Classification: UNCLASSIFIED Caveats: NONE > What happens in practice (and it is bad practice, but it happens often > enough that it usually has to be dealt with) is that the modeler may > subtract a region or combination containing a region from some higher- > level assembly in order to eliminate some overlap problem as seen in > this example hierarchy:
> u toplevel_group > u body_assembly > u left_body.r > - engine_assembly > u top_engine.r > u inner_engine.r > u ... > u right_body.r > u internal_components > u engine_assembly > u top_engine.r > u inner_engine.r > u ... > u armor_assembly > u ... > Here, the modeler subtracts the engine_assembly from the left_body.r > region in order to (presumably/usually) remove an overlap between the > engine and the body. Instead of figuring out exactly what regions > are involved in the overlap, the modeler took the easy route out and > just subtracted the entire region. This is generally very bad > practice (usually due to performance), but it happens and it's still > considered "valid". > When traversing the hierarchy, one of the paths to top_engine.r is > toplevel_group->body_assembly->engine_assembly->top_engine.r .. > which is not a "positive" region, it's a negative one since > engine_assembly is subtracted within body_assembly. It appears that somewhere along the way, BRL-CAD has been made to implement only the old GIFT structures rather than the more general combinatorial geometry that was meant to replace them. Far from being "bad practice", the user really ought to be able to subtract *any* object geometry from any other in the course of modeling, without having to perform any additional work himself. Also, what happens when you subtract a region that itself contains a (nested) subtraction? The software really ought to "optimize" the tree structure to produce a canonical form, if it requires such a form for evaluation. Classification: UNCLASSIFIED Caveats: NONE
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