[fdo-internals] Please review FDO RFC 38 - Multi-polygon support for SHP

[Dan Stoica]

Hi Frank,

> I do feel the RFC is a bit optimistic about the performance impact.  

I'll try to optimize if the case. Indeed, the implementation does not rely on the loops orientation. 

However, this has to be done because the previous approach "speed over correctness" proved to have serious consequences, worse than a performance penalty on loading (aren't the machines faster and faster?).

Regards,
Dan.

-----Original Message-----
From: fdo-internals-bounces at lists.osgeo.org [mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Frank Warmerdam
Sent: Thursday, July 16, 2009 10:09 AM
To: FDO Internals Mail List
Subject: Re: [fdo-internals] Please review FDO RFC 38 - Multi-polygon support for SHP

Orest Halustchak wrote:
> Hi,
> 
>  
> 
> We've run into issues where SHP multi-polygons are not being returned as 
> FDO multi-polygons by the FDO SHP provider. The issue stems from the SHP 
> specification having only a polygon type defined, but that definition 
> includes polygons with any number of outer rings which encompasses both 
> simple polygons and multi-polygons.
> 
>  
> 
> RFC 38, drafted by Dan Stoica, proposes to fix the problem by adding the 
> capability to the FDO SHP provider.
>
> See http://trac.osgeo.org/fdo/wiki/FDORfc38 .
> 
> Have a look at the RFC and provide your comments.

Dan / Orest,

I'm in support of this change.  I had to do the same thing in the OGR shape
driver a few years ago.  I do feel the RFC is a bit optimistic about the
performance impact.  I find it is common to have polygons with holes in
shapefile format, and they will all require fairly expensive processing to
determine if the extra rings are interior or exterior.

In theory I believe you are supposed to be able to tell from the ring
winding direction whether additional rings are interior or exterior.  However,
many shapefiles produced by crappy libraries (like my shapelib!) did not
set the winding direction properly so it is unwise to depend on this.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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