[Tilecache] one possible cause of broken ("pink tile") WMS images

[David Herbert]

My colleagues and I have been investigating what (by looking at both the
OL and Tilecache lists) seems to be a fairly common problem, namely WMS
images that fail to load, seemingly at random.  For example, see the
thread:

http://www.nabble.com/TileCache-Random-Pink-Tiles-td14105104.html 

from December last year, among others.  We have an Intranet application
based on Geoserver, with OL 2.5 as the client and Tilecache 2.0.1 for
caching WMS images.  One particular user was getting broken images
("pink tiles") often in Firefox 2.0.0.12 on Win XP, particularly in
evidence at high zoom levels.  This was the case however high I cranked
the value of IMAGE_RELOAD_ATTEMPTS in OL - it was failing still with
this value at 10.   It appeared as if it might be network-related at
first, but this was eliminated by other users trying the app on the same
Windows XP PC and it worked fine in the same version of Firefox.  

We eventually tracked it down to this particular user's Firefox
profile.  The app worked fine if he created a brand new profile, and put
back all the plugins and extensions he had one by one.  So it looks like
it was caused by a corrupt profile, or some kind of "relic" setting from
a previous version which was still influencing browser behaviour
adversely.  The articles:

http://support.mozilla.com/kb/Managing+profiles 
http://kb.mozillazine.org/Issues_related_to_plugins 
http://kb.mozillazine.org/Problematic_extensions 
http://kb.mozillazine.org/Standard_diagnostic_-_Firefox 

were all excellent and helped us a great deal.  It didn't appear to be
any one particular extension alone which caused the problem.

So, I offer this as something to try for anyone who has a broken tile
problem that happens for users but isn't reproducible in their own
environment.  I'm sure it's just one possible cause among many, but
worth a go?

David Herbert
British Antarctic Suvey.


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