Geoserver CSS Styling: Mark Dynamic URL

Question

In the following CSS styling:

* {
   mark: url(smileyface.png);
   mark-mime: "image/png";
   mark-rotation: 45;
}

How can I make the URL value dynamic (to be read from a layer's field image_url)?

I've tried the following:

* {
   mark: url([image_url]);
   mark-mime: "image/png";
   mark-rotation: [rotation_angle];
} 

But didn't work. The server console says:

WARN [renderer.style] - Unable to read image at file:/Applications/GeoServer.app/Contents/Java/data_dir/workspaces/tst/styles/%5Bdescription%5D : Can't get input stream from URL! 

The auto-generated SLD is:

<sld:PointSymbolizer>
            <sld:Graphic>
              <sld:ExternalGraphic>
                <sld:OnlineResource xmlns:xlink="http://www.w3.org/1999/xlink" xlink:type="simple" xlink:href="[image_url]"/>
                <sld:Format>image/png</sld:Format>
              </sld:ExternalGraphic>
              <sld:Rotation>
                <ogc:PropertyName>rotation_angle</ogc:PropertyName>
              </sld:Rotation>
            </sld:Graphic>
</sld:PointSymbolizer>

Although the same works for mark-rotation.

Answer 1

In the SLD, you would need to use a CQL filter for the dynamic external graphic. One trick in CSS is to write this CQL filter as plain text that will later be interpreted in SLD.

* {
   mark: url('${image_url}');
   mark-mime: "image/png";
   mark-rotation: [rotation_angle];
}