This class computes and (optionally) renders the intersection of the geometry in the specified scene graph with the specified plane. In curve mode it computes a set of line segments corresponding to the intersection. In polygon mode it computes a set of polygon boundaries that can be used as a "cap" for clipped geometry. The intent is that the specified scene graph is one shape. In fact it can be any scene graph, but the result is always a single polygon with multiple boundaries (if necessary). The computed vertices (and indices in polygon mode) can be accessed using the GetShape() method. The curve or polygon may be exported to a file like any other Open Inventor geometry using SoWriteAction, SoSTLWriteAction, etc.

Curve mode:

By default we compute and render the curve(s) defined by the intersection of the scene geometry and the plane. This is fast and works reliably for any Open Inventor geometry that produces triangles when an SoCallbackAction is applied (almost all Open Inventor geometry nodes). The appearance of the intersection curve can be controlled like any Open Inventor geometry using SoMaterial, SoDrawStyle, etc. nodes. The application may need to use an SoPolygonOffset node to "push" the scene geometry behind the intersection curve (polygon offset does not affect the intersection curve because it is a line set). Curve mode is useful to see where a clip plane intersects the geometry. Because it is much faster than polygon mode it is useful as a preview of where the polygons will be generated (if possible).

Polygon mode:

When the polygon field is set to true, the node converts the intersection curves to polygon boundaries. All the boundaries are stored in an SoIndexedFaceSet node and an SoShapeHints node is created with the 'windingType' field set to ODD_TYPE. All boundaries have counterclockwise vertex ordering and the face normal vector is the opposite of the current plane normal vector (because for a clip plane the plane normal points toward the unclipped half space which is effectively toward the interior of the shape). The appearance of the polygon can be controlled like any Open Inventor geometry using SoMaterial, etc. nodes. There are cases where polygons cannot be constructed, for example if the geometry is self-intersecting.

The coordinates of the intersection boundaries can be accessed using the GetShape() method. This is an SoLineSet in curve mode and an SoIndexedFaceSet in polygon mode. In either case the coordinates are in the shape's attached SoVertexProperty node. Note that in curve mode the line segments are in random order and do not form a connected polyline. This allows the intersection geometry to be computed in minimum time and does not (visually) effect rendering. If you need the boundary(s) as a connected polyline for some purpose, use the polygon mode. Note that in polygon mode there are redundant vertices in the vertex list because it still contains all the line segments. To follow the boundaries in connection order use the coordinate indices in the SoIndexedFaceSet node.

Notes:

• Multiple boundaries in polygon mode.

  Currently does not test if whether multiple boundaries are "holes" or "islands". Rendering should be correct in either case, but eventually some optimization could be done if all boundaries are disjoint.

• Memory.

  Memory allocated for vertices and indices is re-used on each recomputation and is not freed until the node is destroyed. This is best for performance. Eventually there should be a method to force memory to be freed.

Limitations:

• Valid geometry.

  For polygon mode to work correctly, the geometry must obey the same rules as a valid STL geometry. In particular the geometry must form a closed solid and must not be self-intersecting. All the intersection boundaries must be closed and must not intersect any other boundary. However in curve mode, there are no limitations. For example, the node can be used to render the intersection of a surface with a plane.

• Triangulation of the polygon.

  This node simply computes the intersection boundary(s). For rendering it relies on Open Inventor to convert the polygon to triangles. In many cases the resulting triangles have a poor (large) aspect ratio.

• Floating point precision.

  Curve mode should work reasonably well with any geometry and any coordinates. However constructing polygon boundaries is very sensitive to floating point precision problems. If the results are not good, try adjusting the tolerance value.

• Redundant coordinates.

  In polygon mode the SoVertexProperty node contains all the coordinates computed for the intersection line segments. The SoIndexedFaceSet node only indexes a subset of these vertices (typically about half). Not removing the unused vertices saves time and has no effect on rendering. To get only the coordinates that are actually used, extract vertices based on the index values in the SoIndexedFaceSet node. Eventually could be an option.

• Performamce.

  This is currently a "brute force" implementation and performance could be better.

FILE FORMAT/DEFAULT

PlaneGeometryIntersection {

scene	NONE
plane	1 0 0 0
polygon	FALSE
automatic	TRUE
tolerance	1e-6

}

Reference