# ExteriorPyr

## DESCRIPTION

Creates a pyramid composed of the unshared faces or unshared edges of the input.The exterior of a pyramidal structure may be useful to give the visual sense of the object without the cost of rendering the entire object. ExteriorPyr culls out shared faces in the input to form a face-compressed output pyramid of the unshared faces -- the exterior of the input pyramid is defined to be the set of unshared faces. If the unshared edges option is chosen, ExteriorPyr culls out shared edges in the input to form a edge-compressed output pyramid of the unshared edges.

If the input is uncompressed (or compressed at or below the face level), the cxPyrActive() routine is used to count the number of accesses of each face. Faces with two or more accesses are suppressed in the output. An input formed by expanding a compressed pyramid may have multiple, unshared copies of faces. In this case cxPyrMerge is called at each level to remove duplicate elements.

If the input is compressed above the face level, ExteriorPyr uses a hash table to count the number of times each face is represented. As an optimization, a face is removed from the hash table when it is found for the second time. This means that the module actually outputs faces that appear an odd number of times (in practice, the only odd number of occurrences is one); inputs that share a face three times will thus create an unexpected output.

The same algorithm applies for the unshared edges option. If the input is uncompressed (or compressed at the edge level), the cxPyrActive() routine is used to count the number of accesses of each face. If the input is compressed above the edge level, ExteriorPyr uses a hash table to count the number of times each edge is represented.

## INPUTS

**Port:** Input

**Type:** Pyramid

**Constraints:** 1..-layer

**Constraints:** optional-baseLat

**Constraints:** n-D compression

**Constraints:** n-compression type

The pyramid structure from which to copy exterior faces.

## WIDGETS

**Port:** Unshared

**Type:** Option Menu

**Menu Item:** Edges

**Menu Item:** Faces

Select using unshared faces or unshared edges to generate the exterior.

## OUTPUTS

**Port:** Output

**Type:** Pyramid

**Constraints:** 1..2-layer

**Constraints:** optional-baseLat

**Constraints:** 1..2-D compression

**Constraints:** unique..multiple-compression type

The face-compressed output of unshared faces.

## KNOWN PROBLEMS

For compressed input, the output is the set of faces or edges shared an odd number of times.## SEE ALSO

BoundBoxPyr, CompressPyr, ExpandPyr, IsosurfacePyr, PyrToGeom, SlicePyr.[Documentation Home]

© The Numerical Algorithms Group Ltd, Oxford UK. 2000