[OpenLayers-Dev] offline caching issues

Andreas Hocevar ahocevar at opengeo.org
Tue Mar 19 22:52:00 PDT 2013

Which version of OpenLayers are you using? Both issues you describe here
should be fixed in master.

On Mar 20, 2013 1:24 AM, "Chris Berkhout" <chrisberkhout at gmail.com> wrote:

> Hi All,
> I've been trying to get offline (localStorage) tile caching to work
> like the example
>     http://openlayers.org/dev/examples/offline-storage.html
> and I've run into a couple of problems.
> I'm restricting myself to a single layer, same origin via proxy and I
> always set read from cache first (fall back to network).
> The first problem is that after populating the cache and going
> offline, failed tile fetches continue to populate the cache. These
> appear as black squares in the example (non-cached failures are pink).
> Manually turning off "Write to cache" in the example resolves this.
> I've been able to avoid this by patching OpenLayers.Control.CacheWrite
> to not write failed tiles to the cache, by checking the condition:
>     !evt.tile.imgDiv.classList.contains("olImageLoadError")
> I guess there's probably a better way to do that.
> The other problem I found is that after populating the cache and going
> offline, then panning away to non-cached areas and back to the cached
> area, things around the edges of the cached areas are disappearing.
> This was more obvious when working with my own layer (at detailed
> zoom), but could also be replicated using the openlayers example. The
> problem seems to be variation (rounding errors?) in the bbox
> parameters of the WMS tile request.
> For example, this URL was a cache miss:
> http://m1.pozi.com/geoserver/gwc/service/wms?LAYERS=VicmapClassic&FORMAT=image%2Fpng8&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG%3A900913&BBOX=16133716.431963,-4554423.89271,16136162.416867,-4551977.9078052&WIDTH=256&HEIGHT=256
> Even though the tile it refers to was cached with a URL of:
> http://m1.pozi.com/geoserver/gwc/service/wms?LAYERS=VicmapClassic&FORMAT=image%2Fpng8&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG%3A900913&BBOX=16133716.431963,-4554423.89271,16136162.416868,-4551977.9078052&WIDTH=256&HEIGHT=256
> That's a maxx (3rd bbox value) with last digit 7 instead 8.
> I could avoid this by patching the cache read/write code to use an
> optional URL normalization function, which the user could set to trim
> off the last digits of the bbox parameters.
> So, I think I've straightened out what's going on, but the only
> solutions I can see involve various hacks in the openlayers code.
> Has anyone else run into these issues or got some advice?
> Cheers,
> Chris
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