[Tilesathome] Avoiding name duplications on dual carriageways

[Frederik Ramm]

Hi,

    I have an improvement for or/p that I presented to you a while ago 
and then forgot about it ;-) it is meant to reduce the number of text 
duplications (name and ref tags) on dual carriageways and similar 
situations.

What it does is this:

If a text rule in the style file is flagged "avoid-duplicates=yes", then 
whenever this rule leads to a text being drawn, the first and last node 
of the corresponding way and the text are stored in memory. Before a 
text is drawn for another way, the list is checked to see whether the 
same text has already been written to a way whose start and end nodes 
are close to the start and end nodes of the way being looked at.

(The distance between the two nodes that will trigger dropping of labels 
is currently a hard-coded number in metres, should probably be 
configurable per zoom level, e.g. "avoid-duplicates=1000" or so?)

This is quite effective at removing double labels, while at the same 
time not doing full text collision avoidance (i.e. if you have two roads 
with *different* names next to each other, both names will be drawn no 
matter what). The drawback is that results may look arbitrary; whichever 
label gets drawn first, gets drawn first (and blocks out another label 
in a similar position). Although I have not observed this during my 
tests, it is conceivable that a "ref" label along a motorway that sits 
exactly on the line between two z12 tiles would be drawn for one lane 
on one tile and for the other lane on the other.

Osmarender/XSLT compatibility: Any software not interpreting the 
"avoid-duplicates" tag will simply draw *all* labels as before, so again 
if you are on the border of two z12 tiles and one is rendered with 
Osmarender/XSLT and the other with or/p, it might happen that the XSLT 
tile has both names and one of them simply stops at the tile boundary. 
This is probably the only adverse effect.

What do people think - should this be used for tiles at home, would it make 
the maps better on the whole, or do the drawbacks outweigh the 
advantages? - Since the method has to be activated for each text rule we 
could start using it only for names/refs on motorways and trunk roads 
(which are most likely to be dual carriageways) and see how it works

Bobkare, do you have any plans for doing something similar in the XSLT 
version (in which case I might shelve this plan and copy your's for the 
sake of compatibility)?

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"