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In the examples up till now, we have used methods like drawOval()
or drawRect()
to draw these common geometric forms.
What about arbitrary shapes?
One approach is to use multiple calls to drawLine()
to draw a sequence of straight line segments that outlines the desired
shape.
But a faster approach is instead to use a single call to drawPolyline().
This will speed up the drawing since the process needs only one
invocation of the method, rather than one for each line segment.
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import
javax.swing.*;
import java.awt.*;
/** Example demonstrating drawPolyline().*/
public class Polygon1Applet extends JApplet
{
public void init () {
Container content_pane = getContentPane
();
int width = getSize ().width;
int height= getSize ().height;
int num_points = 21;
// Create an instance of DrawingPanel
Polygon1Panel polygon1_panel =
new Polygon1Panel
(width,height,num_points);
// Add the DrawingPanel to the contentPane.
content_pane.add (polygon1_panel);
}
}
/** Draw a polygon with drawPolyline() on
* this JPanel subclass. **/
class Polygon1Panel extends JPanel
{
int fWidth,fHeight;
int fNumPoints;
double fFactor;
Polygon1Panel (int width, int height, int nPoints)
{
fNumPoints = nPoints;
fWidth = width;
fHeight= height;
fFactor = 2.0 * Math.PI / fWidth;
} // ctor
public void paintComponent (Graphics g) {
// First paint background unless you
will
// paint whole area yourself.
super.paintComponent (g);
// Create arrays of points for each
// segment of the polygon
int [] x = new int[fNumPoints];
int [] y = new int[fNumPoints];
// Select horizontal step size
double x_del= ((double)fWidth)/
(fNumPoints-1);
// Find coordinates of the display
center
int x_offset = fWidth/2;
int y_offset = fHeight/2;
// Choose amplitude for the sine curve
int amp = (int) (y_offset
* 0.9);
// Create a sine curve from a sequence
// of short line segments
for (int i=0; i < fNumPoints;
i++) {
x[i] = (int)
(i * x_del);
y[i] = (int)
(amp * Math.sin (fFactor * x[i]) )
+
y_offset;
}
// Set the line color to red
g.setColor (Color.red);
// Draw curve with single call to
drawPolyline
g.drawPolyline (x,y,fNumPoints);
// Change the line color and draw
the x-y axes
g.setColor (Color.green);
g.drawLine (0,y_offset,fWidth-1,y_offset);
g.drawLine (x_offset,0,x_offset,fHeight-1);
} // paintComponent
} // class Polygon1Panel
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drawPolygon(int[]
x, int[] y, int nPoints)
will work similarly, except it will connect the last
point to the first if they are not the identical.
An interesting overriden version of this method
drawPolygon(Polygon
poly)
uses an instance of the Polygon
class to hold the points.
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import
javax.swing.*;
import java.awt.*;
/** Example demonstrating drawPolygon() method. */
public class Polygon2Applet extends JApplet
{
public void init() {
Container content_pane = getContentPane();
int width = getSize().width;
int height= getSize().height;
int num_points = 21;
// Create an instance of DrawingPanel
Polygon2Panel polygon2_panel =
new Polygon2Panel(width,height,num_points);
// Add the DrawingPanel to the contentPane.
content_pane.add(polygon2_panel);
} // init
} // class Polygon2Applet
/** Draw a polygon with drawPolygon() on
* this JPanel subclass. **/
class Polygon2Panel extends JPanel
{
int fWidth,fHeight;
int fNumPoints;
double fFactor;
Polygon2Panel (int width, int height, int numPoints) {
fNumPoints = numPoints;
fWidth = width;
fHeight= height;
fFactor = 2.0 * Math.PI / (fNumPoints-1);
} // ctor
public void paintComponent(Graphics g) {
// First paint background
super.paintComponent(g);
// Create arrays of points for each
// segment of the polygon
int [] x = new int[fNumPoints];
int [] y = new int[fNumPoints];
// Find coordinates of the display
center
int x_offset = fWidth/2;
int y_offset = fHeight/2;
// Choose radius for the circle
int radius = (int)(y_offset * 0.9);
// Create a sine curve from a sequence
// of short line segments
for (int i=0; i < fNumPoints; i++)
{
x[i] = (int)( radius
* Math.cos(fFactor * i) )
+
x_offset;
y[i] = (int)( radius
* Math.sin(fFactor * i) )
+
y_offset;
}
// Set the line color to red
g.setColor(Color.red);
// Create a polygon object from
these point arrays
Polygon poly = new Polygon(x,y,fNumPoints);
// Then pass the polygon object
for drawing
g.drawPolygon(poly);
// Change the line color and draw
the x-y axes
g.setColor(Color.green);
g.drawLine(0,y_offset,fWidth-1,y_offset);
g.drawLine(x_offset,0,x_offset,fHeight-1);
} // paintComponent
} // class Polygon2Panel
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This graphics technique - creating a shape object, here a polygon,
and then telling it to draw itself - is a key feature of the Java2D
approach to graphics that we will briefly discuss in Chapter
6: Java : Supplements.
The Polygon
class also has several interesting methods such as
public
boolean contains(int x, int y);
that indicates if the given point is within the polygon
and
public
void translate(int dx, int dy);
that adds dx
and dy to
each of the points of the polygon and thus move the shape by that
amount.
Latest update: Oct. 26, 2004
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