Feature Line Picking

This is a special mode in which a pick on a single element or node is propagated across a connected surface mesh, and all such elements or nodes on that surface are selected. The edges of the surface are defined by "feature lines", which are edges at which the difference in angle between adjacent elements is greater than the stipulated Feature Angle.

It is only applicable to meshes of 2D and 3D elements (ie Shells, Solids, Thick shells and Segments) and to Nodes on such a mesh.

In this example using a crudely meshed seat, made from a single Part, the user has selected a single Shell element in the middle of the backrest with the feature angle set to 20 degrees.

Selection has propagated to the top and side edges, but has stopped where the backrest meets the bottom because the "break angle" in the mesh at this point exceeds 20 degrees.

The following rules apply to feature line picking:

• Feature line propagation only applies to single picks on eligible elements or nodes. It will be ignored if an area pick of any type is used.
  
  
• For 3D elements (solids and thick shells) propagation will be from the selected element face only, and will not "track" across multiple faces of this or subsequent elements. This has been found to give a more natural determination of a "surface".
  
  
• For 2D elements (shells and segments) the winding order of the nodes in elements meeting at an edge are compared, and the computed normal of an element with nodes numbered in the opposite direction will be reversed before angular comparison. This means that adjacent elements can be "upside down" and still pass the angle test.
  
  
• For nodes the surface outward normal is determined from the average of the elements meeting at the node initially picked, and then extrapolation across the surface proceeds as for elements above with the nodes on eligible elements being selected. The results may be unpredictable if the initial node is badly chosen, for example a node on an edge or corner. If a node is connected to both 2D and 3D elements the 2D elements only will be used for normal determination and subsequent propagation.

Feat_Abs : Optional "absolute" feature angle

An option in Feature Line picking above is the definition of an "Absolute" angle. This applies exactly the same logic as described above with the additional restriction that:

• the angular difference between the outward normal of the original node/element and this one must also not exceed the feature angle.

This has the effect of limiting propagation over curved surfaces where the angular difference between adjacent elements may be small, but the overall surface curvature exceeds the specified angle.

In the first image here, using a feature angle of 30 degrees, no "absolute" angle has been specified. A pick on the top of the head has propagated down to all the elements visible here since the angular difference between adjacent elements is < 30 degrees even though that between the top of the head and the chin is closer to 90 degrees.

The second image is exactly the same, except that the "absolute" angle option has been set, meaning that only elements with normals within 30 degrees of the original element's normal (shown approximately in red here) have been selected.

As this example shows propagation across the curved surface has been limited, which can be useful for selecting a geometrical subset of elements on a curved surface.