Geometry Boost List: [ggl] bg::unique and precision

Subject: [ggl] bg::unique and precision
From: Barend Gehrels (barend)
Date: 2011-10-14 02:31:55

Hi,

On 13-10-2011 7:09, Simonson, Lucanus J wrote:
>
> *From:*ggl-bounces_at_[hidden] 
> [mailto:ggl-bounces_at_[hidden]] *On Behalf Of *V D
> *Sent:* Wednesday, October 12, 2011 9:57 PM
> *To:* Generic Geometry Library Discussion
> *Subject:* [ggl] bg::unique and precision
>
> ...
>
> >And indeed, creating a polygon from this WKT and running bg::unique 
> will not drop the duplicate point (see points in bold).
>
> >Is there any way of specifying some kind of epsilon for the unique 
> algorithm ? 1e-10 would be more than enough in my case.
>
> An easy thing to try is the simplify algorithm we were just discussing 
> with a threshold of 1e-10.  This would eliminate both duplicate and 
> collinear points.  If you want to retain collinear but drop duplicate 
> then you'll need to create your own predicate that uses an epsilon I 
> would guess.
>

This is indeed the easiest thing to do.

The unique algorithm was designed to work with a user-defined strategy 
as well. But there is no version yet which takes it. You might try to call

dispatch::unique
<
             typename tag<Geometry>::type,
             Geometry,
             policy
 >::apply(geometry, my_policy);

where my_policy is your own policy (a strategy is a policy - in 
Boost.Geometry strategies are normally coordinate system dependant, 
policies are not, but that is a detail). You could copy it from 
geometry::equal_to and give it a tolerance. This will be harder of 
course than just call simplify, but if you have it, it is welcome to be 
contributed back. ;-)

Regards, barend

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