Geometry Boost List: Re: [geometry] Dissolve rescale_policy

Subject: Re: [geometry] Dissolve rescale_policy
From: Barend Gehrels (barend_at_[hidden])
Date: 2016-03-16 11:50:07

Hi,

Op 16-3-2016 om 3:36 schreef Adam Wulkiewicz:
> Hi Johan,
>
> Johan Doré wrote:
>>
>> Dear Boost Geometry
>>
>> In version 1.60 wonder if there is an reason for the 
>> *no_rescale_policy* in the dissolve function:
>>
>
> Do you mean extensions from develop branch? dissolve() is not a part 
> of the official release, or am I missing something?
>
> <snip>
>>
>> Changing it to the robust policy:
>>
>> inlinevoiddissolve(Geometry const& geometry, Collection& 
>> output_collection)
>>
>> {
>>
>> concept::check<Geometry const>();
>>
>> typedeftypenameboost::range_value<Collection>::type geometry_out;
>>
>> concept::check<geometry_out>();
>>
>> typedeftypenamegeometry::rescale_policy_type
>>
>> <
>>
>> typenamegeometry::point_type<Geometry>::type
>>
>> >::type rescale_policy_type;
>>
>> rescale_policy_type robust_policy = 
>> geometry::get_rescale_policy<rescale_policy_type>(geometry);
>>
>> dispatch::dissolve
>>
>> <
>>
>> typenametag<Geometry>::type,
>>
>> typenametag<geometry_out>::type,
>>
>> Geometry,
>>
>> geometry_out
>>
>> >::apply(geometry, robust_policy, std::back_inserter(output_collection));
>>
>> }
>>
>> Seems to correctly dissolve a larger range of invalid polygons.
>>
>> I therefore wonder if this change I recommendable and should be included?
>>
>
> There are some problems with rescaling we discovered recently. 
> Functions taking two input geometries rescale both geometries in the 
> same time, functions taking one input geometry rescale only one 
> geometry. In both cases the geometries are rescaled WRT the size of a 
> bounding box of all input geometries (so 2 or 1). So the same geometry 
> may be rescaled differently in both cases depending on a function to 
> which it's passed and e.g. the size of the other geometry if a 
> function takes two geometries. And an interesting things may happen 
> during the rescaling. E.g. the vertices of a geometry may be moved 
> relativelly to each other. So in rare cases some edges detected e.g. 
> as collinear after being rescaled differently may be seen as 
> intersecting which means that, e.g. the output of a set operation may 
> be detected as invalid.
>
> We plan to experiment with a different solution. In a long run I think 
> this change is not a prefered. However the rest of the library use the 
> rescaling for Areal geometries so after applying your change dissolve 
> would be consistent with other algorithms.
>
> Barend what do you think about it?

I agree with Adam's answer. I think currently the rescaling can be 
included in the dissolve extension, it makes it consistent with the 
other algorithms and apparently it helps dissolving problematic polygons.

On longer term we hope to remove the rescaling again, indeed.

Note that the new approach (using clusters instead of handling 
tangencies individually) should also help in the dissolve process, 
though I did not try it there.

Regards, Barend