Re: [boost] [geometry] area, length, centroid, behavior

Subject: Re: [boost] [geometry] area, length, centroid, behavior
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2009-02-24 12:33:08

Barend Gehrels a écrit :
> I would like to hear the opinion of potential geometry users on the list 
> about the following.
> 
> For the three geometries, point, linestring (= sequence of points), 
> polygon:
> 
> What should the generic "area" algorithm return for point and linestring?
> - 0 (zero)
> - compiler error (so this function is not defined for them at all)
> - exception
> Personally I would feel for 0 (zero) because you can then request the 
> "area" in a generic collection, regardless of what is inside.

0, indeed.

> 
> What should the generic "length" algorithm return for polygon?
> - 0 (zero)
> - its perimeter
> - compiler error
> - exception

Rename length to perimeter, since that is more generic.

> 
> What should the generic "centroid" algorithm return for point and 
> linestring?
> - the point / the average its points
> - compiler error
> - exception

For all geometries, which are only sets of points, the centroid should 
be the average point.

> and for an invalid polygon (e.g. no points at all, or not closed)?
> - exception
> - return "false" (if centroid is passed as a reference parameter)
> - ? (we've not always the compiler error option here because points are 
> usually stored in a runtime collection)

Undefined behaviour.
An invalid polygon shouldn't be allowed in the first place.

Ideally, that should be prevented by the type system.