Apache™ FOP Design: Renderers

A renderer is primarily designed to convert a given area tree into the output document format. It should be able to produce pages and fill the pages with the text and graphical content. Usually the output is sent to an output stream.

Some output formats may support extra information that is not available from the area tree or depends on the destination of the document.

Each renderer is given an area tree to render to its output format. The area tree is simply a representation of the pages and the placement of text and graphical objects on those pages.

The renderer will be given each page as it is ready and an output stream to write the data out. All pages are supplied in the order they appear in the document. In order to save memory it is possble to render the pages out of order. Any page that is not ready to be rendered is setup by the renderer first so that it can reserve a space or reference for when the page is ready to be rendered.The renderer is responsible for managing the output format and associated data and flow.

Renderers are Responsible

Each renderer is totally responsible for its output format.

Send Output to a Stream

Because font metrics (and therefore layout) are obtained in two different ways depending on the renderer, the renderer actually sets up the fonts being used. The font metrics are used during the layout process to determine the size of characters.

The render context is used by handlers. It contains information about the current state of the renderer, such as the page, the position, and any other miscellanous objects that are required to draw into the page.

A document may contain information in the form of XML for an image or instream foreign object. This XML is handled through the user agent. A standard extension for PDF is the SVG handler.

If there is XML in the SVG namespace it is given to the handler which renders the SVG into the pdf document at the given location. This separation means that other XML handlers can easily be added.

Document level extensions are handled with an extension handler. This handles the information from the AreaTree and adds renders it to the document. An example is the pdf bookmarks. This information first needs to have all references resolved. Then the extension handler is ready to put the information into the pdf document.

You can add other renderers by implementing the Renderer interface. However, the AbstractRenderer does most of what is needed, including iterating through the tree parts, so it is probably better to extend this. This means that you only need to implement the basic functionality such as text, images, and lines. AbstractRenderer's methods can easily be overridden to handle things in a different way or do some extra processing.

The relevent AreaTree structures that will need to be rendered are:

• Page
• Viewport
• Region
• Span
• Block
• Line
• Inline

A renderer implementation does the following:

• render each individual page
• clip and align child areas to a viewport
• handle all types of inline area, text, image etc.
• draw various lines and rectangles

The layout of the document depends mainly on the font being used. If two renderers have the same font metrics then it is possible to use the same Area Tree to render both. This can be handled by the AreaTree Handler.

To Do

Work In Progress

Completed

• new renderer model
• new interface for structured documents, rtf and mif
• added handlers for xml in renderer