This section provides a simple overview of the Styled Layer Descriptor (SLD) specification from the Open GIS Consortium (OGC). The implementation of SLD in GeoTools2 is then discussed and a series of examples are shown.

A synbolizer is used to convert a feature to a "picture", that is, it describes what the feature is to look like when rendered to an output device. Since different renderers have different capabilities the descriptions of the symbolization are necessarily platform independent and a bit absract in places. A symbolizer is also resposnsible for determining what sort of symbol is drawn. There are, currently, five types of symbolizer defined in the SLD spec. These are Point symbolizer Line symbolizer Polygon symbolizer Text symbolizer Raster symbolizer GeoTools2 currently implements the first four of these but not the Raster symboliser. A SLD document will request a default symboliser and then override the default behaviour with supplied parameters. In Geotools2 this is reperesented using the Symbolizer class in the org.geotools.styling package. Each of the specific types of symbolizer is implemented as a DefaultNameSymbolizer class, e.g. DefaultLineSymbolizer. Each type of symbolizer has different parameters that are used to describe the way to draw the feature.

A point symbolizer describes a "graphic" which is to be drawn at some point. The default symbolizer (org.geotools.styling.DefaultPointStyler.java) will draw nothing, so you must add a Graphic element, The whole of a point symbolizer is a an optional geometry and an optional graphic. The geometry element is to tell the symbolizer which geometry attribute of the feature to use. If it is missing then the default geometry of the feature will be used. A graphic (org.geotools.styling.DefaultGraphic.java) is an object which holds a list of zero or more external graphics and marks, and infomation on the size, rotation and opacity of the graphic. A renderer will work through the list of external graphics comparing the formats it can render with the graphics format, if it can the image will be fetched and rendered, if not then the next external graphic in the list will be tryed. If the list of external graphics is exhausted the renderer will begin to work through the list of marks agian checking if it can render each mark in turn, if that fails a default mark of a square will be used. The size parameter sets what size the external graphic or mark will be drawn. If it is not set then external graphics will be drawn at thier inherent size if they have one or 16 pixels high if not, the default size of a mark is 6 pixels square. Opacity describes how opaque the graphic is with 0.0 being completly transparent and 1.0 being completly solid. The default opacity is 1.0. Rotation gives the rotation of the graphic in degrees clockwise about the centre of the graphic. All three of these elements are expressions so can be dependent on attributes of the feature being rendered or can be static litterals.

An external graphic (org.geotools.styling.DefaultExternalGraphic.java) simply holds a url refering to the image to be drawn (using xlink) and a format string that describes the image's mime type. The format string allows the renderer to determine if it can render an image of that type.

A mark describes a shape which can be filled and stroked. Basically the fill describes the colour of the mark and the stroke describes the outline of it. Figure 1 shows a collection of marks rendered using the following sld this is a program list