[ka-Map-users] Layer Interaction/Overlays

[Paul Spencer]

Chris,

ka-Map cvs version (sorry still not on the maptools server!!! hopefully 
today) has this capability.  It is currently targetted at groups of 
layers being the level of control but will also work with individual layers.

There is a big performance hit when separating the map file this way. 
First, you need to download (n) times more tiles.  Second, you need to 
generate and store those tiles.  Third, most browsers seem to suffer 
performance degradation when adding (n) times more tiles (either from 
the stacked layer rendering or increased memory useage, not sure?)

The label issue is also quite a problem.

Using groups to make the separation helps somewhat because you can try 
to group layers intelligently to minimize these problems.  As it is 
configured, all layers not in a group are considered part of the 
__base__ group.  This group is not intended to be turned on/off by the 
user.

The current code base could be optimized to help with this problem, 
including:

* turn off all non-base layers during dragging
* do not request tiles in layers that are off

Ultimately, the combination of tiling and stacking in HTML may not be a 
winning combination until browsers can handle it better and everyone is 
on broadband ;)

Re cross-browser compatibility, I think most browsers should handle this 
ok.  Are you aware of a browser that would not work at all in this scenario?

Cheers

Paul

Chris Tweedie wrote:
> 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 J
> 
>  
> 
> ---------------------------------------------------------------------------
> 
> **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: chris.tweedie at dli.wa.gov.au <mailto:chris.tweedie at dli.wa.gov.au>
> 
> Website: www.dli.wa.gov.au <http://www.dli.wa.gov.au>
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|Paul Spencer                           pspencer at dmsolutions.ca   |
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|Applications & Software Development                              |
|DM Solutions Group Inc                 http://www.dmsolutions.ca/|
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