Dec 08, 2009
Recently I had chance to work on new features for Mapnik, thanks to sponsorship from Richard Weait. I'm so excited about these new features, I even named the new branch
Anyone familiar with the inner workings of
There are lots of shields in Northern America. In fact, hundreds if not thousands of them. Richard Weait has written about it
Welcome to the new expressions engine. It's based on
This naturally begs the question: 'can we evaluate other things at run-time,too?'. And, sure, we can! Currently, the mapnik2 branch allows 'filters', 'name' and 'file' attributes to be arbitrary expressions. Well, 'file' actually uses special type of expressions - 'path expression' (more about this later on). And we are planning to add more support in the future. Expressions themselves canbe any valid combination of the following :
Primary expressions | |
---|---|
Boolean | true, false |
Integer | 123 |
Float | 123.456 or -1.23e-04 |
Unicode String | 'unicode' |
Attribute | [attribute name] |
Multiplicative expressions | |
Mult | 2 * 2 |
Div | 100 / 1.23e-4 |
Mod | 12 % 10 |
Additive expressions | |
Plus | 123 + 456 |
Minus | 123 - 456 |
Relational expressions | |
Less than or equal to | <= , le |
Less than | < , lt |
Greater than or equal to | >= , ge |
Greater than | > , gt |
Regular expressions | |
regex_match | (123 + 1).match('124') [name].match('^United.*') |
Equality | |
Equals | = , eq |
Not equals | != , <>, neq |
Logic | |
Not | not , ! |
And | and , && |
Or | or , || |
[val] ge 1000.00looks better then
[val] > &eq; 1000.0
file="/opt/mapnik/osm/symbols/[highway]/[length].png"While this is not complicated it reduces the number of redundant entries in osm.xml by a factor! It also paves the way to the multi-shield world: if only we could know which country we're in:)OSM data is not very helpful here, maybe this will encourage adding new k:v's , we'll see. Enough talking, let's have an example.
Let's imagine you want to create a map of the world, where each country has its own pattern based on that country's national flag. I know this is a bit far fetched, but nevertheless it shows the concept quite clearly and the resulting map is fun. With current filter/rule logic you'll have to create more than 250 rule objects, each with its own filter to catch individual countries - hmm.. nope. With the new expressions approach we will get there with just one simple rule :
<PolygonPatternSymbolizer type="png" file="/opt/mapnik/mapnik-flags/flags/Flag_of_[cntry_name].png"/>The [cntry_name] attribute is evaluated at run-time and we end up with a map like this one, great!: