.. _css_tutorial:

Tutorial: Styling data with CSS
===============================

This tutorial will show using CSS to style a layer, along with the equivalent SLD code.

To use this tutorial, you will need the :ref:`CSS extension <css_install>` as well as the ``states`` layer from the `default GeoServer configuration <https://github.com/geoserver/geoserver/tree/master/data/release/data/shapefiles>`_.

Creating a style for the states layer
-------------------------------------

.. highlight:: xml

The SLD file for the default ``states`` layer looks like this::

    <?xml version="1.0" encoding="ISO-8859-1"?>
    <StyledLayerDescriptor
      version="1.0.0"
      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"
      xmlns:gml="http://www.opengis.net/gml"
      xsi:schemaLocation="http://www.opengis.net/sld
        http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd
    ">
      <NamedLayer>
        <Name>USA states population</Name>
        <UserStyle>
          <Name>population</Name>
          <Title>Population in the United States</Title>
          <Abstract>A sample filter that filters the United States into three
            categories of population, drawn in different colors</Abstract>
          <FeatureTypeStyle>
            <Rule>
              <Title>&lt; 2M</Title>
              <ogc:Filter>
                <ogc:PropertyIsLessThan>
                 <ogc:PropertyName>PERSONS</ogc:PropertyName>
                 <ogc:Literal>2000000</ogc:Literal>
                </ogc:PropertyIsLessThan>
              </ogc:Filter>
              <PolygonSymbolizer>
                 <Fill>
                    <!-- CssParameters allowed are fill (the color) and fill-opacity -->
                    <CssParameter name="fill">#4DFF4D</CssParameter>
                    <CssParameter name="fill-opacity">0.7</CssParameter>
                 </Fill>     
              </PolygonSymbolizer>
            </Rule>
            <Rule>
              <Title>2M - 4M</Title>
              <ogc:Filter>
                <ogc:PropertyIsBetween>
                  <ogc:PropertyName>PERSONS</ogc:PropertyName>
                  <ogc:LowerBoundary>
                    <ogc:Literal>2000000</ogc:Literal>
                  </ogc:LowerBoundary>
                  <ogc:UpperBoundary>
                    <ogc:Literal>4000000</ogc:Literal>
                  </ogc:UpperBoundary>
                </ogc:PropertyIsBetween>
              </ogc:Filter>
              <PolygonSymbolizer>
                 <Fill>
                    <!-- CssParameters allowed are fill (the color) and fill-opacity -->
                    <CssParameter name="fill">#FF4D4D</CssParameter>
                    <CssParameter name="fill-opacity">0.7</CssParameter>
                 </Fill>     
              </PolygonSymbolizer>
            </Rule>
            <Rule>
              <Title>&gt; 4M</Title>
              <!-- like a linesymbolizer but with a fill too -->
              <ogc:Filter>
                <ogc:PropertyIsGreaterThan>
                 <ogc:PropertyName>PERSONS</ogc:PropertyName>
                 <ogc:Literal>4000000</ogc:Literal>
                </ogc:PropertyIsGreaterThan>
              </ogc:Filter>
              <PolygonSymbolizer>
                 <Fill>
                    <!-- CssParameters allowed are fill (the color) and fill-opacity -->
                    <CssParameter name="fill">#4D4DFF</CssParameter>
                    <CssParameter name="fill-opacity">0.7</CssParameter>
                 </Fill>     
              </PolygonSymbolizer>
            </Rule>
            <Rule>
              <Title>Boundary</Title>
              <LineSymbolizer>
                <Stroke>
                  <CssParameter name="stroke-width">0.2</CssParameter>
                </Stroke>
              </LineSymbolizer>
              <TextSymbolizer>
                <Label>
                  <ogc:PropertyName>STATE_ABBR</ogc:PropertyName>
                </Label>
                <Font>
                  <CssParameter name="font-family">Times New Roman</CssParameter>
                  <CssParameter name="font-style">Normal</CssParameter>
                  <CssParameter name="font-size">14</CssParameter>
                </Font>
                <LabelPlacement>
                  <PointPlacement>
                    <AnchorPoint>
                      <AnchorPointX>0.5</AnchorPointX>
                      <AnchorPointY>0.5</AnchorPointY>
                    </AnchorPoint>
                  </PointPlacement>
                </LabelPlacement>
              </TextSymbolizer>
            </Rule>
         </FeatureTypeStyle>
        </UserStyle>
        </NamedLayer>
    </StyledLayerDescriptor>

Now, let's start on a CSS file that accomplishes the same thing. 

First, got to the styles page and click on :guilabel:`Add a new style` link to start a new style. In the "New style" page, do the following:

* Name the new style anything you'd like, such as ``csstutorial``
* Choose ``CSS`` as the format
* In the :guilabel:`Generate a default style` dropdown choose ``Polygon`` and click on :guilabel:`Generate...`

.. figure:: images/tutorial_new_style.png

   Creating a new CSS style
    

.. highlight:: css
    
This creates an example style with a source similar to this one (the colors may differ)::

    /* @title cyan polygon */
    * {
        stroke: #000000;
        stroke-width: 0.5;
        fill: #0099cc;
    }


This demonstrates the basic elements of a CSS style:

A **selector** that identifies some part of the data to style. Here, the selector is ``*``, indicating that all data should use the style properties.

**Properties** inside curly braces (``{}``) which specify how the affected features should be styled. Properties consist of name/value pairs separated by colons (``:``).

We can also see the basics for styling a polygon (``fill``), and its outline (``stroke``).

.. seealso:: 

    The :ref:`css_filters` and :ref:`css_properties` pages provide more information about the options available in CSS styles.

.. highlight:: xml

Before moving on, let's save the style and preview it with the states layer:

* Click on "Apply" to save the layer and enable the style preview
* Now on the "Style Editor page", switch to the "layer preview" tab and click on the "previewing on layer" link, then choose the "states" layer in the dialog
* The style editor should now show the states layer filled and stroked

.. figure:: images/tutorial_preview.png

   Previewing the CSS style with the state layer

Let's use these basics to start translating the states style. The first rule in the SLD applies to states where the ``PERSONS`` field is less than two million::

    <Rule>
      <Title>&lt; 2M</Title>
      <ogc:Filter>
        <ogc:PropertyIsLessThan>
         <ogc:PropertyName>PERSONS</ogc:PropertyName>
         <ogc:Literal>2000000</ogc:Literal>
        </ogc:PropertyIsLessThan>
      </ogc:Filter>
      <PolygonSymbolizer>
         <Fill>
            <!-- CssParameters allowed are fill (the color) and fill-opacity -->
            <CssParameter name="fill">#4DFF4D</CssParameter>
            <CssParameter name="fill-opacity">0.7</CssParameter>
         </Fill>     
      </PolygonSymbolizer>
    </Rule>

.. highlight:: css

Using a :doc:`CQL</tutorials/cql/cql_tutorial>`-based selector, and copying the names and values of the CssParameters over, we get::

    [PERSONS < 2000000] {
      fill: #4DFF4D;
      fill-opacity: 0.7;
    }

.. highlight:: xml

For the second style, we have a ``PropertyIsBetween`` filter, which doesn't directly translate to CSS::

    <Rule>
      <Title>2M - 4M</Title>
      <ogc:Filter>
        <ogc:PropertyIsBetween>
          <ogc:PropertyName>PERSONS</ogc:PropertyName>
          <ogc:LowerBoundary>
            <ogc:Literal>2000000</ogc:Literal>
          </ogc:LowerBoundary>
          <ogc:UpperBoundary>
            <ogc:Literal>4000000</ogc:Literal>
          </ogc:UpperBoundary>
        </ogc:PropertyIsBetween>
      </ogc:Filter>
      <PolygonSymbolizer>
         <Fill>
            <!-- CssParameters allowed are fill (the color) and fill-opacity -->
            <CssParameter name="fill">#FF4D4D</CssParameter>
            <CssParameter name="fill-opacity">0.7</CssParameter>
         </Fill>     
      </PolygonSymbolizer>
    </Rule>

.. highlight:: css

However, ``PropertyIsBetween`` can easily be replaced by a combination of two comparison selectors. In CSS, you can apply multiple selectors to a rule by simply placing them one after the other. Selectors separated by only whitespace must all be satisfied for a style to apply. Multiple such groups can be attached to a rule by separating them with commas (``,``). If a feature matches any of the comma-separated groups for a rule then that style is applied. Thus, the CSS equivalent of the second rule is::

    [PERSONS >= 2000000] [PERSONS < 4000000] {
      fill: #FF4D4D;
      fill-opacity: 0.7;
    }

