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

Martin Dobias wonder.sk at gmail.com
Fri Jan 24 00:29:46 PST 2014

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.


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