[Gdal-dev] OGR: C API for Polygon/Multipolygon?

[gdal-dev-admin at remotesensing.org]

gdal-dev-admin at remotesensing.org wrote:
> I'm attempting to use a 'C' interface to OGR.  I have points and
> polylines getting drawn on my map, but don't see how to easily do a
> 'C' interface to the polygon functions.
> 
> I have a parallel working implementation using Shapelib alone, so
> I've done this before, including the CW/CCW ring stuff (fill/hole
> polygons).  I need to do the same thing now in OGR so I can switch
> to it and gain the advantages of the OGR library.
> 
> Anyone have a reference implementation in 'C' that shows how to
> access the elements in polygons/multipolygons for reading?
> 
> I see "OGRLinearRing", "getExteriorRing", and "getInteriorRing", but
> those appear to be C++ objects/methods.  I see no 'C' interface for
> them.  Accurate?  Do I need to get into the wkt/wkb calls in order
> to do it?

Curt,

For reasons that seemed good at the time, I did not make some of the C
geometry functions mirror the C++ that closely.  To access the rings of
polygons you use OGR_G_GetGeometryRef().  To find out how many rings there
are use OGR_G_GetGeometryCount().  This essentially treats polygons as a
contain of rings.  You then fetch the points from the rings the same way
you would with a linestring.

Best regards,
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