On 9/21/2011 4:16 PM, Etides wrote:
Ok but when moving the mouse fastly over the map it really
highlight the polygon feature under the cursor without
any query to database so it seems that they really
> have all the map already loaded in vector format
Nice. This is totally off topic from OpenLayers, though, so if we get
asked to knock it off, well okay. :)
I'm looking at their site now with Firebug. I heard last week a trick
which sounded quite similar to what I think I'm seeing here. (thank you
Dane!)
The short version:
Wax and TileStream and MapBox can apparently provide this sort of
functionality. Last I heard this new feature was still in the hacking
stage, but if you're interested, check it out. I know we intend to keep
tabs on this and get involved as soon as workload permits.
http://developmentseed.org/blog/2011/jun/10/wax-custom-advanced-ui-web-maps/
https://github.com/mapbox/wax
The long version:
The page loads a set of JSON files, each one being a tile.
http://cft1.giscloud.com/t/1316552561/map16581/layer80785/7/63/62.json
The JSON tiles have pixel information basically being 4-point polygons
of pixels, a style (R, G, B, width), and classification-ID. The
JavaScript effectively draws these points into the canvas, then provides
a mouseover callback for those pixels on canvas.
It's much more efficient to and parse 20 JSON documents at 25-75 kB
apiece than a single 2 MB document (the cost of parsing documents is
more than linear to size), and several thousand event callbacks are
lighterweight than several thousand Feature.Vector instances (which are
basically bloated copies of the document that was just parsed). So skip
straight to step 3: blit the simple polygons onto canvas and add callbacks.
I'd heard about it in theory, but seeing it in action makes me quite
hungry to get more involved. I gotta get back to work, empty my inbox,
and get into TileMill and TileStream...
--
Greg Allensworth, Web GIS Developer
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