Mirroring

Is there an nice way to accommodate mirroring objects in the schema?

If not, is it possible to propose one for the NextGen?

cross posted to: Mirroring · Issue #86 · buildingSMART/NextGen-IFC · GitHub

Isn’t that one of the simpler cases of a Cartesian transformation operation carried out with mapped items?

Indeed it is already possible using a non-uniform cartesian transformation within a mapped item, but I wouldn’t exactly call it “nice” :slight_smile:

With this method, perhaps some of the object is mirrored, and others not. This adds to the confusion about the implied constraints of how mapped representations should be used.

Further reading on this topic:

On this topic I’m a proponent of “less is more” and would want to rethink how mirroring (and other common geometric modifiers, like arrays) can be applied without overcomplicated the already complex representation tree.

Philosophical response coming up.

I think the ability to simply represent mirrored objects is what differentiates CAD from BIM. And by that, I mean that it is a CAD ability, not a BIM ability. What real world object is the same if you mirror it? I believe - and happy to be wrong on this - that a mirror of an object is really just a simplification/shortcut when modelling, and not representative of real-life part numbers, etc.

Ditto non-uniform transforms. Unless your house is made out of silly putty, you shouldn’t model all walls as cubes with non-uniform transforms. Maybe a beam or column with a non-uniform scale along the axis? But that should be explicit and not buried inside a geometric entity somewhere in your tree.

Personally, then, I’d like to see non-uniform mirrored transforms removed from IFC.

Cheers @angel.velez - I think everybody agrees on the distinction between CAD ability and BIM ability.

Personally, though I’m not quite sure I buy your argument. The utility to designers is just too great to ignore. Every BIM tool has a mirror feature, and it is semantically meaningful to the designer that when they author one, the mirrored version is also authored, therefore I would expect to see it in IFC too.

Since it is a CAD ability and not a BIM ability, that implies it should be a non-rooted entity, but does not disqualify its utility from IFC.