[mapguide-internals] fusion legend and sprited legend images

[Walt Welton-Lair]

Hi Paul,

I was looking at your email and fusion ticket 131 (http://trac.osgeo.org/fusion/ticket/131), and I have a question / concern.  Making a single request to generate an image strip containing all legend images is certainly much faster than making individual requests for the images, but it has the drawback that *all* images for *all* layers need to be initially generated.  This alone can take considerable time for maps contains many layers.  There are users that have maps containing 500+ layers, and many of the those layers are themed.  So ultimately you might end up having to generate 1000's of legend images in the image strip.

In MapGuide a fix was made this summer to improve legend rendering performance in basic layouts (non-fusion).  This was for ticket 1023 (https://trac.osgeo.org/mapguide/ticket/1023).  The approach used there was different: legend images are only requested for layers which are visible, i.e. for top-level layers or for layers whose parent groups are all expanded.  If a layer has a collapsed parent group, then those images are only requested when the user expands that group.

For any decent map containing 500+ layers, the bulk of these layers would be organized into groups, most of which would initially be collapsed.  So the number of initial legend image requests ends up being reasonable with the MapGuide approach.  But in the case of the fusion mechanism, my understanding is that all images would have to be generated up front.  Or can we do something similar to MapGuide?

Walt


-----Original Message-----
From: mapguide-internals-bounces at lists.osgeo.org [mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Paul Spencer
Sent: Thursday, October 08, 2009 1:06 PM
To: MapGuide Internals Mail List
Subject: [mapguide-internals] fusion legend and sprited legend images

I've been working on implementing http://trac.osgeo.org/fusion/ticket/131 
  which allows for the possibility of generating a sprite containing  
all the legend icons and thus eliminating the individual calls to  
generate legend icons for each layer/class.  This is now implemented  
and working for MapServer and the wiring is in place for it to work  
with MapGuide.

This code provides a big apparent performance boost for maps with lots  
of layers/themed layers by using a single image for all (or most)  
legend icon images and using the CSS sprite technique for positioning  
the image such that the correct sub-image is visible for the  
appropriate layer/class/scale range.

If there is interest in having this feature supported for MapGuide  
maps, I need some help from MapGuide gurus to modify fusion/Layers/ 
MapGuide/php/LoadScaleRanges.php to do the following:

1) create an image that contains all the icons for each layer/class/ 
scale range combination

* The image can be organized either vertically or horizontally
* the individual images need to be the same size within the strip

2) save the image to a web-accessible location and add the url and  
size of each icon to the output json object, i.e.

"icons_url":"http:\/\/localhost\/ms_tmp\/4ace0feb_1a23_0.png",
"icons_width":18,
"icons_height":12

3) each scale range that has an image in the strip needs to add to its  
json representation values for icon_x and icon_y which are the offsets  
into the image that the icon can be found at

If a scale range doesn't have icon_x/_y then the legend will retrieve  
the legend image using the existing mechanism

There is php code in MapServer's LoadScaleRanges.php which can do the  
image manipulation to create the strip etc, just need to figure out  
how to get MapGuide to generate the individual icons.

I am willing to work with someone on this code but I need a MapGuide- 
savvy developer to figure out how to make the individual icons

Cheers

Paul
__________________________________________

    Paul Spencer
    Chief Technology Officer
    DM Solutions Group Inc
    http://research.dmsolutions.ca/

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