[geos-devel] [GEOS] #609: GEOSPointOnSurface returns EMPTY for single-point linestring

[Martin Davis]

I just realized some corner cases, which require some clarification to 
deal with.

One is geometries with multiple zero-length lines.  In this case I think 
the geometries should be treated as points, and the centroid and 
interior point computed accordingly.

A similar situation exists for polygons with no area (which are invalid, 
but it's still nice to return a sensible answer).  A similar strategy 
should work - treat them as points.  Polygons with extent but no area 
(eg POLYGON ((200 200, 200 300, 200 200, 200 200))) can be treated as if 
they are lines.

In all cases, if geometry components of higher dimension are present, 
the lower-dimension geometries are ignored.

This is going to require more significant change to handle - in 
particular, it points toward having single class for Centroid to handle 
all geometry types.

On 1/11/2013 9:32 AM, Martin Davis wrote:
> Just to clarify my original response, the primary constraint on 
> functions that return geometry values is that they return a valid 
> geometry that makes some sort of sense.
>
> So in the case of centroid, regardless of what is "mathematically 
> correct" a valid sensible geometry should be returned (as opposed to a 
> meaningless geometry with NaN ordinates, as now happens).
>
> In the case of interiorPoint the situation is even more clearcut - 
> obviously the interior point should be one of the points in the geometry.
>
> On 1/11/2013 6:01 AM, GEOS wrote:
>> #609: GEOSPointOnSurface returns EMPTY for single-point linestring
>> ------------------------+--------------------------------------------------- 
>>
>>   Reporter:  strk        |       Owner:  geos-devel@…
>>       Type:  defect      |      Status:  new
>>   Priority:  major       |   Milestone:  3.3.7
>> Component:  Default     |     Version:  3.3.6
>>   Severity:  Unassigned  |    Keywords:
>> ------------------------+--------------------------------------------------- 
>>
>>
>> Comment(by strk):
>>
>>   So for GEOSCentroid the answer is correct, in that by definition the
>>   CentroidLine class computes the centroid of all segment's midpoint
>>   weighted by segment length, so a zero-length segment is not 
>> considered.
>>
>>   Now, the algorithm of InteriorPointLine [1] is:
>>
>>      - Find an interior vertex which is closest to the centroid of the
>>   linestring.
>>      - If there is no interior vertex, find the endpoint which is 
>> closest to
>>   the centroid.
>>
>>   [1]
>> http://www.vividsolutions.com/jts/javadoc/com/vividsolutions/jts/algorithm/InteriorPointLine.html
>>
>>   But if there's no interior vertex both endpoints are equidistant to 
>> the
>>   centroid, aren't them ?
>>   Martin: am I missing anything ?
>>
>
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