Graphics -- Introduction

Java Software Solutions Lewis and Loftus

Graphics -- Introduction

• The use of graphics is common among modern software systems

• Java has strong API support for graphics in the java.awt

(abstract windowing toolkit) package

• Chapter 7 focuses on:

– the coordinate system for Java graphics

– the use of color

– drawing shapes such as lines, ovals, rectangles, etc.

– the use of fonts

– basic animation techniques

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Chapter 7 Copyright 1997 by John Lewis and William Loftus. All rights reserved.


The Graphics Class

• An object of the Graphics class represents a particular drawing surface

• It defines a graphics context in which drawn shapes will be rendered

• The Graphics class contains methods for drawing various shapes and controlling visual aspects like font and color

• An applet has a graphics context, which is automatically passed to the paint method when it is called

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The Coordinate System

• A simple two-dimensional coordinate system exists for each graphics context (or drawing surface)

• Each point on the coordinate system represents a single pixel

• The top left corner of the area is coordinate <0, 0>

• A drawing surface has a particular width and height

• Anything drawn outside of that area will not be visible

• See Coordinates.java


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The Coordinate System

<0, 0> x y

<x, y>

X

<width-1, height-1>

Y


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Color

• The Color class is used to define and manage the color in which shapes are drawn

• Colors are defined by their RGB value , which defines the relative contribution of the primary colors red, green, and blue

• Each drawing surface has a foreground color and a background color

• The setColor method of the Graphics class defines the foreground color, and the setBackground method of the component (the applet) sets the background color

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Color

• The Color class contains several predefined colors, defined as public, static constants

• See Nature.java

• Many other colors can be defined using the constructor of the Color class

• Over 16 million colors can be defined, but humans cannot distinguish between many similar colors

• Furthermore, the hardware of most systems has limitations to the color options available


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XOR Mode

• Drawing in normal mode causes shapes of the same color to blend together

• Drawing in XOR mode causes the overlapping portions of the shapes to be rendered in a contrasting color

• This effect can be used to "erase" a shape by redrawing it in the same color in the same spot while in XOR mode

• See XOR_Demo.java

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Drawing Shapes

• The Graphics class contains methods for drawing several specific shapes: lines, ovals, rectangles, arcs, polygons, and polylines

• Most shapes can be drawn filled or unfilled

• A line, drawn with the drawLine method, is always one pixel wide and cannot be filled


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Ovals

• An oval is defined by its bounding rectangle : width height

• The methods that draw an oval take four parameters, all integers: drawOval (x, y, width, height) fillOval (x, y, width, height)


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Ovals

• The first two parameters are the <x, y> coordinate of the top-left corner of the bounding rectangle

• The third and fourth parameters specify the width and height of the bounding rectangle

• The drawOval method draws an unfilled oval and the fillOval method draws a filled (opaque) oval

• See Ovals.java

and Rotating_Disk.java

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Rectangles

• Rectangles can be drawn

– filled or unfilled

– with squared or rounded corners

– with a slight three-dimensional effect or not

• The primary parameters for all rectangle drawing methods define the upper left corner of the rectangle and its width and height

• The shape of the rounded corner of a rounded rectangle are defined by an arc width and height

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Rectangles

• A three dimensional rectangle is shown using a small highlight on two sides of the rectangle

• The highlight appears on the bottom and right or the top and left as specified by a boolean parameter to the 3D drawing methods

• See Rectangles.java

,

Rounded_Rectangles.java

, and

Three_D_Rectangles.java

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Arcs

• An arc is defined as a segment of an oval

• The first four parameters to the arc drawing methods define the bounding rectangle of the oval (top left corner, width, and height)

• The other two parameters define the start angle and the arc angle

• The start angle indicates where the arc begins and the arc angle determines how far the arc sweeps across its defining oval

• See Arc.java

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Arcs

• The start angle can be specified using both positive and negative values:

90

-270

45

-315

180

-180

0

360

-360

270

-90

Copyright 1997 by John Lewis and William Loftus. All rights reserved.

Chapter 7

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Arcs

• An arc angle can also be positive or negative

• A positive arc angle sweeps counterclockwise, and a negative arc angle sweeps clockwise

• Therefore, the same arc can be specified using four different combinations of start and arc angles

• Arcs can also be filled or unfilled

• See Arcs.java


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Polygons

• A polygon is a multisided figure defined by a series of ordered points

• Line segments connecting the points form the polygon

• The points are defined by corresponding arrays of x and y coordinate values, and can already be incorporated into an object of the Polygon class

• Polygons are closed, forming a line segment from the last point back to the first

• See Polygons.java

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Polylines

• A polyline is similar to a polygon except that it is not closed

• That is, there is no line segment from the last point back to the first unless explicitly specified

• They are convenient for specifying certain kinds of complex shapes

• Polylines cannot be filled

• See Polylines.java

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Fonts

• A font defines the look of each character when it is printed or drawn

• The Font class provides methods for specifying fonts in a Java program

• Each computer system supports a specific set of fonts

• See Font_Lister.java

• The setFont method defines the current font for a program

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Fonts

• A font is defined using the Font class constructor and a combination of:

– font name

– font style: plain, bold, italic, or bold+italic

– font size, in points

• Constants are defined in the Font class to specify the font style

• See Entropy.java

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Animations

• Simple animations can be accomplished by drawing a shape, then erasing it, then drawing it again in a slightly altered position

• Erasing can be accomplished through careful use of

XOR mode

• Timing must be controlled so that the animation does not move too fast

• See Bouncing_Ball.java

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Graphics -- Introduction

Graphics -- Introduction

The Graphics Class

The Coordinate System

The Coordinate System

Color

Color

XOR Mode

Drawing Shapes

Ovals

Ovals

Rectangles

Rectangles

Arcs

Arcs

Arcs

Polygons

Polylines

Fonts

Fonts

Animations

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Graphics -- Introduction