValue() interface for these generic types, i.e. every property,
no matter in what format it stores the data, can be used as if it used the
generic format. Of course, this is not as efficient as directly accessing the
data, but if speed is not the highest priority or as a fall-back it's quite
useful. And, finally, osg::GeoProperty features an interface for OpenGL vertex
arrays, giving access to the data and the types involved, which is used for
rendering.

In addition to the above-mentioned data there are some other osg::GeoProperty.
OpenSG allows multiple primitive types per geometry, i.e. you can freely mix
triangles, triangle strips and polygons in a single geometry node. The
osg::GeoPTypes property defines the type of the primitives used. Right now, it only
exists as a osg::GeoPTypesUI8 variant, but others may follow. The number of
vertices per primitive is defined by another property, the osg::GeoPLengths
property. This, too, only exists in a osg::GeoPLengthsUI32 variant right now. 

\subsection PageSystemGeoPropertyClasses Class Structure 

The basic idea of the GeoProperty class structure is very simple. The actual
implementation looks complicated, but is primarily so to simplify reuse of
code and simplify extensions. If the ideas behind the structure are
understood, the actual code doesn't look so bad any more. 

All properties are derived from osg::Attachment, so they can directly be used as
attachments and attached to an osg::AttachmentContainer.

There are two primary types of GeoProperties: osg::AbstractGeoProperty and
osg::GeoProperty.

osg::AbstractGeoProperty describes the different kinds of attributes a Geometry
can have: Positions, Colors, Normals, TexCoords, Indices, Types and
Lengths. These are abstract, they have no specified data type and thus
cannot be instantiated. They are the kinds of properties, not the actual
properties. Use them in cases where you want to be able to use any actual type
of data for a specific kind of property.

osg::GeoProperty describes the concrete versions of the properties. These are
typed, can be instantiated, and these are the versions normally used by an
application to set up their osg::Geometry node cores. All the concrete classes
like osg::GeoPositions3f and osg::GeoTexCoords2f are typedefs of this class.

The actual interface to access the Properties has been factored out into
separate classes, osg::GeoPropertyArrayInterface and
osg::GeoPropertyInterface. Both of these are supported by all kinds and types
of properties, the distinction is made for future extensions.

osg::GeoPropertyArrayInterface defines a general, type-independent interface to
all kinds of properties. It is very similar to the interface used by
OpenGL's Vertex Arrays, i.e. it is possible to access the type,
dimensionality and stride of the data, as well as getting access to the
base pointer. This interface is primarily used for rendering. \dev The main
reason for this interface is future extensibility to allow non-Property
data to be used in osg::Geometry nodes, e.g. to use data that is stored in
external libraries inside OpenSG. \enddev 

osg::GeoPropertyInterface is the typed interface. Thus there are specific
classes for every kind of property, which use the GenericType of the specific
kind in their interface. This interface is the one that models the osg::MField
interface, it has functions to add values to and change values of the property.

\dev 

Do we need a subValue here? DR 

\enddev

\hint 

Some rules of thumb:

- Everywhere you actually want to create a new property you have to use the
  typed versions like osg::GeoPositions3f, as they are the only ones that
  actually contain data.

- To write functions that can handle arbitrary types of data, use abstract
  property pointers and the generic interface to access the data.
  \code
  GeoColorsPtr col = GeoColorsPtr::dcast(geo->getColors());
  if(col == NullFC)
  {
      FWARNING(("Downcast failed!\n"));
      return;
  }
  beginEditCP(col, Geometry::ColorsFieldMask);
  col->push_back(Color3f(1, 0, 1));
  ...
  endEditCP(col, Geometry::ColorsFieldMask);
  \endcode
  This may incur some overhead, as the data might have to be converted to the
  actual data type of the property.

- If you know that all the geometry your function has to work on has been
  created yourself using a single type of property you can just downcast to
  that type and use the interface of the osg::MField that holds the data
  directly. For safety reasons you should make sure the downcast suceeded.
  \code 
  GeoColors3ubPtr col = GeoColors3ubPtr::dcast(geo->getColors());
  if(col == NullFC)
  {
      FWARNING(("Downcast failed!\n"));
      return;
  }
  MFColor3ub *c = col->getFieldPtr();
  beginEditCP(col, Geometry::ColorsFieldMask);
  c->push_back(Color3ub(255, 0, 255));
  ...
  endEditCP(col, Geometry::ColorsFieldMask);
  \endcode
  This is the most efficient way to access the data.
  
- The most general is the osg::GeoPropertyArrayInterface. If you want to be
  able to support all kinds of osg::Geometry properties this is the way to go.
  It doesn't differentiate between property types, so this is as general as it
  gets. There are probably not many situations that need this kind of
  generality, the renderer and some very generic osg::Geometry optimizations
  like the osg::createSharedIndex and osg::createSingleIndex are the only
  places where it's used right now.

