Symbolizers

The basic unit of visualization is the symbolizer. There are five types of symbolizers: Point, Line, Polygon, Raster, and Text.

Symbolizers are contained inside rules. A rule can contain one or many symbolizers.

Note

The most common use case for multiple symbolizers is a geometry (point/line/polygon) symbolizer to draw the features plus a text symbolizer for labeling these features.

../../../../_images/symbolizers.svg

Use of multiple symbolizers

Drawing order

The order of symbolizers significant, and also the order of your data.

For each feature the rules are evaluated resulting in a list of symbolizers that will be used to draw that feature. The symbolizers are drawn in the order provided.

Consider the following two symbolizers:

symbolizers:
- point:
    symbols:
    - mark:
        shape: square
        fill-color: '#FFCC00'
- point:
    symbols:
    - mark:
        shape: triangle
        fill-color: '#FF3300'

When drawing three points these symbolizers will be applied in order on each feature:

  1. Feature 1 is drawn as a square, followed by a triangle:

    ../../../../_images/symbolizer-order1.svg

    Feature 1 buffer rendering

  2. Feature 2 is drawn as a square, followed by a triangle. Notice the slight overlap with Feature 1:

    ../../../../_images/symbolizer-order2.svg

    Feature 2 buffer rendering

  3. Feature 3 is drawn as a square, followed by a triangle:

    ../../../../_images/symbolizer-order3.svg

    Feature 3 buffer rendering

Note

In the final image, Feature 1 and Feature 2 have a slight overlap. This overlap is determined by data order which we have no control over. If you need to control the overlap review the Feature Styles section on managing “z-order”.

../../../../_images/symbolizer-order4.svg

Feature style controlling z-order

Matching symbolizers and geometries

It is common to match the symbolizer with the type of geometries contained in the layer, but this is not required. The following table illustrates what will happen when a geometry symbolizer is matched up with another type of geometry.

Points

Lines

Polygon

Raster

Point Symbolizer

Points

Midpoint of the lines

Centroid of the polygons

Centroid of the raster

Line Symbolizer

n/a

Lines

Outline (stroke) of the polygons

Outline (stroke) of the raster

Polygon Symbolizer

n/a

Will “close” the line and style as a polygon

Polygons

Will “outline” the raster and style as a polygon

Raster Symbolizer

n/a

n/a

n/a

Transform raster values to color channels for display

Text Symbolizer

Label at point location

Label at midpoint of lines

Label at centroid of polygons

Label at centroid of raster outline

Syntax

The following is the basic syntax common to all symbolizers. Note that the contents of the block are not all expanded here and that each kind of symbolizer provides additional syntax.

geometry: <cql>
uom: <text>
..
x-composite: <text>
x-composite-base: <boolean>

where:

Property

Required?

Description

Default value

geometry

No

Specifies which attribute to use as the geometry.

First geometry attribute found (usually named geom or the_geom)

uom

No

Unit of measure used for width calculations

pixel

The following properties are equivalent to SLD “vendor options”.

Property

Required?

Description

Default value

x-composite

No

Allows for both alpha compositing and color blending options between symbolizers.

N/A

x-composite-base

No

Allows the rendering engine to use the symbolizer mapping to define a “base” buffer for subsequent compositing and blending using x-composite. See the section on Feature Styles for more details.

false

See the following pages for details:

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Next: Line symbolizer