BOT_REPAIR(nged)

It is quite common in CAD modeling to encounter a triangle mesh (a Bag of Triangles or "BoT" object in BRL-CAD terms) that does not define an enclosed volume in space. Sometimes this is intentional (such as "plate mode" meshes whose volume is implicitly defined) but more commonly it represents a modeling error - i.e. the intent was to define a closed volume in space, but the mesh elements as defined do not successfully do so.

In the fully general case it is impossible to repair such a mesh without manual intervention - if a huge majority of a spherical mesh is cut away, for example, there is no way to know automatically if the original modeling intent was to define the whole sphere or to define (say) a lens shape. However, in many cases the flaws are small compared to the overall shape, and there are simply "holes" in the mesh that can be filled in to produce a valid manifold. There are also even more basic categories of problem where the geoemtry itself does (or rather, can) define a closed volume, but the topology of the mesh itself is incorrect.

For those cases which may be suitable for automatic repair, the bot repair command attempts to produce valid manifold solid meshes automatically using mesh repair algorithms. By default, topological repairs and hole filling for holes with a size up to five percent of the overall mesh area are the techniques that bot repair will attempt to employ. There are also options to adjust the hole sizes considered viable for repair attempts. However, be aware that attempting to repair holes that are very large may produce unexpected results - an output may be closed and manifold, but end up changing the shape of a mesh. Parts of the mesh that are intended to be separate may end up being connected to "close" the shape, losing concave features. Outputs should be inspected - particularly when large hole sizes are specified - to insure the new shape is true to the original intent as well as topologically valid.

A "repaired" mesh may either be output as an overwrite of the input mesh (an "in-place" repair) or (if only one input mesh is specified) written to a user-specified object name as a new mesh. Note that in-place repair is destructive - the original mesh is lost - but that method of repair is very useful when correcting large numbers of meshes in a hierarchical model.

If an input mesh is already a valid solid, no work is done. If a repair cannot be successfully made, an error is returned.

A number of checks are made on a repaired mesh to try and ensure the processing results are valid - in particular, if the bounding box size changes or the surface area of the mesh decreases that is interpreted as an error.

Note that this command will not (currently) perform any work on plate mode or surface mode BoTs. If a user is not aware that they have those types of BoT objects in a database, they may be surprised at bot repair not operating on those objects. To verify the type of an individual bot, use the command bot get type objname. To find all plate mode bots in a database the command search -type bot -type plate may be used and to find all surface mode BoTs the command search -type bot ! -type plate ! -type solid will work. If a BoT is incorrectly identified as surface, its type may be changed with the command bot set type objname solid. Note, however, that changing a BoT’s type and repairing it when it is intended to be plate mode or surface may result in incorrect geometry per modeling intent, even if the result ends up topologically valid.

BRL-CAD Team

Reports of bugs or problems should be submitted via electronic mail to devs@brlcad.org