[OpenLayers-Dev] [Users] Markers performance

[David William Bitner]

The problem that this breeds is that while it would work great when zoomed
in, you still have the same problem when zoomed out unless you do some kind
of point clustering.

Which leads me to .... http://wiki.osgeo.org/index.php/Point_Clustering ..
If you have any thoughts on methods for point clustering (or any code that
implements point clustering) please add them to this wiki.  This is a
problem that a lot more folks than just openlayers are facing.

My personal take on this after a fair bit of playing around with clustering
on the client side is that it just is not scaleable to thousands of points.
Given that with GeoRSS, WFS, or other text feeds, either the data must exist
on the server or there must be a proxy in place -- it makes a lot of sense
to have a customized proxy that takes a feed along with a request for a tile
size (in pixels and geographic) and returns the points clustered
appropriately with some sort of attribute system that can show aggregated
details.

Last night I started playing around with Python Numeric and PyCluster with
kmeans and hierarchical clustering.  I think that this approach has quite a
bit of promise and it seemed pretty quick to me -- and would give better (at
least subjectively to me) clumping of the data than gridded or interleaved
approaches.

With a clustered point server (either proxy or direct from data -- think
points equivalent to tile.php), all we would need to do is to define our
output format (JSON anyone?) and then create an OL layer that can take in
that output.

David

On 10/24/06, Erik Uzureau <euzuro at gmail.com> wrote:
>
> So on thinking this over, it has occurred to me something that might
> be a good solution to this problem:
>
> A sort of blend of the OpenLayers.Layer.Markers layer and the
> OpenLayers.Layer.WMS.Untiled Layer.
>
> Basically, the problem now is this:
>
> Let's say you are using OpenLayers.Layer.Text and load 1000
> features/markers (like Andy is doing)
>
> Each and every one of those markers gets added to the DOM,
> even though given the current view, maybe only 20 or 30 of them
> are visible.
>
> The genius idea behind WMSUntiled is that it loades a tile that is
> [variably] larger than the current map extent, and only loads the data
> for that tile... and it is not until the user changes the extent (by
> panning
> or zooming) such that the original tile no longer covers the entire map
> extent that the layer has to reload data.
>
> Well, we could really do the same thing for markers, right? Keep all
> the markers in the OpenLayers.Layer.Makers.markers[] array and then
> each time we need to load a tile, scan the markers array to see which
> of them fall inside the bounds.
>
> ...which of course leads me to think that we could do some restructuring
> of the layers classes. I have for long felt that WMS/Untiled.js was a bit
> off.
>
> First of all, it *is* tiled. It's just that there's only ever *one* tile.
> So to start
> with, it might make more sense to call it  SingeTile or UnGridded or
> something along those lines.
>
> Furthermore, if we are going to have an UnGridded WMS, why shouldn't
> we have an UnGridded WFS?
>
> Obviously, the class rearrangements would require a significant amount
> of work and rewriting and testing, etc.
>
> For Andy's problem, writing a simple UnGridded/SingleTiled Marker layer
> would not be particularly difficult.
>
> Is anyone out there interested in any of this?
>
> Erik
>
>
>
> On 10/18/06, Erik Uzureau <euzuro at gmail.com> wrote:
> >
> > Dear andy,
> >
> > How many text markers are we talking about here? It is a known
> > limitation
> > of browsers that they cannot handle displaying (and then moving around)
> > large numbers of Div DOMElements... especially with IE.
> >
> > If you can tell us a little more about what you're trying to do, maybe
> > we can
> > help you better. :-)
> >
> >
> > --Erik
> >
> > On 10/18/06, andy <aborruso at tin.it> wrote:
> > > Dear all,
> > > I need an advice. I must use a marker layer for its tooltip feature. I
> > > have used "OpenLayers.Layer.Text", I have had what I need but I have
> > > slowness when I have a lot of text markers.
> > > Are the other methods to create markers faster? Which is fastest?
> > >
> > > Thank you,
> > >
> > > a
> > >
> > > P.S. In the openlayers home page there is an inaccessible link:
> > > http://openlayers.org/presentations/foss4g2006/examples/
> > > _______________________________________________
> > > Users mailing list
> > > Users at openlayers.org
> > > http://openlayers.org/mailman/listinfo/users
> > >
> >
>
>
> _______________________________________________
> Dev mailing list
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>
>
>


-- 
************************************
David William Bitner
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