CGAL users discussion list

[Ben Supnik <>]

Hi,

> The observer is used to construct the first 2 arrangements.

> > I can't quite see how the observer can keep track of the original
> > face, as
> > when the time when I input the arrangement, I don't input in the original
> > face information. Mind to be more explicit about it?

Right - if your arrangement isn't _already_ tagged on a per face basis, 
there's nothing you can do.  If it is tagged, the overlay traits make 
tracking easy, because the overlay gets a call for every face, and its 
inputs are the two faces in the two original arrangements that cover 
this face.  (And by definition of the overlay, each overlay face is 
small enough that it will overlap exactly one face in each of the source 
arrangements.)

So the real question is how to capture face information when you 
construct your original arrangement.

If you create your arrangement with piece-wise construction (e.g. 
inserting individual curves, possibly with the helpers that let you 
accelerate) the observer will get called when you add a segment that 
creates a face.

If you create your arrangement by inserting a whole pile of curves at 
once then the observer won't help - in that case you have to somehow 
recover the polygon information in some other way.  I do this by tagging 
data on my curve type - this lets me look at a face and pull meta data 
out of its boundary.  Note that in this case (inserting lots of curves) 
if your data is junk the set of faces you get out might be surprising, 
e.g. if there were intersecting curves.  I use different algorithms to 
interpret the curve data depending on how I want to cope with bogus input.

Whether bulk insert or piece-wise construction is fastest is pretty 
dependent on your data.

cheers
ben