# 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.

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:

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

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

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”.

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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