[ka-Map-users] Layer Interaction/Overlays

[Paul Spencer]

Moran,

Moran Ben-David wrote:
> 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)?

One problem with MapServer is that transparency on layers only works 
between layers drawn in the same image ... and MapServer doesn't output 
PNGs with alpha channels, just with transparent on/off ... so you really 
need to do it the way ka-Map does it now, rendering each layer (or group 
of layers) into a separate DIV and then setting the opacity on the DIV 
(only Opera doesn't seem to support opacity in the latest browsers)

> 
>  
> 
> 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.

yep, latest CVS version of ka-Map does this (right now, it renders 
groups rather than individual layers).  Also, MapBuilder does this (its 
an entirely javascript client based on loading map layers via OGC 
services ... pretty cool)

Performance is the big drawback, hence the use of groups to consolidate 
as many layers as possible and have only a few separately controllable 
layers on the client side.

Cheers

Paul

> 
>  
> 
> 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 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>
> 
> ---------------------------------------------------------------------------
> 
>  
> 
> 
> 
> This e-mail and any files transmitted with it are intended only for the 
> use of the addressee(s). It may contain information that is confidential 
> and privileged. If you are not an intended recipient, any use, 
> interference with, disclosure, distribution or copying of this material 
> is unauthorised and prohibited. If you receive this in error, please 
> notify the author by Return email to the sender. Information in this 
> message not relating to the official business of DLI shall be understood 
> as neither given nor endorsed by it. While every care is taken, it is 
> recommended that you scan any attachments for viruses. DLI liability is 
> limited to re-supplying affected attachments.
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> ka-Map-users mailing list
> ka-Map-users at lists.maptools.org
> http://lists.maptools.org/mailman/listinfo/ka-map-users

-- 
+-----------------------------------------------------------------+
|Paul Spencer                           pspencer at dmsolutions.ca   |
+-----------------------------------------------------------------+
|Applications & Software Development                              |
|DM Solutions Group Inc                 http://www.dmsolutions.ca/|
+-----------------------------------------------------------------+