HLR

• definition
• about HLR algorithms
• restrictions
• behavior in specific applications

Definition

Hidden Line Removal (HLR) lets you print in vector mode the viewed edges and outlines of 3D objects. It is an extension of the Wireframe rendering mode where (or segments of lines) visually occluded by surfaces are not drawn. Note that in HLR mode, the color of drawn edges is the color of the surface they belong to.

About HLR Algorithms

Two HLR algorithms can be used to compute a HLR scene:

• Exact mode which uses exact geometry
• Polyhedral mode which enables to compute HLR on non-exact geometry that can be found in third-party data, in CGR files (when working with the cache system, for instance) or in specific Version 4 elements.

The Exact mode is more CPU- and memory-intensive than the Polyhedral mode but gives a more accurate result.

Note also that HLR and Dynamic Hidden Line Removal are not computed the same way because Dynamic Hidden Line Removal is primarily based on the graphics card. Therefore, discrepancies might appear between the two results.

Restrictions

Exact mode

As this mode is based on exact geometry, the result is more precise. However as explained above, this mode is CPU- and memory-intensive and some restrictions might occur in specific cases.
For instance, you might encounter problems with clashing bodies because some geometrical elements might not be generated. To by-pass this limitation, you can manually resolve the intersection topology using an appropriate sequence of Boolean operations.

In Exact mode, the smoothness of edges is not taken into account and therefore, cannot be switched off or on.

Polyhedral mode

In this mode, the geometry's complexity (in terms of number of triangles for each face of a surface) is limited. Attributes applied to edges, such as the linetype or the thickness, are those of the original edges as far as possible. When it is not possible to use the original attributes, default attributes are applied instead.

In Polyhedral mode, the smoothness of edges is not taken into account and therefore, cannot be switched off or on.

Behavior in Specific Applications

When working with the Activating clipping frame, Clip View or Sectioning command in a 2D Layout or DMU Sectioning context, the polyhedral HLR mode is used. In this case, some specific geometry elements such as curved pipes or cylinders might not be affected by clipping views, sections or polygonal traps during the HLR rendering process.