[Qgis-developer] On the fly simplication of point layers

Andreas Neumann a.neumann at carto.net
Fri Jan 24 00:48:32 PST 2014

Hi Martin,

So you think the same code of the point displacement renderer can also
be used for clustering?

It was developed exactly for the opposite: if multiple geometries are on
top of each other they are displaced so you can see all of the points. A
use-case is where several people live at the same address but have
different attributes and you want to see them all.

I am a bit surprised that the same code could be used for the opposite -
to cluster points that are not at the same position. Wouldn't it require
some additional code/work to do that?

Just wondering.


Am 24.01.2014 09:29, schrieb Martin Dobias:
> Hi Tim
> On Fri, Jan 24, 2014 at 2:31 PM, Tim Sutton <lists at linfiniti.com> wrote:
>> On Fri, Jan 24, 2014 at 1:12 AM, A Huarte <ahuarte47 at yahoo.es> wrote:
>>> Hi!
>>> About simplification (no clustering), I think that the If we discard
>>> points based on the distance to the last fetched and rendered point
>>> (https://github.com/qgis/QGIS/blob/master/src/core/qgsfeatureiterator.cpp#L36)
>>> it will be very effective to render big dense points layers. I have a GIS
>>> application using this technique and it draws LAS files (> 30mb) "fast".
>>> I would like write a "point simplifier" in QGIS to validate results.
>>> Do you agree ?
>> I think this would be great. Since you are doing a full scan of the dataset,
>> you should be able to count how many points each aggregate point represents
>> right? I was thinking we could pass that the the renderer as a 'virtual'
>> attribute so that we can use it to e.g. scale the symbol size.
>> +1 from me to implement this anyway as a new cluster renderer (or a patch
>> the point displacement renderer if that seems workable).
> Actually in the threading branch I have significantly reworked the way
> how the clustering is done in the point displacement renderer. A quick
> test with ~75K points shows that QGIS 2.0 takes ~10 minutes to render
> with point displacement, while in threading branch it is matter of a
> second or two. In the threading branch the renderer uses spatial index
> for clustering. Another optimization could be to build the spatial
> index for a layer just once, for even better performance.
> So I believe now it's mainly a matter of adding more configuration
> options.to allow output similar to "usual" marker clustering - and
> make such options default.
> I would prefer not to have "point simplifier" somewhere in the data
> access API - like Tim suggests, it is best to keep such functionality
> in a renderer.
> Regards
> Martin
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