\endhint

\section PageSystemGeoIndexing Indexing

Using these properties it is possible to define geometry. Note that OpenSG
inherits the constraints and specifications that concern geometry from OpenGL.
Vertex orientation is counterclockwise when seen from the outside, and concave
polygons are not supported.

\image html geo_nonindexed.png "Non-Indexed Geometry"

\image latex geo_nonindexed.eps "Non-Indexed Geometry" width=8cm

One additional advantage of separating properties from Geometry is the ability
to share properties between geometry osg::NodeCore s. As geometries can only
have one material right now that's useful for simplifying the handling of
objects with multiple materials.

This simple geometry has one problem: there is no way to reuse vertex data.
When a vertex is to be used multiple times, it has to be replicated, which can
increase the amount of memory needed significantly. Thus, some sort of
indexing to reuse vertices is needed. You can guess what's coming? Right,
another property.

Indices are stored in the osg::GeoIndices property, which only exists in the
osg::GeoIndicesUI32 variant right now. When indices are present the given lengths
define how many indices are used to define the primitive, while that actual
data is indexed by the indices.

\image html geo_indexed.png "Indexed Geometry"

\image latex geo_indexed.eps "Indexed Geometry" width=8cm

Indexed geometry is very close to OpenGL, and probably the most often used
type of geometry. It doesn't handle all the cases, though.

Sometimes vertices need different additional attributes, even though they have
the same position. One example are discontinuities in texture coordinates,
e.g. when texturing a simple cube. The edges of the cube don't necessarily use
the same texture coordinate. To support that a single indexed geometry has to
replicate the vertices.

To get around that you need multiple indices per vertex to index the different
attributes. Adding an index for every attribute would blow up the geometry
significantly and not necessarily make it easier to use. We decided to use
another way: interleaved indices.

\image html geo_multiindexed.png "Multi-Indexed Geometry"

\image latex geo_multiindexed.eps "Multi-Indexed Geometry" width=8cm

Interleaved indices require every vertex to hold multiple indices. Which index
is used for what attribute is defined by a separate indexMapping field. The
indexMapping field is a osg::UInt32 osg::MField. The possible values are bitwise
combinations of the available attribute masks: osg::Geometry::MapPosition,
osg::Geometry::MapNormal etc. The length of the indexMapping defines how many
indices are used per vertex. If it's not set a single index for all available
properties is used (or none at all).

In addition to the properties geometry keeps a osg::MaterialPtr to define the
material that's used for rendering the geometry (see \ref PageSystemMaterial) and
a flag that activates caching the geometry in OpenGL display lists. As
geometry rendering is not optimized very much right now that's the best way to
get decent performance. Display lists are turned on by default.

\section PageSystemGeometryIterators Geometry Iterators

The osg::Geometry setup is very nice and flexible to define: you can mix different
kinds of primitives in an object, you can have properties and different kinds
and the indexing allows the reuse of some or all of the data.

From the other side of the fence things look different: if you want to walk
over all triangles of a geometry to calculate the average triangle size or the
surface area, or for calculating face normals or for whatever reason you have
to take care of all the flexibility and be prepared for lots of different ways
to define geometry.

To simplify that the concept of a geometry iterator has been introduced. A
geometry iterator allows to iterate over a given geometry primitive by
primitive, face by face (a face being a triangle or quad), or triangle by
triangle. 

All of them are used like STL iterators: the osg::Geometry has methods to return
the first or last+1th iterator, and to step from one
element to the next. They can also unify the different indexing
variants: when using an iterator you can access the index value for each
attribute of each vertex of the iterator separately. Or you can directly
access the data that's behind the index in its generic form, which is probably
the easiest way of accessing the data of the osg::Geometry.

\example The following loop prints all the vertices and normals of all the
triangles of a geometry:

\code
for(it = geo->beginTriangles(); it != geo->endTriangles(); ++it)
{
    std::cout << "Triangle " << it.getIndex() << ":" << std::endl;
    std::cout << it.getPosition(0) << " " << it.getNormal(0) << std::endl;
    std::cout << it.getPosition(1) << " " << it.getNormal(1) << std::endl;
    std::cout << it.getPosition(2) << " " << it.getNormal(2) << std::endl;
}
\endcode

\endexample

If you're used to having a separate Face object that keeps all the data for a
face, the Iterators pretty much mimic that behavior. The one thing you can't
do using iterators is changing the data. To do that you have to use the Indices
the Iterators give you and access the Properties directly. Be aware that the
Iterators hide all data sharing, so manipulating data for a face the iterator
gives you can influence an arbitrary set of other faces.

