'''Intent'''

Separate the mechanics of drawing from the act of drawing. 

'''Motivation'''

In building an OO graphics system you must connect to an 
existing host windowing system.  The host windowing system 
API usually contains operations for drawing graphics 
primitives (lines, rectangles, arcs, etc.) in a window.  
Mapping these API calls to a set of objects often relies 
upon a basic conception of the way people draw.

'''Structure'''
 
(* Diagram to be supplied later -- send to ward*)

'''Participants'''

	* '''Pen'''
		* performs drawing commands for graphics primitives (lines, arcs, rectangles)
		* maintains an internal state (color, line width, etc.)

	* '''Paper'''
		* represents (fronts for) the operating system surface being drawn on (bitmap, screen, window)
		* often responsible for creating pens

	* '''Person'''
		* draws meaningful shapes and figures using the Pen

'''Collaborations'''

	* The Person knows how to draw a specific visual (be it a bar chart, or a simple line of text).  It performs this by sending drawing commands to the Pen.  The Person is isolated from the connection to a particular type of Paper, allowing the same figures to be drawn on bitmaps, screens or printers.
	* The act of drawing is separted from the mechanics of how the drawing is done.

'''Implementation'''

Pen will often directly wrapper the API calls necessary 
to implement the drawing primitives.  Most windowing 
systems require a window handle to send these calls, 
so it obtains that from Paper.  Paper is often subclassed 
to represent the different types of graphics mediums 
(bitmaps, screens and printers).  Person will be subclassed
to represent each specific type of visual object.

'''Known Uses'''

Smalltalk/V and VisualWorks utilize this pattern in their 
graphics systems.  In Smalltalk/V, Pen is represented by 
the class Pen,  Paper is represented by the class 
GraphicsMedium and its subclasses.  The Person is 
represented by subclasses of SubPane.  In VisualWorks Pen 
is the class GraphicsContext.  Paper is the class 
DisplaySurface and its subclasses.  The Person is 
represented by the VisualComponent hierarchy.
IBM Smalltalk has a similar concept, but it does not 
map quite as well as Smalltalk/V and VisualWorks.

'''Related Patterns'''

The Paper, Pen and Person pattern can be combined with 
Decorator and Composite to form a graphics hierarchy of 
visuals that can contain other visuals.

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CategoryGui CategoryUserInterface