[postgis-devel] Collapsing behavior for simplify

[Paul Ramsey]

I think we should probably move the Simplify behavior to return minimum values for dimensions, start-end lines, start-middle-middle-end polygons, etc. 


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On April 4, 2015 at 2:00:12 PM, Nicklas Avén (nicklas.aven at jordogskog.no) wrote:
> Hallo
>  
> When adding tests to ST_SimplifyVW (the Visvalingam-Whyatt version of
> simplify) I noticed some differences in behavior between ST_Simplify and
> ST_SimplifyVW. Since the ST_Simplify behavior somewhat strange I want to
> discuss it before I copy that behavior to ST_SimplifyVW.
>  
> It is discussied in many tickets, and I haven't taken the time to read
> them all. But I think #1698 comes closest to a solution
> http://trac.osgeo.org/postgis/ticket/1698
>  
>  
> First an inconsistency with ST_Simplify that I don't understand if is
> intentional:
> in regression test nr 9 it shows that a polygon collapses and returns
> nothing:
> http://trac.osgeo.org/postgis/browser/trunk/regress/simplify_expected
>  
> But if you take the last test, nr 12 and change last parameter to 200 so
> it collapses it returns an empty multipolygon.
> SELECT '12', ST_astext(ST_Simplify('MULTIPOLYGON(((0 0, 10 0, 10 10, 0
> 10, 0 0),(5 5, 5 6, 6 6, 8 5, 5 5)),((100 100, 100 130, 130 130, 130
> 100, 100 100)))', 200));
>  
>  
> Then from what i understand in ticket #1698 ST_Simplify is expected not
> to remove the entire boundary but instead return something.
>  
> In Visvalingam-Whyatt this is the situation right now (which I am quite
> happy with):
> For linestrings they always return at least start and end point.
>  
> Polygons can collapse inner rings.
> Boundaries is never collapsed. THey always return 4 points. That is
> start and end point of pointarray (same point) and the two points with
> highest effective area. So the result will be a triangle with as big
> area as possible (not too thin triangle).
>  
> in multipolygons all inner rings can be collapsed but all sub-geometry
> boundaries will remain.
>  
> Is there any reason to leave that concept?
>  
> I guess it have to be explained in doc as a difference from
> ST_Simplify.
>  
> The doc now for the new functions is:
> http://postgis.net/docs/manual-dev/ST_SetEffectiveArea.html
> and
> http://postgis.net/docs/manual-dev/ST_SimplifyVW.html
>  
>  
> Regards
>  
> Nicklas
>  
>  
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