[OpenLayers-Users] strange problems with out of sequence/out of range map tiles

[Linda Rawson]

Just to add some more info to this topic.  I had very strange behavior when
I switched the tilecaching to a linux box running with mod_python.  I never
saw the problem using CGI.  That could have been a speed problem or
something else.  In addition I also found a slight problem when openlayers
sends the url encoded string to the WMS service.  Some handle them and some
do not.

Linda

On Dec 28, 2007 7:33 AM, cmose <cjmose at gmail.com> wrote:

>
> I just wanted to see if anyone had come across any more causes of the
> Random
> Tile issue. I'm encountering that quite often using OL 2.6 and TileCache
> 1.9. Numerous tiles load out of sequence - typically they are copies of
> tiles already loaded into the map (e.g., a big land mass tile showing up
> in
> the middle of the atlantic, one country's capital ending up in another
> country due to bad tile, etc.,). I am seeing this quite frequently -
> unfortunately I can't pop up an example site as this is an internal app.
>
> (I'm also not seeing any spiral loading - quite typically the tiles in the
> middle of the screen are the last to load).
>
> Thanks much
>
>
>
> Christopher Schmidt wrote:
> >
> > On Thu, Oct 05, 2006 at 09:52:49AM +0100, David Herbert wrote:
> >> I don't see this at all.  What I get is a rather random loading, and
> >> often a critical inside tile is last to load.  And sometimes as I said
> >> above, that tile doesn't load at all, though the WMS has served it.
> >> Presumably the requests for tiles are fired off in separate threads
> >> rather than synchronously? So differences in WMS loading and response
> >> times could theoretically mess up spiral gridding?  Doesn't explain the
> >> blank tiles though.
> >
> > Although the problem is apparently intermittent and hard to reproduce
> > (neither myself, Erik, or Schuyler have ever seen it), I do want to
> > address the issue of how the tiles are loaded.
> >
> > Loading the actual images is not performed by OpenLayers. The mechanism
> > in which tiles are loaded is:
> >
> >   * Create a set of 'img' elements in a grid, and set their
> >     style.display to 'none'. An 'onload' event is attached to the image,
> >     which will unset its display style, and an onerror event is attached
> >     which unsets its display style and changes the background color.
> >   * Start at the center of the grid and 'spiral' outward through the
> >     grid: set the img src on each image.
> >
> > When panning:
> >   * A column is taken from the non-visible edge of the grid, and added
> >     onto the newly panned part of the grid. The style on these tiles is
> >     set to display:none, and the tiles then  have their img.src
> >     modified.
> >
> > The second step is probably done more quickly than any of the tiles
> > load, and the loading of the tiles is then handled entirely by the
> > browser.
> >
> > So, the step to start looking is probably to add some logging in the
> > Tile.Image class, possibly in the 'draw' function.
> >
> > Beyond that, I've not yet been able to reproduce this, so tracking the
> > bug down is very hard: Is it possible that your internet connection is
> > relatively slow, and so the tile onload events get confused?
> >
> > I would expect that you would *not* have gotten this behavior under 1.0
> > or most of the pre 2.0 svn: Not until we implemented img/tile reuse.
> > Unfortunately, this can't just be removed -- the memory ramifications
> > for not reusing the image elements are just too high.
> >
> > Hopefully, this will help you begin to investigate the problem.
> > Unfortuantely, until I can reproduce it, I can't figure out how to help
> > debug it.
> >
> > Good luck, and please report back if you do find anything.
> >
> > Regards,
> > --
> > Christopher Schmidt
> > Web Developer
> > _______________________________________________
> > Users mailing list
> > Users at openlayers.org
> > http://openlayers.org/mailman/listinfo/users
> >
> >
>
> --
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> http://www.nabble.com/strange-problems-with-out-of-sequence-out-of-range-map-tiles-tp6603260p14526518.html
> Sent from the OpenLayers Users mailing list archive at Nabble.com<http://nabble.com/>
> .
>
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-- 
Linda Rawson
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