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Exploring Java

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13. Drawing With the AWT

Contents:
Basic Drawing
Colors
Fonts
Images
Drawing Techniques

If you've read the last few chapters and seen the examples in the tutorial in Chapter 2, A First Applet, then you've probably picked up the basics of how graphical operations are performed in Java. Up to this point, we have done some simple drawing and even displayed an image or two. In this chapter, we will finally give graphics programming its due and go into depth about drawing techniques and the tools for working with images in Java. In the next chapter, we'll explore image processing tools in more detail and we'll look at the classes that let you generate images pixel by pixel on the fly.

13.1 Basic Drawing

The classes you'll use for drawing come from the java.awt package, as shown in Figure 13.1.[1].

[1] The current set of drawing tools has many limitations. In the near future, JavaSoft will be releasing packages for advanced 2D graphics, which will have much greater capabilities. A 3D package is also planned. See Chapter 1, Yet Another Language? for information about likely future Java enhancements.

An instance of the java.awt.Graphics class is called a graphics context. It represents a drawable surface such as a component's display area or an off-screen image buffer. A graphics context provides methods for performing all basic drawing operations on its area, including the painting of image data. We call the Graphics object a graphics context because it also holds contextual information about the drawing area. This information includes parameters like the drawing area's clipping region, painting color, transfer mode, and text font. If you consider the drawing area to be a painter's canvas, you might think of a graphics context as an easel that holds a set of tools and marks off the work area.

There are four ways you normally acquire a Graphics object. Roughly, from most common to least, they are:

  • From AWT, as the result of a painting request. In this case, AWT acquires a new graphics context for the appropriate area and passes it to your component's paint() or update() method.

  • Directly from an off-screen image buffer. In this case, we ask the image buffer for a graphics context directly. We'll use this when we discuss techniques like double buffering.

  • By copying an existing Graphics object. Duplicating a graphics object can be useful for more elaborate drawing operations; different copies of a Graphics object draw into the same area on the screen, but can have different attributes and clipping regions.

  • Directly from an on-screen component. It's possible to ask a component to give you a Graphics object for its display area. However, this is almost always a mistake; if you feel tempted to do this, think about why you're trying to circumvent the normal paint()/repaint() mechanism.

Each time a component's update() or paint() method is called, AWT provides the component with a new Graphics object for drawing in the display area. This means that attributes we set during one painting session, such as drawing color or clipping region, are reset the next time paint() or update() is called. (Each call to paint() starts with a tidy new easel.) For the most common attributes, like foreground color, background color, and font, we can set defaults in the component itself. Thereafter, the graphics contexts for painting in that component come with those properties initialized appropriately.

If we are working in a component's update() method, we can assume our on-screen artwork is still intact, and we need only to make whatever changes are needed to bring the display up to date. One way to optimize drawing operations in this case is by setting a clipping region, as we'll see shortly. If our paint() method is called, however, we have to assume the worst and redraw the entire display.

Drawing Methods

Methods of the Graphics class operate in a standard coordinate system. The origin of a newly created graphics context is the top left pixel of the component's drawing area, as shown in Figure 13.2.

The diagram above illustrates the default coordinate system. The point (0,0) is at the top left corner of the drawing area; the point (width, height) is just outside the drawing area at the bottom right corner. The point at the bottom right corner within the drawing area has coordinates (width-1, height-1). This gives you a drawing area that is width pixels wide and height pixels high.

The coordinate system can be translated (shifted) with the translate() method to specify a new point as the origin. The drawable area of the graphics context can be limited to a region with the setClip() method.

