| 3.7. Geometry nodes features | ||
|---|---|---|
 | Chapter 3. Reading, writing, processing VRML scene graph |  |
An important descendant of TVRMLNode is
the TVRMLGeometryNode class. This is an abstract
class. All visible VRML nodes (in VRML 1.0 and 2.0) are descendants of this class.
TVRMLGeometryNode class defines a couple of
important methods,
overridden in each descendant. All of these methods take
a State parameter that describes
VRML state at given point of the graph
(this is typically obtained by a traverse callback),
since we need this to have full knowledge about node's geometry.
LocalBoundingBox and
BoundingBox methods calculate axis-aligned
bounding box of given node.
Axis-aligned bounding box is one of the
simplest bounding volume types. It's a cuboid with axes aligned
to base coordinate system X, Y and Z axes. It can be easily
expressed as a pair of 3D points. In our engine we require
that the points' coordinates are correctly ordered, i.e.
X position of the first point must always be less or equal than
the X position of the second point, and analogously for Y and Z
values. We also have the special value for designating empty
bounding box. And while we're talking about empty bounding boxes,
remember to not confuse empty box with a box with zero
volume: a box with zero volume still has some position.
For example, a PointSet VRML node with only
one point has a non-empty bounding box with a zero volume.
A PointSet without any points has empty
bounding box.
I chose axis-aligned bounding boxes just because they are very simple to calculate and operate on. They have some disadvantages — as with all bounding volumes, there is some compromise between how accurately they describe bounding volumes and how comfortable it is to operate on them. But in practice they just work fast and are enough accurate.
LocalBoundingBox method returns a bounding
box of given object without transforming it (i.e. assuming that
State contains an identity transformation).
BoundingBox method
takes current transformation into account. Each descendant
has to override at least one of these methods.
If you override only LocalBoundingBox then
BoundingBox will be calculated by transforming
LocalBoundingBox (which can give poor bounding
volume, much larger than necessary).
If you override only BoundingBox then
LocalBoundingBox will be calculated by
calling BoundingBox with transformation matrix
set to identity matrix (this can make LocalBoundingBox
implementation much slower than a potential special
LocalBoundingBox implementation that knows that
there is no transformation, so no matrix multiplications have to be done).
VerticesCount and
TrianglesCount calculate triangles
and vertices count of given geometry.
LocalTriangulate and
Triangulate methods are available in the TShape
class. They calculate all the triangles of given geometry.
Use TShape.GeometryArrays if you want the full
information about every shape (including indexes, colors,
and all the other information required for efficient rendering).
If you want to control how detailed the triangulation should be:
Programmers can use
DefaultTriangulationSlices,DefaultTriangulationStacksandDefaultTriangulationDivisionsglobal variables.VRML / X3D authors can use the Geometry3D component - extensions: custom triangulation fields in node to control this.
Finally, my programs view3dscene and rayhunter allow you to control this by command-line options
--detail-quadric-slices <integer> --detail-quadric-stacks <integer> --detail-rect-divisions <integer>
| |  | |
| 3.6. Traversing VRML graph |  | 3.8. WWWBasePath property |