[OSM-dev] [Tilesathome] lowzoom

[Frederik Ramm]

Hi,

> On the negative side: either the captions are done with XSLT osmarender which has projection issues in lowzoom (see how city names get moved in z6) or we use or/p which gets projection right but cannot suppress text labels when there are too many. Both issues are something that a potential developer could/should have a look at.

I'm very much willing to build overlap detection into or/p. However I'm 
unsure whether the method choosen by 80n for Osmarender/XSLT is really 
the way to go. As far as I can see, it does not take projection into 
account and will thus yield a different object spacing on different 
latitudes. Has it given good results in practice? Then I'll just copy 
his algorithm and be done with it. (Or even better, someone else could ;-)

The algorithm also seems to allow proximity spacing only within one 
rule, i.e. it would not be possible to, say, first add all city captions 
and then add towns if there is still room or so.

The way you'd normally do this is simply place the text, record the 
bounding box, and before placing any other text, check whether it 
overlaps with an existing bbox. (Really good software would, on 
detecting an overlap, probably try to move the text around a bit to fit 
it in.) You would make sure to sort your objects in descending order of 
importance, so you drop the less important objects near the important ones.

The problem with this approach is that you can only make rough guesses 
at how many pixels wide and high a certain text will be, because you 
don't have access to proper font metrics at this point in processing. It 
is only at a later stage, during SVG rendering, that the font name is 
resolved and the font selected. I don't know if the SVG JavaScript 
support is so exhaustive that you could actually do this in SVG-embedded 
JavaScript but anyway I wouldn't want to go there.

80n's current approach solves this by making sure that the centre points 
of labels are spaced a minimum distance apart; it doesn't look at the 
extents of a label. Probably a reasonable workaround?

Also,

Bye
Frederik