=== Style

Styles contain a set of rendering rules to render data elements provided by a data source. Each style can contain one or more rules for rendering specific features.

.Style attributes
[options="header",cols="m,d,d,d"]
|====================
| Attribute                 | Value | Default | Description
| comp-op | SVG compositing |  |  
| filter-mode | "all", "first" | "all" |  
| opacity | float | 1 |  
| image-filters-inflate | boolean | false |  
| image-filters | string |  |  
| direct-image-filters | string |  |  
|====================


=== Rule

A style rule filters data elements provided by a data source, and renders the matching elmements using one or more symbolizers. Rules can also be limited to specific scale factor ranges only, e.g. to implement different zoom level styles.

.Rule attributes
[options="header",cols="m,d,d,d"]
|====================
| Attribute                 | Value | Default | Description
| name | string | none |  
|====================


=== Filter

Filters specify on what data elements a rule should operate, by evaluating expressions. The expression language supports simple comparisons, logic, math, and regular expression matching.

There are also two special filters that do not evaluate expresions but depend on whether other previous filters in the style found a match:

<ElseFilter> matches if no other filter in a style matched so far, so it can be used as a fallback default.

<AlsoFilter> only matches if at least one previous filter in a style matched.

The `<Filter>` tag has no tag attributes.

==== Filter expressions

The filter expression language supports the following constants, operators, and functions. It also allows for referencing data element attributes by putting the attribute name in square brackets `[attribute]` and to change evaluation order by putting expression parts in regular round brackets `(...)`.

TODO: operator precedence
TODO: regular expression match and replace

===== Constants

`true`:: logical true
`false`:: logical false
`pi`:: 3.14159...
`deg_to_rad`:: 0.01745... (pi/180)
`rad_to_deg`:: 57.295... (180/pi)

===== Operators

`+`::
`-`::
`*`::
`/`::
`%`::
`and`, `&&`::
`or`, `||`::
`not`, `!`::
`=`, `eq`, `is`::
`!=`, `<>`, `neq`::
`<`, `lt`::
`\<=`, `le`::
`>`, `gt`::
`>=`, `ge`::
  ...

===== Functions

`sin(x)`:: Sinus
`cos(x)`:: Cosinus
`tan(x)`:: Tangens
`atan(x)`:: Arc Tangens
`exp(x)`:: `e^x^`
`log(x)`:: natural logarithm of `x`
`pow(x,y=`:: `x^y^`
`abs(x)`:: positive absolute value of `x`
`min(x,y)`:: minimum value
`max(x,y)`:: maximum value
`length(str)`:: string length