Archive for the 'SLD' Category

SLD Rule Evaluation Followup

Posted in Locality and Space, OpenStreetMap, SLD on October 2nd, 2007 at 17:21:55

Andreas posted earlier today with a followup on the SLD Rules front: Though I had consigned myself to expressions, he had not, and went on to create a very nice looking rules system which supports the full range of SLD operators.

More information is available in his post to the mailing list: the code is in his sandbox (though this URL will move once it moves to trunk).

Three rule types are used to build up rules: comparison rules, for comparing values against feature attributes, FeatureID rules, for limiting by fid, and Logical rules, which group the other rule types together. Logical rules can be AND, OR, or NOT.

One key benefit to this — which I had not thought about until Andreas brought it up — is the ability to serialize these rules back out to SLD. It’s possible to use something like yacc/lexx (C, etc.) or eval() to evaluate an expression — but in Javascript, at least, it’s hard to turn that expression back into rules. This saves us that problem, and means we can serialize out SLD from user-created styling rules — clearing the way for the in-browser SLD editor that many people have been talking about for a long time.