The third rule can be handled in much the same manner as the first::

    [PERSONS >= 4000000] {
      fill: #4D4DFF;
      fill-opacity: 0.7;
    }

.. highlight:: xml

The fourth and final rule is a bit different. It applies a label and outline to all the states::

    <Rule>
      <Title>Boundary</Title>
      <LineSymbolizer>
        <Stroke>
          <CssParameter name="stroke-width">0.2</CssParameter>
        </Stroke>
      </LineSymbolizer>
      <TextSymbolizer>
        <Label>
          <ogc:PropertyName>STATE_ABBR</ogc:PropertyName>
        </Label>
        <Font>
          <CssParameter name="font-family">Times New Roman</CssParameter>
          <CssParameter name="font-style">Normal</CssParameter>
          <CssParameter name="font-size">14</CssParameter>
        </Font>
        <LabelPlacement>
          <PointPlacement>
            <AnchorPoint>
              <AnchorPointX>0.5</AnchorPointX>
              <AnchorPointY>0.5</AnchorPointY>
            </AnchorPoint>
          </PointPlacement>
        </LabelPlacement>
      </TextSymbolizer>
    </Rule>

.. highlight:: css

This introduces the idea of rendering an extracted value (``STATE_ABBR``) directly into the map, unlike all of the rules thus far. For this, you can use a CQL expression wrapped in square braces (``[]``) as the value of a CSS property. It is also necessary to surround values containing whitespace, such as ``Times New Roman``, with single- or double-quotes (``"``, ``'``). With these details in mind, let's write the rule::

    * {
      stroke-width: 0.2;
      label: [STATE_ABBR];
      label-anchor: 0.5 0.5;
      font-family: "Times New Roman";
      font-fill: black;
      font-style: normal;
      font-size: 14;
    }

Putting it all together, you should now have a style that looks like::

    [PERSONS < 2000000] {
      fill: #4DFF4D;
      fill-opacity: 0.7;
    }
    
    [PERSONS >= 2000000] [PERSONS < 4000000] {
      fill: #FF4D4D;
      fill-opacity: 0.7;
    }
    
    [PERSONS >= 4000000] {
      fill: #4D4DFF;
      fill-opacity: 0.7;
    }

    * {
      stroke-width: 0.2;
      label: [STATE_ABBR];
      label-anchor: 0.5 0.5;
      font-family: "Times New Roman";
      font-fill: black;
      font-style: normal;
      font-size: 14;
    }

Click the :guilabel:`Apply` button at the bottom of the form to save your changes.

.. figure:: images/tutorial_noborder.png

   CSS style applied to the ``states`` layer

You will see that the borders are missing! In the GeoServer CSS module, each type of symbolizer has a "key" property which controls whether it is applied. Without these "key" properties, subordinate properties are ignored. These "key" properties are:

* **fill**, which controls whether or not Polygon fills are applied. This specified the color or graphic to use for the fill.
* **stroke**, which controls whether or not Line and Polygon outline strokes are applied. This specifies the color (or graphic fill) of the stroke.
* **mark**, which controls whether or not point markers are drawn. This identifies a Well-Known Mark or image URL to use.
* **label**, which controls whether or not to draw labels on the map. This identifies the text to use for labeling the map, usually as a CQL expression. 
* **halo-radius**, which controls whether or not to draw a halo around labels. This specifies how large such halos should be.

.. seealso:: 

    The :ref:`css_properties` page for information about the other properties.

Since we don't specify a ``stroke`` color, no stroke is applied. Let's add it, replacing the final rule so that it will now look like this::

    * {
      stroke: black;
      stroke-width: 0.2;
      label: [STATE_ABBR];
      label-anchor: 0.5 0.5;
      font-family: "Times New Roman";
      font-fill: black;
      font-style: normal;
      font-size: 14;
    }

.. figure:: images/tutorial_border.png

   Border added to style

Refining the style
------------------

Removing duplicated properties
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The style that we have right now is only 23 lines, a nice improvement over the 103 lines of XML that we started with. However, we are still repeating the ``fill-opacity`` attribute everywhere.

We can move it into the ``*`` rule and have it applied everywhere. This works because the GeoServer CSS module emulates *cascading*: While SLD uses a "painter's model" where each rule is processed independently, a cascading style allows you to provide general style properties and override only specific properties for particular features.

