Portrayal is the use of rules to display and convert data such as GML, coverages, JDBC result sets or COM objects into an image or formatted text document. For Web Application developers using OGC interfaces, data is accessed through a WFS or WCS. The ERDAS APOLLO Portrayal Engine transforms a collection of features or coverages into the required output format. Output formats can be vector format (SVG), image formats (GIF, JPEG, PNG, WBMP and, GeoTIFF) or even textual formats (text, HTML, XML and, PDF). The ERDAS APOLLO Portrayal Engine uses server-side rules to portray information. These rules can be expressed in several languages: Property, SLD and Java.
ERDAS APOLLO includes the ERDAS APOLLO Style Editor which provides the ability to style data using ERDAS APOLLO predefined rules. The ERDAS APOLLO Style Editor can be used to portray data using these predefined styles and rules or using custom designed ones. Custom rules can be written using the Portrayal API.
Portrayal rules and styles are two distinct concepts. Each entity provides a different level of service but both are needed to portray data. Rules are pieces of program code that provide a specific way to portray data using a classification scheme, such as classes representing a numeric attribute. Styles are text files that contain parameters defining how to portray a dataset. For example, a Style may define which field the ERDAS APOLLO Portrayal Engine will classify and what fill color and stroke width to use.
Rules define the behavior to be used when portraying any kind of compatible feature or coverage collection. They are written once and used as many times as requested on as many different data sets as required.
A rule resides in a Java class written using the ERDAS APOLLO API and is dedicated to render a defined kind of data, feature and coverage collections. A rule may also use a property or SLD file which is called a style. The Java code may be either generic to allow the rule to be used with as many feature or coverage types as possible or written to portray a specific feature or coverage collection in a more efficient way. ERDAS APOLLO provides a set of generic rules to be used with any kind of feature. Developing new portrayal rules requires advanced Java knowledge.
The ERDAS APOLLO predefined rules have been extensively tested and optimized to provide open and powerful portraying of any feature collection. Rules also support advanced logic such as generalization, classification, and new feature creation to render an updated feature collection.
Styles are collections of parameters that are used by a rule to render a specific set of data in a predefined way. They are different for each set of data and are only used by the specified data set.
A style defines a set of parameters to portray data using a selected rule and the properties for use in labeling and classifying and in determining the geometry to render, the colors for fill, the stroke to use and/or the image band to display. Styles are tied to the data being styled as well as to the rule it uses. Styles do not include any kind of logic and do not affect performance. These issues are addressed during the rules development process. The styling process only focuses on portraying the data.
The ERDAS APOLLO Style Editor simplifies the process of building styles. It allows you to define, preview and deploy styles.