[geos-devel] Exposing PrecisionModel, GeometryPrecisionReducer in the C API

[Sandro Santilli]

Sandro, I've been looking at the wrapper you added to QGIS
but from what I read it doesn't seem that the geometry coming out
of GEOSGeometryPrecisionReducer_reduce has an updated PrecisionModel,
which means overlay operations won't be using that new model.

Do you confirm ?

If that's the case, the wrapper doesn't do much more than a
grid-snapping (plus some topological fixes of the snapping),
basically exposing no real management of PrecisionModels...

Are there tests to verify that ?

REF: https://trac.osgeo.org/geos/ticket/713

--strk;

On Wed, Jan 14, 2015 at 10:38:23AM +0100, Sandro Santilli wrote:
> On Tue, Jan 13, 2015 at 11:21:34AM +0100, Sandro Mani wrote:
> > 
> > On 08.01.2015 13:08, Sandro Mani wrote:
> > >So to wrap up, things currently look as follows (which I actually
> > >think is quite neat!):
> > >
> > >
> > >/** GEOSContextHandle_t internally keeps a reference count, set to
> > >1 on creation
> > > * - *All* methods producing a GEOSGeom increase the reference count
> > > * - GEOSGeom_destroy decreses the GeometryFactory reference
> > >count, if 0, it destroys the context
> > > * - finishGEOS_r decreses the reference count, if 0, it destroys
> > >the context
> > > */
> > >
> > >/** GEOSGeom_clone() clones the geometry and applies
> > >PrecisionReducer::reduce
> > > *  if the PrecisionModel of the current context is different than
> > >the that of
> > > *  the passed geometry
> > > */
> > >
> > >/** Set context handle precision model **/
> > >void GEOSContextHandle_t
> > >GEOSContext_setPrecisionDouble(GEOSContextHandle_t* handle);
> > >void GEOSContextHandle_t
> > >GEOSContext_setPrecisionFixed(GEOSContextHandle_t* handle, double
> > >scale);
> >
> > So, what do you think? Worth pursuing this approach?
> 
> Sorry I'm not having much time to dedicate to this.
> I'm not very sure about the approach, to be honest.
> There are indeed 2 possible things one would want to do with PM:
> 
>  1. "advertise" the PM of a geometry, w/out rounding coords
> 
>  2. "set" the PM of a geometry, rounding coords
> 
> I'm not sure we ever want to allow for 1, to avoid wrong advertisement.
> The signature in the current proposal makes 2 clear for the cloning
> of a geometry, but we're still missing clear documentation about what
> happens from the constructors. Would/should they round ?
> 
> Example, what should happen here:
> 
>   GEOSContextHandle_t h = initGEOS_r(...);
>   GEOSCoordSequence *s = GEOSCoordSeq_create_r(h,2,2);
>   GEOSContext_setPrecisionDouble(h);
>   GEOSCoordSeq_setX_r(h, s, 0, 0);
>   GEOSCoordSeq_setY_r(h, s, 0, 0);
>   GEOSContext_setPrecisionFixed(h, 1);
>   GEOSCoordSeq_setX_r(h, s, 1, 0.2);
>   GEOSCoordSeq_setY_r(h, s, 1, 0.2);
> 
> Should the coordinate sequence contain 2 equal points after the above ?
> And what about WKB/WKT parsing ?
> 
> It would help to gather more comments from others.
> I've added Oliver and Sean in Cc as they took part of a precedent
> discussion about this:
> http://lists.osgeo.org/pipermail/geos-devel/2014-February/006780.html
> 
> --strk;
> 
>   ()   Free GIS & Flash consultant/developer
>   /\   http://strk.keybit.net/services.html