[QGIS-Developer] Handling "complex" geometries

[Andreas Neumann]

Hi Loïc, 

As a cartographer and QGIS user I'd love to see spline or nurbs support
in QGIS. But from a developer point of view (and data format point of
view) it probably opens a can of worms. I do hope that we can get there
eventually. 

As you have discovered, besides GML3 and DXF/DWG and ESRI FGDB no other
format seems to support spline/nurbs. 

We would have to enhance Postgis as well to handle them - and the
geometry libraries (or we segment them as we do with circular arcs) as
an intermediate step. 

I am sure that other devs (e.g. Marco Hugentobler, Nyall, Martin and
others) would have more thoughts on that topic. And maybe Even could
comment from a OGR point of view - what would have to be done to support
splines/nurbs. 

Good to see discussion starting on that topic! 

Andreas 

On 2017-10-16 23:54, L.Bartoletti wrote:

> Hi,
> 
> I'm working on an AutoCAD to QGis migration topic for town planners and landscapers that made me think about something that I would like your opinion.
> 
> I recently committed new features to draw and I have just proposed QEPs to improve the drawing "a la" CAD in QGis, but I'm wondering about the representation of some geometries and how to manage them well.
> 
> There is no known method to transform an ellipse into an arc, so geometry is always segmented. Similarly, a circle is represented by several arcs which, when a node is modified, can deform the circle.
> I've often seen landscapers or town planners (like me) drawings with splines. We don't yet have the possibility to draw it (TODO ;) ) but clearly, how to manage it in QGis afterwards?
> 
> Even if it's boring to take up again, a circle or ellipse is simple to redraw if you want to modify them, but a spline is more complex.
> 
> Apart from DXF/DWG formats, I am not aware of the direct support of these geometries in GIS formats (maybe GML 3 and those compliant with ISO 19107:2003? I didn't know this standard until today), in any case, they don't exist in the standard OGC "simple feature access" which seems to be our basis, right? Thus, we must convert them: arcs or segments for circles, segments for splines and ellipses.
> 
> So, how can we keep track of the originals geometries without having to convert them?
> Here are some thoughts mixed up ...
> 
> - Never mind, we stay with primitive types
> - Added support for existing geometries (circles, ellipses) when formats offer it. There is maybe a link to do with GDAL/OGR.
> - We try to be full ISO and we develop all the geometries in QGis :D
> 
> For formats that do not support these types, we record at best (arc, segments) but for editing:
> - The action is stored in memory and until the editing session is finished, the geometry can be modified by moving the quadrants (circles, ellipses) or control points (splines)
> - Information is stored in an auxiliary or base file for RDBMS. Thus, when it is opened in QGis you can find the information and modify the geometries correctly, otherwise it remains open as it is today in arc or segmented. It's like an external/"proprietary" feature.
> 
> How do you feel about that?
> 
> Regards.
> 
> Loïc
> 
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