[ka-Map-users] Efficient Point Collision

[Christopher Brown]

Hi Josh,

I just read through the whole thread on the PostGIS list more thoroughly
than I did before I made the last post and the discussion was very
interesting especially in relation to breaking the extent into a grid as
this is the only way I can think to reduce the size of the server-side
loops. I have a couple of questions though, the example in the thread states
that the interleave is produced in the following format - "42.987654,
-072.123456 => -07422.192837465564 LLLlLl.LlLlLlLlLlLl" this will work for
collision when it is just a case of number order but for determining which
marker is in which square within a grid wouldn't it need to be in the
following format "042.987654, -072.123456 => -007422.192837465564
LILlLl.LlLlLlLlLlLl" as to query if a point is somewhere within the extent
x1/y1 and x2/y2 we need x and y to have an equal unit length if we are to
achieve this through the interleave value (as apposed to a series of If
statements) by appending zero's to the front of the value? This way we can
determine if the point is within the grid square extent simply by checking
if our point interleave is bigger than extent interleave one and smaller
than extent interleave two.

Did you arrive at a final solution after the discussions on this PostGIS
list, if so what was your end solution and how did it work out?

Regards,

Chris

 

~~~~~~~~~~~~~~~~~~~~~~~~~~~

Christopher Brown

Head of Internet Development

Base Bloc Cambodia

#33, 123, Phnom Penh, Cambodia. 

P.O. Box 2086

www.basebloc.com

Mobile (+855) 12 315 302

Office (+855) 23 222 015

 

  _____  

From: Josh Livni [mailto:josh at livniconsulting.com] 
Sent: 21 October 2006 22:34
To: 'Christopher Brown'; ka-map-users at lists.maptools.org
Subject: RE: [ka-Map-users] Efficient Point Collision

 

I think client side is always going to be slow for large numbers of markers
(more than a few hundred); just downloading all the marker pts and a most
basic javascript to go over them is reasonably time consuming.  I personally
think some kind of clustering on server side might be the most reasonable
thing for large numbers of markers.  

 

That said, one thing that might add to your current method of ordering of
markers by the xcoord, server or client-side, is to order by an interleave
instead.  I once implemented something along the lines of what is described
here, and it was quite fast:
http://www.postgis.org/pipermail/postgis-users/2006-March/011430.html 

 

  -Josh

 

 

From: ka-map-users-bounces at lists.maptools.org
[mailto:ka-map-users-bounces at lists.maptools.org] On Behalf Of Christopher
Brown
Sent: Friday, October 20, 2006 11:53 PM
To: ka-map-users at lists.maptools.org
Subject: [ka-Map-users] Efficient Point Collision

 

Hi Guys,

Whilst we're on the subject I have also been working on a point collision
system, except I'm using addObjectGeo() to lay down the marker instead of
any of the XML related methods. I have the script working fine and it
behaves in the same way as flickr ( http://flickr.com/map/ ) with discs of
relative sizes which show the number of points within. 

To break it down each time a marker is placed it loops through each already
placed marker stored in a makers array to test for collision based on pixel
distance using "var distance = Math.sqrt(Math.pow((Math.pow((thisNodeLeft -
otherNodeLeft),2) + Math.pow((thisNodeTop - otherNodeTop),2)),1));", this
system works great for around 50 markers but often my apps will place 500+
makers which runs just fine until I collision check them as the loop just
becomes huge and obviously causes the browser to hang.

To counter this I made SQL output the markers in order of their X coordinate
then instead of checking every marker, I just check against the last 10
markers to me drawn and the last 10 will generally be the closest ones as
the X positions will all be similar if they are colliding, this method works
99% of the time at scales where the marker density is not too high, but
clearly wouldn't work as it does with flickr when zoomed right out so the
density is extremely high and the disks can contain 100+ markers, I would
like to be able to achieve this effect but just can't think of a way that is
both dynamic and not going to bring the server or the client to a crawl.

I don't need code, just some inspiration on how to achieve this, all ideas
and suggestions welcome!

Cheers,

Chris

 

~~~~~~~~~~~~~~~~~~~~~~~~~~~

Christopher Brown

Head of Internet Development

Base Bloc Cambodia

#33, 123, Phnom Penh, Cambodia. 

P.O. Box 2086

www.basebloc.com

Mobile (+855) 12 315 302

Office (+855) 23 222 015

 

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