[ka-Map-users] Layer Interaction/Overlays

[Moran Ben-David]

Chris this is a great idea.  Especially in my line of work/applications
where we display thematic maps that don't allow even near complete caches of
maps or tiles.  Our work in http://www.dataplace.org
<http://www.dataplace.org/>  (for fannie mae foundation - along with Vinq)
has relied on the simple 

 

1.       check tile cache

2.       generate complete map (with thematic shading and layers)

3.       slice tiles

4.       cache tiles

 

approach.  Our cache though currently large enough to have some recurring
cache hits, won't be able to be large enough for the future of our app.
That is we want to add more indicators to our system (and thus many more
potential maps being generated).  As well we want to add more complicated
maps and chart shields as labels.   The universe of maps even with Terabyte
drives won't be covered by pre-rendering.

 

We have been aiming towards a client side rendering solution to increase our
cache hits.  I'm curious, has anyone tried rendering a map with
transparencies created by map server?   I.e. essentially doing what Chris
suggested?  Are the maps as pretty on Firefox (given that IE doesn't support
full alpha-channel usage in PNG's)?

 

As well, has anyone looked at the approach of actually rendering the layers
(placing them on the map) one by one on the client?  I know this delves into
fattening the client beyond the abilities of javascript, but I'm curious if
this has been discussed in the ka-map group.

 

Thanks,

Moran Ben-David

Placebase (www.place-base.com)

 

  _____  

From: ka-map-users-bounces at lists.maptools.org
[mailto:ka-map-users-bounces at lists.maptools.org] On Behalf Of Chris Tweedie
Sent: Tuesday, May 24, 2005 3:05 AM
To: ka-map-users at lists.maptools.org
Subject: [ka-Map-users] Layer Interaction/Overlays

 

To Paul / rest of the devs, what's your thoughts on adding transparency to
the tiles and allowing tile/layer overlays? I see you have commented out the
following, so you may be already looking at this?

 

        // TODO: We don't need transparency for now

        //$oMap->outputformat->set("transparent", MS_ON );

 

Im looking at instead of creating the tiled images based on all layers in
the map file, I would add a new config switch to specify particular layers
that you wish to split into separate tiles. Ideally, this would allow
particular layers in the map to be toggled on/off and be moved
hierarchically all on the client side.

 

Advantages:

-          Complete client side transparency / z-index position using CSS /
DOM / JS

-          Same benefits with cached tiles but further functionality

-          Remove reliance on the server regenerating images to perform
simple layer ordering functions

-          Will suit the planned cache refresh function seen on bugzilla. If
a layer data source is updated, only related layer images need to be
flushed, not everything.

 

Disadvantages:

-          Cross browser support issues

-          Integration with current code would require directory and tile
algorithm change

-          Annotation clashes due to the images being created independently.
ie. labels and symbols may become illegible

-          Increased overhead downloading images

 

While the download overhead may be significant depending on the number of
images you wish to overlay, typically you'd limit the amount of layers
available to be moved/turned off. However as the xmlhttprequest handles
concurrent downloading of images, I'd expect most users will not notice the
increased data as they can continue to zoom/pan.

 

Thoughts/Opinions? Thought I'd throw the idea out there and gauge a
response. Afterall, no point reinventing the wheel :-)

 

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Chris Tweedie

Technical Officer

Shared Land Services

Department of Land Information

1 Midland Square, Midland, Western Australia 6056

PO Box 2222 Midland 6936

Telephone: (08) 9273 7520 Facsimile (08) 9273 7638

Email:  <mailto:chris.tweedie at dli.wa.gov.au> chris.tweedie at dli.wa.gov.au

Website:  <http://www.dli.wa.gov.au> www.dli.wa.gov.au

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