\subsection PageSystemPrimitiveIterator Primitive Iterator

The osg::PrimitiveIterator is the basic iterator that just iterates through the
osg::GeoPTypes property and gives access to the primitive's data. It is useful
to solve the index mapping complications and to get access to the generic
data, but it's primarily a base class for the following two iterator types.

\subsection PageSystemFaceIterator Face Iterator

The osg::FaceIterator only iterates over polygonal geometry and ignores
points, lines and polygonal primitives with less than three vertices. It also
splits the geometry into triangles or quads. 

\subsection PageSystemTriangleIterator Triangle Iterator

The osg::TriangleIterator behaves like the osg::FaceIterator, but it also splits
Quads into two triangles, thus it does an implicit triangulation. As OpenSG
just like OpenGL doesn't support concave geometry that's not as hard as it
sounds.

\subsection PageSystemLineIterator Line Iterator

The osg::LineIterator only iterates over line geometry and ignores points,
polygonal primitives and line primitives with less than two vertices. It
splits line strips and loops into single lines.

\subsection PageSystemEdgeIterator Edge Iterator

The osg::EdgeIterator (currently) only iterates over line geometry and ignores
points, polygonal primitives and line primitives with less than two
vertices like the osg::LineIterator does, but it leaves line strips and loops
as they are. This iterator will make more sense in a future version, where it
returns the edges of the other primitives as single lines or line loops as
well.

\dev For all polygonal primitives probably except GL_POLYGON itself the edges
could be returned as single lines conceptually similar to the implicit
triangulation of the osg::TriangleIterator. For a polygon it seems reasonable
to me to reinterpret it as line loop. Shouldn't we overload getType() to return
the interpretation? \enddef

The iterators can also be used to indicate a specific primitive/face/triangle/line.
Each of these has an associated index that the iterator keeps and that can be
accessed using getIndex(). A new iterator can be used to seek() a given
primitive/face/triangle again and work on it. This is used for example in the
osg::IntersectAction.


\subsection PageSystemSimpleGeometry Simple Geometry

OpenSG does not have NodeCores for geometric primitives like spheres, cones,
cylinders etc. Instead there are a number of utility functions that can create
these objects. They can be created as a ready-to-use node and as a naked node
core. In most cases we tried to mimic the VRML primitive semantics, so if
you're familiar with VRML you will feel right at home.

\dev We definitely need a Teapot here... :) \enddev

\subsubsection PageSystemSimpleGeometryPlane Plane

osg::makePlane creates a single subdivided quad.

\subsubsection PageSystemSimpleGeometryBox Box

osg::makeBox creates a box around the origin with subdivided sides.

\subsubsection SimpleGeometryCone Cone

osg::makeCone create a cone at the origin.

\subsubsection PageSystemSimpleGeometryCylinder Cylinder

osg::makeCylinder create a cylinder at the origin.

\subsubsection PageSystemSimpleGeometryTorus Torus

osg::makeTorus create a torus at the origin.

\subsubsection PageSystemSimpleGeometryConicalFrustum ConicalFrustum

osg::makeConicalFrustum creates a truncated cone at the origin.

\subsubsection PageSystemSimpleGeometrySphere Sphere

There are two ways to create a sphere. osg::makeSphere uses a icosahedron as a
base and subdivides it. This gives a sphere with equilateral triangles, but
they do not correspond to latitude or longitude, which makes it hard to get
good texture mapping on it. As every subdivision step quadruples the number of
triangles, it is also hard to control the complexity of these kinds of spheres.

osg::makeLatLongSphere on the other hand creates a
sphere by simply usign a regular subdivision of latitude and longitude. This
creates very small polygons near the poles, but is more amendable to texture
mapping and gives finer control of the resoltuion of the sphere. 

\subsubsection PageSystemSimpleGeometryExtrusion Extrusion Geometry

osg::makeExtrusion creates a pretty general extruded geometry. It works by sweeping a given cross
section, which can be given clockwise or counterclockwise, across a spine. For
every spine point an orientation and a scale factor are specified. The
beginning and the end of the object can be closed by caps, but for the capping
to work the cross section has to be convex. The resulting geometry can
be refined as a subdivision surface (no idea which subdivision scheme is
applied, anyone care who knows care to take a look?). Optionally normals and
texture coordinates can be generated


\subsection PageSystemGeoFunctions Helper Functions

A number of helper functions can be used in conjunction with manipulating and
optimizing geometry.

\subsubsection PageSystemGeoFunctionsCalcNormals Normal Calculation

A common problem for self-created geometry or for geometry loaded from simple
file formats are missing normals. Normals are needed for proper lighting,
without them objects will either be black or uniformly colored.

Normals can be calculated either for every face or for every vertex.