The basic drawing and painting methods should seem familiar to you if you've done any graphics programming. The following applet, TestPattern, exercises most of the simple shape drawing commands; it's shown in Figure 13.3.

import java.awt.*;
import java.awt.event.*;
public class TestPattern extends java.applet.Applet { 
    int theta = 45;
    public void paint( Graphics g ) {
        int Width = size().width;
        int Height = size().height;
        int width = Width/2;
        int height = Height/2;
        int x = (Width - width)/2;
        int y = (Height- height)/2;
        int [] polyx =  { 0, Width/2, Width, Width/2 };
        int [] polyy =  { Height/2, 0, Height/2, Height };
        Polygon poly = new Polygon( polyx, polyy, 4 );
        
        g.setColor( Color.black );
        g.fillRect( 0, 0, size().width, size().height );
        g.setColor( Color.yellow );
        g.fillPolygon( poly );
        g.setColor( Color.red );
        g.fillRect( x, y, width, height );
        g.setColor( Color.green );
        g.fillOval( x, y, width, height );
        g.setColor( Color.blue );
        int delta = 90;
        g.fillArc( x, y, width, height, theta, delta );
        g.setColor( Color.white );
        g.drawLine( x, y, x+width, x+height );
    }
    public void init() {
        addMouseListener( new MouseAdapter() {
            public void mousePressed( MouseEvent e ) {
                theta = (theta + 10) % 360;
                repaint();
            }
        } );
    }
}

TestPattern draws a number of simple shapes and responds to mouse clicks by rotating the filled arc and repainting. Compile it and give it a try. If you click repeatedly on the applet, you may notice that everything flashes when it repaints. TestPattern is not very intelligent about redrawing; we'll examine some better techniques in the upcoming section on drawing techniques.

With the exception of fillArc() and fillPolygon(), each method takes a simple x, y coordinate for the top left corner of the shape and a width and height for its size. We have picked values that draw the shapes centered, at half the width and height of the applet.

The most interesting shape we've have drawn is the Polygon, a yellow diamond. A Polygon object is specified by two arrays that contain the x and y coordinates of each vertex. In our example, the coordinates of the points in the polygon are (polyx[0], polyy[0]), (polyx[1], polyy[1]), and so on. There are simple drawing methods in the Graphics class that take two arrays and draw or fill the polygon, but we chose to create a Polygon object and draw it instead. The reason is that the Polygon object has some useful utility methods that we might want to use later. A Polygon can, for instance, give you its bounding box and tell you if a given point lies within its area.

AWT also provides a Shape interface, which is implemented by different kinds of two dimensional objects. Currently, it is only implemented by the Rectangle and Polygon classes, but it may be a sign of things to come, particularly when JavaSoft releases the extended 2D drawing package. The setClip() method of the Graphics class takes a Shape as an argument, but for the time being, it only works if that Shape is a Rectangle.

The fillArc() method requires six integer arguments. The first four specify the bounding box for an oval--just like the fillOval() method. The final two arguments specify what portion of the oval we want to draw, as a starting angle and an offset. Both the starting angle and the offset are specified in degrees. Zero degrees is at three o'clock; a positive angle is clockwise. For example, to draw the right half of a circle, you might call:

g.fillArc(0, 0, radius * 2, radius * 2, -90, 180);  

See the Dial example in Chapter 11, Using and Creating GUI Components (widgets?) for an example of some trigonometric gymnastics with arcs().

Table 13.1 shows the shape-drawing methods of the Graphics class. As you can see, for each of the fill() methods in the example, there is a corresponding draw() method that renders the shape as an unfilled line drawing.

Table 13.1: Shape Drawing Methods in the Graphics Class
Method Description
draw3DRect() Draws a highlighted, 3D rectangle
drawArc() Draws an arc
drawLine() Draws a line
drawOval() Draws an oval
drawPolygon() Draws a polygon, connecting endpoints
drawPolyline() Draws a line connecting a polygon's points
drawRect() Draws a rectangle
drawRoundRect() Draws a rounded-corner rectangle
fill3DRect() Draws a filled, highlighted, 3D rectangle
fillArc() Draws a filled arc
fillOval() Draws a filled oval
fillPolygon() Draws a filled polygon
fillRect() Draws a filled rectangle
fillRoundRect() Draws a filled, rounded-corner rectangle

draw3Drect() automatically chooses colors by "darkening" the current color. So you should set the color to something other than black, which is the default (maybe gray or white); if you don't, you'll just get black on both sides. For an example, see the PictureButton in Chapter 11, Using and Creating GUI Components.

There are a few important drawing methods missing from Table 13.1. For example, the drawString() method, which draws text, and the drawImage() method, which draws an image. We'll discuss these methods in detail in later sections.


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