[Tilecache] TC called if OL maxextent bigger than a particular layer's ?

[Yves Moisan]

Hi All,

We have a case where the maxExtent of our map needs to be larger than 
the area for which we have air photos (the layers we are tilecaching).  
Up to now, we have set both OL's maxExtent and tilecache.cfg's bbox 
element of the layers in question to the same value, which is to say we 
are deliberately restricting the map extent to the area covered by the 
tilecached layer.  However we need to use a larger mapExtent than that 
of the image layers, so the question is do we need to align the TC'ed 
layers bbox with the new maxExtent ?  According to the docs 
(http://tilecache.org/readme.html) there's no way around that.

We would of course like to avoid doing that because the series of images 
we have is only a thin diagonal line in a huge bbox and we already have 
way too many blank tiles already so making the maxExtent larger would 
only generate more blank tiles.  Speaking of blank tiles, is there 
another way than to specify a bunch of bboxes to separate 
tilecache_seed.py calls to render only areas that have valuable info at 
very high zoom levels ?  The idea is that in the very unlikely event 
someone zooms in to level 10 in an area where ther is no image, the WMS 
server will be called to render blank images and that should not be too 
costly.  The most likely event of course is that people will zoom in 
where there are visible images and that would be taken care of by a 
series of bbox seeds.  How do people cope with OL.mapExtent > TC.bbox ?

TIA,

Yves