Face normals, as calculated by osg::calcFaceNormals, are only unique for a given
triangle or quad. The resulting object will look faceted, whcih may or may not
be the desired effect. This will also work for striped or fanned models, as
OpenSG doesn't have a per-face binding and uses multi-indexed per-vertex
normals for this.

Vertex normals are calculated for every vertex and allow a shape to look
smooth, as the lighting calculation is done using the vertex normals and
interpolated across the surface. They can be calculated using two different
methods.

osg::calcVertexNormals(GeometryPtr geo) will just average all the normals of
the faces touching a vertex. It does not unify the vertices, i.e. it does not
check if a vertex with given coordinates appears in the position property
multiple times, the geometry has to be created correctly or be run thrugh
osg::createSharedIndex.

The disadvantage of osg::calcVertexNormals(GeometryPtr geo) is its
indiscriminative nature, it will average out all the edges in the object. The
alternative is osg::calcVertexNormals(GeometryPtr geo,  Real32 creaseAngle),
which uses a crease angle criterion to define which edges to keep. Edges that
have an angle larger than \a creaseAngle will not be averaged out. It won't
always work for striped geometry. It will process it, but if a stripe point
needs to be split because it has two normals, that won't be done. The same
sharing caveat as given above applies. 

Calculating vertex normals with a crease angle sounds simpler than it is,
especially if the calculation should be independent of the triangulation of the
object. Thus the algorithm is relatively expensive and should be avoided in a
per-frame loop.  There are some ideas to do the expesive calculations once and
quickly reaverage the normals when needed. These have not been realized, if you
need this or even better want to implement it, notify us at info@opensg.org.

\subsubsection PageSystemGeoFunctionsMakeGeo Geometry Creation

Setting up all the objects needed to fully specify an OpenSG Geometry can be a
bit tedious. So to simplify the process there are some functions that take data
in other formats and create the corresponding OpenSG Geometry data.

Right now there is only one function to help with this,
osg::setIndexFromVRMLData. It takes separate indices for the different
attributes, as given in the VRML97 specification, together with the flags that
influence the interpretation of these indices, and sets up the indices, lengths
and types properties of the given geometry. 


\subsubsection PageSystemGeoFunctionsOptimizeGeo Geometry Optimization

OpenSG's Geometry structure is very flexible and pretty closed modeled on
OpenGL. But not all of the Geometry data specification variants are similarly
efficient to render. The functions in this group help optimizie different
aspects of the Geometry.

osg::createOptimizedPrimitives takes a Geometry and tries to change it so that
it can be rendered using the minimum number of vertex transformations. To do
that it connects triangles to strips and fans (optionally). It does not change
the actual property values, it just creates new indices, types and lengths. The
algorithm realized here does not try a high-level optimization, instead it is
optimized for speed. Due to its pseudo-random nature it can be run multiple
times in the same time a more complex algorithm needs, allowing it to try
different variants and keeping the best one found. Or, if execution time is a
problem, it can be run only once and create a very quick result that is good,
but not optimal.

osg::createSharedIndex tries to find identical elements in the Geometries
Properties and remove the copies. It will not actually change the Property
data, it will just change the indexing to only use one version of the data.
This is a necessary preparation step to allow osg::createOptimizedPrimitives to
identify the triangles it can connect to form stripes and fans.

osg::createSingleIndex resorts the Geometry's Property values to allow using
a single index (in contrast to interleaved multi-indices) to represent the
Geometry. To do that it might have to remap and copy Property values, as well
as index values. While multi-indexing can be very efficient datawise, as as
much of possible is shared, for rendering it is problematic. OpenGL doesn't
know multi-indexing, thus for multi-indexed Geometry the more efficient OpenGL
geometry specifiers like VertexArrays can't be used, which can have a
significant impact on performance, especially for dynamic objects.

UInt32 calcPrimitiveCount ( GeometryPtr geo,
                            UInt32 &triangle, UInt32 &line, UInt32 &point );


\subsubsection PageSystemGeoFunctionsShowNormals Normal Visualisation

For debugging it can be useful to actually see the normals of an object, as
that allows making sure that the normals point in the expected direction and
that normals are really identical and not just pretty close. Every normal is
represented by a line of a user-defined length. 

As OpenSG doesn't have an explicit face/vertex binding mode there are two
different functions to create an object representing the vertex or face
normals. The application should know whether face or vertex normals are used in
the given geometry. In general it is safe to assume vertex normals are used.

osg::calcVertexNormalsGeo creates an object that shows the vertex normals of
the given geometry, while osg::calcFaceNormalsGeo creates an object that shows
the face normals.

\dev We should add something more general here. Forcing the app to know is not
nice. calcVertexNormalsGeo can already calc normals at the vertices and at the
centers of the tris, but the decision when to use which and which normal to use
is not clear. \enddev

*/