This brings the style down to only 21 lines::

    [PERSONS < 2000000] {
      fill: #4DFF4D;
    }
    
    [PERSONS > 2000000] [PERSONS < 4000000] {
      fill: #FF4D4D;
    }
    
    [PERSONS > 4000000] {
      fill: #4D4DFF;
    }

    * {
      fill-opacity: 0.7;
      stroke-width: 0.2;
      label: [STATE_ABBR];
      label-anchor: 0.5 0.5;
      font-family: "Times New Roman";
      font-fill: black;
      font-style: normal;
      font-size: 14;
    }

Scale-dependent styles
^^^^^^^^^^^^^^^^^^^^^^

The labels for this style are nice, but at lower zoom levels they seem a little crowded. We can easily move the labels to a rule that doesn't activate until the scale denominator is below 2000000. We do want to keep the stroke and fill-opacity at all zoom levels, so we can separate them from the label properties.

Keep the following properties in the main (``*``) rule::

    * {
      fill-opacity: 0.7;
      stroke-width: 0.2;
    }

Remove all the rest, moving them into a new rule::

    [@sd < 20M] {
      label: [STATE_ABBR];
      label-anchor: 0.5 0.5;
      font-family: "Times New Roman";
      font-fill: black;
      font-style: normal;
      font-size: 14;
    }

Setting titles for the legend
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

So far, we haven't set titles for any of the style rules. This doesn't really cause any problems while viewing maps, but GeoServer uses the title in auto-generating legend graphics. Without the titles, GeoServer falls back on the names, which in the CSS module are generated from the filters for each rule. Titles are not normally a part of CSS, so GeoServer looks for them in specially formatted comments before each rule. We can add titles like this::

    /* @title Population < 2M */
    [PERSONS < 2000000] {
    
      ...

    /* @title 2M < Population < 4M */
    [PERSONS > 2000000] [PERSONS < 4000000] {

      ...
    
    /* @title Population > 4M */
    [PERSONS > 4000000] {

      ...


    /* @title Boundaries */
    * {

      ...

Because of the way that CSS is translated to SLD, each SLD rule is a combination of several CSS rules. This is handled by combining the titles with the word "with". If the title is omitted for a rule, then it is simply not included in the SLD output.

The final CSS should looks like this::

    /* @title Population < 2M */
    [PERSONS < 2000000] {
      fill: #4DFF4D;
      fill-opacity: 0.7;
    }
    
    /* @title 2M < Population < 4M */
    [PERSONS >= 2000000] [PERSONS < 4000000] {
      fill: #FF4D4D;
      fill-opacity: 0.7;
    }
    
    /* @title Population > 4M */
    [PERSONS >= 4000000] {
      fill: #4D4DFF;
      fill-opacity: 0.7;
    }
    
    
    /* @title Boundaries */
    * {
      stroke: black;
      stroke-width: 0.2;
      fill-opacity: 0.7;
    }
    
    [@sd < 20M] {
      label: [STATE_ABBR];
      label-anchor: 0.5 0.5;
      font-family: "Times New Roman";
      font-fill: black;
      font-style: normal;
      font-size: 14;
    }

.. figure:: images/tutorial_finalstyle.png

   Final style with rule names

Applying rule nesting
^^^^^^^^^^^^^^^^^^^^^

As a final variation, the style can be made more compact by leveraging rule nesting::

  * {
    stroke: black;
    stroke-width: 0.2;
    fill-opacity: 0.7;
  
    /* @title Population < 2M */
    [PERSONS < 2000000] {
      fill: #4DFF4D;
    };
    /* @title 2M < Population < 4M */
    [PERSONS >= 2000000] [PERSONS < 4000000] {
      fill: #FF4D4D;
    };
    /* @title Population > 4M */
    [PERSONS >= 4000000] {
      fill: #4D4DFF;
    };
    
    /* Labelling */
    [@sd < 20M] {
      label: [STATE_ABBR];
      label-anchor: 0.5 0.5;
      font-family: "Times New Roman";
      font-fill: black;
      font-style: normal;
      font-size: 14;
    }   
  }

CSS Workshop
^^^^^^^^^^^^

For more details, visit the next section, the `CSS workshop <../workshop/index.html>`_. This workshop has been used in the past for classroom settings to teach the CSS extension and has been ported to the user documentation.