CGAL users discussion list

[Efi Fogel <>]

On Wed, Nov 7, 2012 at 10:56 PM, Shriramana Sharma <> wrote:

On Wed, Nov 7, 2012 at 10:08 PM, Efi Fogel <> wrote:
> What does need an explanation?
> The polygon Q in the second case is invalid. All bets are off.

Hi -- I understand that *by the book* the polygon is "invalid", but
I'm asking: if it is invalid, how come the algorithm for intersection
detection still works? Doesn't the algorithm check for validity? And
if it doesn't, then what is the point in having a validity condition?
(I mean, like saying "you need a visa to get into our country" but not
having the customs people actually check for it?)

It's more like saying "you need unleaded petrol for your car" but not having actually check for it before you put it into your car.

Or is it just that for "invalid" polygons the output is undefined, and
maybe sometimes right and sometimes wrong?

IMHO not checking for validity seems a good way to lead to pitfalls,
but I'm not an expert programmer so I'd appreciate an explanation as
to why things are the way they are.

Checking for validity, sometimes, is more expensive then running the procedure itself.
If we could check for validity, perhaps we could have also fix those invalid polygons....
 

Thanks for your patience.

--
Shriramana Sharma

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