[Understanding] Renderers [9]

[Keiron Liddle <ke...@aftexsw.com>]

Renderers
---------

A renderer is used to convert the Area Tree into the output document.
The renderer is given the tree one page at a time. 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 reeady 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 AbstractRenderer does most of the work to iterate through the area 
tree parts. This means that the most renderers simply need to implement 
the specific parts with inserting text, images and lines. The methods can 
easily be overridden to handle things in a different way or do some extra 
processing.


Fonts
-----

The fonts are setup by the renderer being used. The font metrics are used 
during the layout process to determine the size of characters.


Render Context
--------------

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.


XML Handling
------------

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.


Extensions
----------

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.


Types
-----

PDF
This uses the PDFDocument classes to create a PDF document. This supports
out of order rendering as it is possible to reserve a pdf page object that
can be later filled. Most of the work is to insert text or create lines.
SVG is handled by the XML handler that uses the PDFGraphics2D and batik to
draw the svg into the pdf page.
This also allows for font embedding.

SVG
This creates a single svg document that contains all the pages rendered
with page sequences horizontally and pages vertically. This also adds
links between the pages so that it can be viewed by clicking on the page
to go to the next page.

TXT
This simply outputs to a text document.

AWT
This draws the pages into an AWT graphic.

PS & PCL
Similar to PDF.

XML
Creates an XML file that represents the AreaTree.

Print
This prints the document using the java printing facitlities. The AWT
rendering is used to draw the pages onto the printjob.

RTF & MIF
These formats do not use this rendering mechanism.


Adding a Renderer
-----------------

It is also possible to add other renderers. The renderer simply needs to
implement the Renderer interface. The AbstractRenderer does most of what
is needed so it is better to extend this. This means that you only need to
implement the basic functionality such as text and lines.


Multiple Renderers
------------------

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 render
the Area Tree in each renderer. This can be handled by the AreaTree
Handler.


Associated Tasks
----------------
Sort out multiple renderers concept.







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