[Talk-GB] invisible

Graham Jones grahamjones139 at gmail.com
Tue Jan 18 13:00:03 GMT 2011

I think you are describing what my examples do - for example the map at
http://maps.webhop.net/topo uses a simple base map and the OpenLayers
javascript program running in the web browser draws the selectable overlays
on top of it - the overlays are PNG images with a transparent background.
Or maybe you meant some sort of clever bitmap with layers defined within it?

Whether it is efficient or not depends on what you are trying to do with the
overlays - for single points (e.g. the power stations themselves on my map)
holding bitmaps is much less efficient than storing the locations of each
power station - you could have OpenLayers take the locations of the points
and plot them.

Using transparent overlays is easy - you do not need to do any real time
processing - just get an image from a server and dispaly it.   Going for
something cleverer like vector rendering needs a database to give you the
correct data, then the javascript in the web browser needs to process it and
display it.   The difficulty is getting a quick enough database response to
give a nice user experience - this was the subject of another 2010 Google
Summer of Code Project (

Another alternative is that the mapnik renderer now has the capability to
include meta-data in the image tiles, but I haven't looked into this.



On 18 January 2011 12:35, Henry Gomersall <heng at cantab.net> wrote:

> On Tue, 2011-01-18 at 11:02 +0000, Chris Saunter wrote:
> > Pre-generating tiles for every concievable view of the OSM dataset is
> > not a very efficient way of dealing with different people wanting to
> > view different geographic areas in different styles.
> >
> Why can this not be done (to a reasonable extent) with feature bitmap
> layers that can be quickly assembled client side?
> Cheers,
> Henry
> _______________________________________________
> Talk-GB mailing list
> Talk-GB at openstreetmap.org
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Graham Jones
Hartlepool, UK.
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