3.1. CSS Quickstart¶

3.1.1.1. Key properties¶

As we work through CSS styling examples you will note the use of key properties. These properties are required to trigger the creation of an appropriate symbolizer in SLD.

Using just these key properties and the selector *, you will be able to visualize vector data.

For example, here is the key property stroke providing a gray representation for line or polygon data:

* {
   stroke: gray;
}

Here is the key property fill providing a blue fill for polygon data:

* {
   fill: #2020ED;
}

Here is the key property mark showing the use of the well-known symbol square:

* {
   mark: symbol(square);
}

Here is the key property label generating labels using the CITY_NAME feature attribute:

* {
   label: [CITY_NAME];
}

Here is the key property halo-radius providing an outline around generated label:

* {
   label: [NAME];
   halo-radius: 1;
}

Reference:

• CSS Cookbook

• CSS Examples

3.1.1.2. Rules¶

We have already seen a CSS style composed of a single rule:

* {
  mark: symbol(circle);
}

We can also make a rule that only applies to a specific FeatureType:

populated_places {
  mark: symbol(triangle);
}

We can make a style consisting of more than one rule, carefully choosing the selector for each rule. In this case we are using a selector to style capital cities with a star, and non-capital with a circle:

[ FEATURECLA = 'Admin-0 capital' ] {
  mark: symbol(star);
  mark-size: 6px;
}

[ FEATURECLA <> 'Admin-0 capital' ] {
  mark: symbol(circle);
  mark-size: 6px;
}

The feature attribute test performed above uses Constraint Query Language (CQL). This syntax can be used to define filters to select content, similar to how the SQL WHERE statement is used. It can also be used to define expressions to access attribute values allowing their use when defining style properties.

Rule selectors can also be triggered based on the state of the rendering engine. In this example we are only applying labels when zoomed in:

[@scale < 20000000] {
   label: [ NAME ];
}

In the above example the label is defined using the CQL Expression NAME. This results in a dynamic style that generates each label on a case-by-case basis, filling in the label with the feature attribute NAME.

Reference:

• Filter Syntax

• ECQL Reference

3.1.1.3. Cascading¶

In the above example feature attribute selection we repeated information. An alternate approach is to make use of CSS Cascading and factor out common properties into a general rule:

[ FEATURECLA = 'Admin-0 capital' ] {
  mark: symbol(star);
}

[ FEATURECLA <> 'Admin-0 capital' ] {
  mark: symbol(circle);
}

* {
  mark-size: 6px;
}

3.1.1.4. Pseudo-selector¶

Up to this point we have been styling individual features, documenting how each shape is represented.

When a shape is represented using a symbol, we have a second challenge: documenting the colors and appearance of the symbol. The CSS extension provides a pseudo-selector allowing further properties to be applied to a symbol.

Example of using a pseudo-selector:

* {
  mark: symbol(circle);
}

:mark {
  fill: black;
  stroke: white;
}

In this example the :mark pseudo-selector is used select the circle mark, and provides a fill and stroke for use when rendering.

The above pseudo-selectors apply to all symbols, but to be specific the syntax nth-symbol(1) can be used:

* {
  mark: symbol(circle);
}

:nth-mark(1) {
  fill: black;
  stroke: white;
}

Reference:

• Styled Marks (User Guide)

3.1.2.1. SLD Style¶

SLD makes use of a series of Rules to select content for display. Content is selected using filters that support attribute, spatial and temporal queries.

Once selected, content is transformed into a shape and drawn using symbolizers. Symbolizers are configured using CSS Properties to document settings such as “fill” and “opacity”.

Content can be drawn by more than one rule, allowing for a range of effects.

Here is an example SLD file for reference:

<?xml version="1.0" encoding="ISO-8859-1"?>
<StyledLayerDescriptor version="1.0.0"
 xsi:schemaLocation="http://www.opengis.net/sld StyledLayerDescriptor.xsd"
 xmlns="http://www.opengis.net/sld"
 xmlns:ogc="http://www.opengis.net/ogc"
 xmlns:xlink="http://www.w3.org/1999/xlink"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <NamedLayer>
    <Name>airports</Name>
    <UserStyle>
      <Title>Airports</Title>
      <FeatureTypeStyle>
        <Rule>
          <Name>airports</Name>
          <Title>Airports</Title>
            <PointSymbolizer>
              <Graphic>
                <ExternalGraphic>
                  <OnlineResource xlink:type="simple"
                  xlink:href="airport.svg" />
                  <Format>image/svg</Format>
                </ExternalGraphic>
                <ExternalGraphic>
                  <OnlineResource xlink:type="simple"
                  xlink:href="airport.png" />
                  <Format>image/png</Format>
                </ExternalGraphic>
                <Mark>
                  <WellKnownName>triangle</WellKnownName>
                  <Fill>
                    <CssParameter name="fill">#000000</CssParameter>
                  </Fill>
                  <Stroke>
                    <CssParameter name="stroke">#FFFFFF</CssParameter>
                    <CssParameter name="stroke-opacity">0.50</CssParameter>
                  </Stroke>
                </Mark>
              <Size>16</Size>
            </Graphic>
          </PointSymbolizer>
        </Rule>
      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>

3.1.2.2. CSS Style¶

CSS also makes use of rules, each rule making use of selectors to shortlist content for display. Each selector uses a CQL filter that supports attribute, spatial and temporal queries. Once selected, CSS Properties are used to describe how content is rendered.

Content is not drawn by more than one rule. When content satisfies the conditions of more than one rule the resulting properties are combined using a process called inheritance. This technique of having a generic rule that is refined for specific cases is where the Cascading in Cascading Style Sheet comes from.

Here is an example using CSS:

* {
  mark: url(airport.svg);
  mark-mime: "image/svg";
}

In this rule the selector * is used to match all content. The rule defines properties indicating how this content is to be styled. The property mark is used to indicate we want this content drawn as a Point. The value url(airport.svg) is a URL reference to the image file used to represent each point. The mark-mime property indicates the expected format of this image file.

3.1.4.1. Explore Data¶

1. Return to the Data tab and use the Compute link to determine the minimum and maximum for the scalerank attribute.

3.1.4.2. Challenge Compare SLD Generation¶

# The rest API can be used to review your CSS file directly.

Browser:

• view-source:http://localhost:8080/geoserver/rest/styles/airport0.css

Command line:

curl -v -u admin:geoserver -XGET http://localhost:8080/geoserver/rest/styles/airports0.css

1. The REST API can also be used generate an SLD file:

   Browser:

   • view-source:http://localhost:8080/geoserver/rest/styles/airport0.sld?pretty=true

Command line:

curl -v -u admin:geoserver -XGET http://localhost:8080/geoserver/rest/styles/airports0.sld?pretty=true

1. Compare the generated SLD differ above with the handwritten SLD file used as an example?

   Challenge: What differences can you spot?

   Instructor Notes

   Generated SLD does not include name or title information; this can be added by students using an annotation. Encourage students to look this up in the reference material provided.

   The second difference is with the use of a fallback Mark when defining a PointSymbolizer. The CSS extension does not bother with a fallback as it knows the capabilities of the GeoServer rendering engine (and is not trying to create a reusable style).