[OSM-talk] Path rendering in the cycleway

[Andy Allan]

On Sun, Aug 31, 2008 at 9:12 AM, spaetz <osm at sspaeth.de> wrote:

> And second each added tag makes a) parsing slower b) the stylesheet more unreadable (for maintainers).

Yeah - from the mapnik side of things we've already had Steve Chiltern
asking for help. The filter rules become immense quite quickly.

Lets imagine a basic line, that can be on a bridge or in a tunnel - so
needs three filters

foo = bar and tunnel = yes
foo = bar and bridge = yes
foo = bar

but if there's now two ways of tagging the line, and maybe two ways to
tag a tunnel/bridge, then the rules become unreadable:

(foo = bar or (foo = ben and ben = neb)) and (tunnel = yes or (kol =
lok and lok = web)))
(foo = bar or (foo = ben and ben = neb)) and bridge = yes or (blah =
ack and swe = ob))
(foo = bar or (foo = ben and ben = neb)

....and full of devious bugs like the eagle-eyed would notice in the
rules above[1]. And the self same complexity then bubbles up all over
the place (like in mkgmap, and the editors...) which simply
continually raises the barrier to entry and therefore makes the osm
dataset *less* useful, not more.

I'm meeting up with Steve Chiltern next week to look at ways to make
the style rules for mapnik more easily maintainable when we are both
in Aberdeen, and if anyone is interested in helping please jump in and
help.

Cheers,
Andy

[1] the second line is missing some brackets, and matches all
instances of the alternative bridge tagging, even those not on foo=bar
lines).