[georss] Styling GeoRss points

[Charlie Savage]

Hi everyone,

I'd like revisit a discussion that happened in January about styling 
GeoRss points.  I'd like to specify two images/icons for each Atom entry 
that has a GeoRss point - a normal image and a hover image for mouse overs.

I'm wondering if there is any community consensus on how to do this? 
Doing a bit of research, I found a discussion about this on the GeoRss 
mailing list in January. A good starting point is a post by Christopher 
Schmidt who talked about reusing KML, while Mikel Maron asked if reusing 
CSS was more appropriate.

I agree that styling information shouldn't be added to GeoRss and that 
reusing CSS is a good choice. However, CSS doesn't work for points when 
you want to represent them with an image/symbol. Based on its HTML 
heritage, CSS considers images to be markup and not presentation and 
thus does not support changing an image's src attribute. The closest it 
gets is supporting background images, but that seems like the wrong 
solution for this problem.

Thus, we need to find another solution. Some ideas I've pondered include:

1. Use KML as Chris suggested. It would look something like this:

<Style id="highlightPlacemark">
   <IconStyle>
   <Icon>
   <href>http://maps.google.com/mapfiles/kml/paddle/red-stars.png</href>
   </Icon>
   </IconStyle>
</Style>
<Style id="normalPlacemark">
   <IconStyle>
   <Icon>
   <href>http://maps.google.com/mapfiles/kml/paddle/wht-blank.png</href>
   </Icon>
   </IconStyle>
</Style>
<StyleMap id="exampleStyleMap">
   <Pair>
     <key>normal</key>
     <styleUrl>#normalPlacemark</styleUrl>
   </Pair>
   <Pair>
     <key>highlight</key>
     <styleUrl>#highlightPlacemark</styleUrl>
   </Pair>
</StyleMap>

The obvious downside to this is how verbose it is - which is fine for 
KML but doesn't fit the GeoRss philosphy of keeping things simple.

2. Reuse atom's icon element:

<atom:icon>http://www.mapbuzz.com/images/marker.gif</atom:icon>
<atom:icon pseudo-class="hover">
     http://www.mapbuzz.com/images/marker_hover.gif</atom:icon>

The downsides to this approach are:

     * atom:icon is defined only at the feed level.
     * we have to introduce a custom attribute, which I called 
pseudo-class to match CSS's terminology.
     * If Atom ever supports icon at the entry level the semantics 
likely will be a bit different.
     * atom:icon does not specify widths or heights, which is important 
to support SVG symbols.

3. Reuse XHTML's img element:

<xhtml:img href="http://www.mapbuzz.com/images/marker.gif"
   height="32" width="32"/>
<xhtml:img href="http://www.mapbuzz.com/images/marker.gif"
   height="32" width="32"alt="hover"/>

The advantage to this approach is that Atom's content element already 
allows mixing in of XHTML, so there is some precedence. It also supports 
image sizes and we could hijack alt to specify different images types.

4. Reuse SVG's image element:

<xhtml:img xlink:href="http://www.mapbuzz.com/images/marker.gif"
x="100" y="100"height="32" width="32"/>
<xhtml:img xlink:href="http://www.mapbuzz.com/images/marker.gif"
x="100" y="100"height="32" width="32" pseudo-class="hover"/>

An SVG image introduces a funny twist - it let's you specify x and y 
values.I could see this being confused with the x/y values in a GeoRss 
point. Alternatively, it could be helpful to precisely postion this 
image. SVG images also support a number of style related attributes, 
such as opacity, which could be helpful.

Currently, option #4 looks like the best choice to me, but just 
wondering what other people think.

Thanks,

Charlie

(a pretty version of this post is on my blog at 
http://cfis.savagexi.com/articles/2007/06/03/styling-georss-points)
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