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Java 2D Rendering

The basic rendering mechanism is the same as in previous versions of the JDK--the drawing system controls when and how programs can draw. When a component needs to be displayed, its paint or update method is automatically invoked with an appropriate Graphics context.

The Java 2D API introduces java.awt.Graphics2D, a new type of Graphics object. Graphics2D extends the Graphics class to provide access to the enhanced graphics and rendering features of the Java 2D API.

To use Java 2D API features, you cast the Graphics object passed into a component's rendering method to a Graphics2D object.

public void Paint (Graphics g) {
    Graphics2D g2 = (Graphics2D) g;
    ...
}

Graphics2D Rendering Context

The collection of state attributes associated with a Graphics2D object is referred to as the Graphics2D rendering context. To display text, shapes, or images, you set up the Graphics2D rendering context and then call one of the Graphics2D rendering methods, such as draw or fill. As the following figure shows, the Graphics2D rendering context contains several attributes.

	The pen style that is applied to the outline of a shape. This stroke attribute enables you to draw lines with any point size and dashing pattern and to apply end-cap and join decorations to a line.
	The fill style that is applied to a shape's interior. This paint attribute enables you to fill shapes with solid colors, gradients, and patterns.
	The compositing style that is used when rendered objects overlap existing objects.
	The transform that is applied during rendering to convert the rendered object from user space to device-space coordinates. Optional translation, rotation, scaling, or shearing transforms can also be applied through this attribute.
	The clip, which restricts rendering to the area within the outline of the Shape used to define the clipping path. Any Shape can be used to define the clip.
	The font used to convert text strings to glyphs.
	Rendering hints that specify preferences in the trade-offs between speed and quality. For example, you can specify whether antialiasing should be used, if it's available.

To set an attribute in the Graphics2D rendering context, you use the set Attribute methods:

• setStroke
• setPaint
• setComposite
• setTransform
• setClip
• setFont
• setRenderingHints

When you set an attribute, you pass in the appropriate attribute object. For example, to change the paint attribute to a blue-green gradient fill, you would construct a GradientPaint object and then call setPaint.

gp = new GradientPaint(0f,0f,blue,0f,30f,green);
g2.setPaint(gp);

Graphics2D holds references to its attribute objects--they are not cloned. If you alter an attribute object that is part of the Graphics2D context, you need to call the appropriate set method to notify the context. Modifying an attribute object during rendering causes unpredictable behavior.

Graphics2D Rendering Methods

Graphics2D provides the following general rendering methods that can be used to draw any geometry primitive, text, or image:

• draw--renders the outline of any geometry primitive, using the stroke and paint attributes.
• fill--renders any geometry primitive by filling its interior with the color or pattern specified by the paint attribute.
• drawString--renders any text string. The font attribute is used to convert the string to glyphs, which are then filled with the color or pattern specified by the paint attribute.
• drawImage--renders the specified image.

In addition, Graphics2D supports the Graphics rendering methods for particular shapes, such as drawOval and fillRect.

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