[gdal-dev] ISO WKB

Even Rouault even.rouault at mines-paris.org
Fri Dec 13 13:21:22 PST 2013


Le vendredi 13 décembre 2013 21:51:57, Paul Ramsey a écrit :
> Actually, I think changing the contract on getGeometryType to only ever
> returning the basic (non-3d) type would clean up driver implementation
> code a bit. Grepping for it in the drivers, you see a lot of
> 
> CPLAssert(wkbFlatten(poGeom->getGeometryType()) == wkbPoint)
> 
> and
> 
>    || (poLine->getGeometryType() != wkbLineString
>     && poLine->getGeometryType() != wkbLineString25D) )
> 
> and very few unqualified calls to it.

This would have impacts on drivers, but also on external code since 
OGR_G_GetGeometryType() is the C mapping of OGRGeometry::getGeometryType()

There are also OGRGeomField::GetType(), OGRFeatureDefn::GetGeomType() and 
OGRLayer::GetGeomType() (and their C equivalents) that return wkbGeometryType. 
I imagine that if we change the contract of OGRGeometry::getGeometryType(), 
they should also behave similarly, for consistency.

So we have a backward compatibility issue here, which must be carefully 
weighted in.

> 
> P.
> 
> -- 
> Paul Ramsey
> http://cleverelephant.ca
> http://postgis.net
> 
> On December 13, 2013 at 12:00:34 PM, Paul Ramsey
> (pramsey at cleverelephant.ca) wrote:
> 
> I’ve pushed up some work to do this
> 
> https://github.com/pramsey/gdal/tree/isowkb
> 
> I had to change *two* method signatures, which I don’t really like, both
> exportToWkb and getGeometryType needed a variant parameter added. The
> problem is that getGeometryType returns the full WKB type, with
> dimensionality encoded, which makes it somewhat duplicative of
> getCoordinateDimension. In an ideal world, getGeometryType would only
> return the type number, and dimensionally information would be inferred
> from getCoordinateDimension.
> 
> Then only the exportToWkb signature would need a variant parameter.
> 
> importFromWkb also was changed to ingest the ISO variant if that’s what
> it’s fed. No changes to signature required there.
> 
> P.
> 
> -- 
> Paul Ramsey
> http://cleverelephant.ca
> http://postgis.net
> 
> On December 12, 2013 at 2:58:05 PM, Even Rouault
> (even.rouault at mines-paris.org) wrote:
> 
> Le jeudi 12 décembre 2013 23:39:25, Paul Ramsey a écrit :
> > I’ve been looking over the code, and I think I can make importFromWkb
> > consume ISO inputs with relatively minor changes. (during the geometry
> > type determination in the concrete types, it looks easy enough to add a
> > little logic to deal with ISO type numbers) For exportToWkb, I’m
> > wondering if maybe an optional parameter (wkbVariant?) that defaults to
> > wkbVariantOGC would be sufficient?
> 
> Sounds good to me. Is "wkbVariantOGC" the variant currently implemented ?
> Is our 3D support really compliant with an OGC standard ?
> 
> > It also seems the wkb reader/writer have little handling for empty
> > geometry types. I’d like to add some in, if that’s OK.
> 
> Are you sure about that ?
> 
> See :
> >>> from osgeo import ogr
> >>> ogr.CreateGeometryFromWkb(g.ExportToWkb()).ExportToWkt()
> 
> 'LINESTRING EMPTY'
> 
> >>> ogr.CreateGeometryFromWkb(ogr.CreateGeometryFromWkt('LINESTRING
> 
> EMPTY').ExportToWkb()).ExportToWkt()
> 'LINESTRING EMPTY'
> 
> >>> ogr.CreateGeometryFromWkb(ogr.CreateGeometryFromWkt('LINESTRING
> 
> EMPTY').ExportToWkb()).ExportToWkt()
> 'LINESTRING EMPTY'
> 
> >>> ogr.CreateGeometryFromWkb(ogr.CreateGeometryFromWkt('POLYGON
> 
> EMPTY').ExportToWkb()).ExportToWkt()
> 'POLYGON EMPTY'
> 
> >>> ogr.CreateGeometryFromWkb(ogr.CreateGeometryFromWkt('MULTILINESTRING
> 
> EMPTY').ExportToWkb()).ExportToWkt()
> 'MULTILINESTRING EMPTY'
> 
> >>> ogr.CreateGeometryFromWkb(ogr.CreateGeometryFromWkt('MULTIPOLYGON
> 
> EMPTY').ExportToWkb()).ExportToWkt()
> 'MULTIPOLYGON EMPTY'
> 
> >>> ogr.CreateGeometryFromWkb(ogr.CreateGeometryFromWkt('GEOMETRYCOLLECTION
> 
> EMPTY').ExportToWkb()).ExportToWkt()
> 'GEOMETRYCOLLECTION EMPTY'
> 
> The only exception is POINT EMPTY due to the lack of wkb representation for
> it
> 
> >>> ogr.CreateGeometryFromWkb(ogr.CreateGeometryFromWkt('POINT
> 
> EMPTY').ExportToWkb()).ExportToWkt()
> 'POINT (0 0)'
> 
> > Fortunately the only substantial difference for ISO WKB is the type
> > number.
> > 
> > I’m starting an isowkb branch on my git repo to hold this piece of work,
> > I don’t think it’ll be very big.
> > 
> > Then I’ll start asking about ‘m’ dimensions :)
> 
> That's a fresh new land to explore...
> 
> > P.
> > 
> > --
> > Paul Ramsey
> > http://cleverelephant.ca
> > http://postgis.net
> > 
> > On December 12, 2013 at 2:33:41 PM, Even Rouault
> > (even.rouault at mines-paris.org) wrote:
> > 
> > Le jeudi 12 décembre 2013 21:36:42, Paul Ramsey a écrit :
> > > Hey all,
> > > Is there already a facility for writing out ISO-standard WKB in GDAL?
> > > The biggest difference is the wkbGeometryType number for the case of
> > > 3d features. OGR currently uses the old extended SFSQL adopted by
> > > PostGIS. ISO (naturally) did their own thing. As far as I can see,
> > > there’s no support there now, or is there? P.
> > 
> > Hi Paul,
> > 
> > no, there's no support for ISO WKB yet. We still stick to
> > http://home.gdal.org/projects/opengis/twohalfdsf.html
> > 
> > I can imagine we could have exportToISOWkb() method in OGRGeometry*. Or
> > we could add an extra boolean argument bISOWkb to the existing
> > exportToWkb(). Not sure about WkbSize() : does the size of ISO WKB blobs
> > differ from the unofficial WKB ?
> > 
> > Even
> > 
> > --
> > Geospatial professional services
> > http://even.rouault.free.fr/services.html
> 
> --
> Geospatial professional services
> http://even.rouault.free.fr/services.html

-- 
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