Thomas Davie wrote:
I'm just getting going with a renderer that'll use MapCSS,
\o/
If someone could cover these basics I'd appreciate it.
But before that I'll just briefly quote from a mail I sent last July:
I was going to write a brief message outlining the philosophy (FWIW)
of MapCSS, but might as well summarise it here:
- be CSS-like
- but not dogmatically so: if shadowing the original CSS spec would
vastly overcomplicate things, don't. Keep it simple!
- be informed by the vocabulary of previous attempts (Cascadenik,
Mapnik XML, etc.)
- degrade gracefully - it's fine if clients don't support absolutely
everything
So that's the big picture answer. MapCSS should do whatever is CSS-like,
unless it's clearly unworkable or too complex when applied to a map.
That means the answer to this:
> The most important thing that I can't get fixed in my head at the
> moment is whether a MapCSS file specifies a bunch of statements that
> must be executed (causing the side effect of some map being rendered)
> in order, or whether it specifies a bunch of consistent rules that
> together can be queried to find the style of a particular {node | way
> | relation}.
is the latter, because that's how CSS works.
The second case doesn't seem to make much sense either though. No
method of evaluating the style of an object is specified. Suppose one
rule specifies to set tag a to x, while another specifies to set tag a
to y. Which takes precidence? Suppose 3 other rules depend on
(respectively) a not being set, a being x, and a being y, which of these
rules apply? When should styles be merged, and when should they be
applied one on top of each other?
Ignore the 'set' and 'exit' stuff. 'exit' is basically a hack (a speed
optimisation for Halcyon) and 'set' an ill-conceived attempt to mix
TagTransform in to cope with the haphazard nature of OSM tagging. I
think there are probably things we can do to address the latter, but
'set' probably isn't one :)
cheers
Richard
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