CGAL users discussion list

[Graviton <>]

> 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.

This is the route that I'm going to do my arrangement, but I don't really know how to utilize the observer to insert the face information whenever a face is created.

Any ideas?

On Sat, Nov 12, 2011 at 12:59 AM, Ben Supnik [via cgal-discuss] <[hidden email]> wrote:

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

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