About HLR

Hidden Line Removal (also known as 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 lines (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.

This page discusses:

HLR Algorithms

Two HLR algorithms can be used to compute a HLR scene: an exact mode and a polyhedral mode.

  • Exact mode uses exact geometry
  • Polyhedral mode enables to compute HLR on non-exact geometry that can be found in third-party content, in CGR files (when working with the cache system, for example) or in specific V4 elements.

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

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.

For example, on screen in 2D Layout for 3D Design in HRD mode, the intersection profile of an object with a front cutting plane or a back clipping plane is not displayed. However, the HLR print result shows this intersection profile:

Shading with EdgesHRDHLR






Regarding transparent faces, HLR algorithms consider them as opaque which means that objects behind these faces are hidden. On the contrary, the HRD rendering mode considers transparent faces as completely transparent by default. To force the HRD mode to consider transparent faces as opaque, use the Perform transparent faces as opaque in HRD mode option.

Linetype

The linetype and the line thickness of drawn edges are taken into account in HLR rendering mode. This means that the linetype displayed in the print preview is the one you see in the 3D. For example, if the linetype is defined as "empty", edges are not displayed.

Exact and Polyhedral Modes

Some restrictions apply to both Exact and Polyhedral modes.

Exact mode

The behavior might not be correct in the following cases:

  • HLR behaves badly when the Display all elements using Z-buffer depth option is selected.
  • HLR behaves badly with points, certain kinds of line, texts and images as soon as they are in Z mode (that is, they participate in the Z-buffer).
  • Smooth edges cannot be removed when using HLR if the three following conditions are fulfilled at the same time:

    • The object has been designed in V5 (for example a CATPart).
    • The object was first created before V5R16.
    • The session uses the cache management OR only the CGR file of the CATPart object is inserted in the product, not the CATPart itself.

  • Smooth edges cannot be removed when using HLR if the tab viewpoint is CONIC.

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 example, 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.

Exact ModePolyhedral Mode




On top of the green cylinder, the Exact mode avoids SAG effects on printed curves.The clash between the green and blue bodies is supported in Polyhedral mode only.

The result of the Boolean operator between the two blue pad bodies is supported in both Exact and Polyhedral modes:



Note: When working with the Activating clipping frame, the Clip View or the Sectioning command in a 2D Layout for 3D Design or in 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.

See the 2D Layout for 3D Design User's Guide for more information about clipping commands and the Design Synthesis User's Guide for more information about sectioning.