Drawing a line on transparent frame - graphics

Question

I would like to build a simple highlighter program. The idea is to set an image to change the cursor so that it will look like a highlighter. And then, when we move the cursor, it will trace a line along with our movement. The requirement here is to be able to draw the line on a transparent background (not fully transparent though, just about 55%).
My progress so far is being able to draw the line, with transparent background. However, the line is also transparent. Here's my code : left click to begin drawing, right click to stop, and press space to change color.
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
public class FreehandExample extends JFrame implements MouseListener, MouseMotionListener, KeyListener {
private int counter = 0;
private int draw = 0;
private int red[] = {58,71,231,243,255};
private int green[] = {54,224,235,109,40};
private int blue[] = {241,95,61,52,40};
private Point start, end;
private Graphics gd;
public FreehandExample()
{
setUndecorated(true);
setBackground(new Color(255,0,0));
setSize(new Dimension(300,200));
setLocationRelativeTo(null);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
addKeyListener(this);
addMouseListener(this);
addMouseMotionListener(this);
setOpacity(0.55f);
setVisible(true);
}
public void mousePressed(MouseEvent e) {
start = new Point(e.getX(), e.getY());
}
public void mouseClicked(MouseEvent e) {
if(e.getButton() == MouseEvent.BUTTON1) draw = 1;
if(e.getButton() == MouseEvent.BUTTON3) draw = 0;
}
public void mouseEntered(MouseEvent e) {}
public void mouseExited(MouseEvent e) {}
public void mouseReleased(MouseEvent e) {}
public void mouseDragged(MouseEvent e) {}
public void mouseMoved(MouseEvent e) {
gd = this.getGraphics();
if(draw==1){
end = new Point(e.getX(), e.getY());
gd.setColor(new Color( red[counter],green[counter],blue[counter]));
gd.drawLine(start.x, start.y, end.x, end.y);
start = end;
}
}
public static void main(String []args){
new FreehandExample();
}
public void keyPressed(KeyEvent e) {
if(e.getKeyCode() == KeyEvent.VK_SPACE) {
counter++;
if(counter>4) counter=0;
}
}
public void keyReleased(KeyEvent e) {}
public void keyTyped(KeyEvent e) {}
}
I have tried the concept of per-pixel transparency... but the line drawing is not being drawn immediately... there is a delay before the line is drawn. However, it indeed produced the correct result e.g the line is not transparent while the frame is transparent.
Could someone please help me modify this code to meet the requirement...?
Thanks.

Answer 1

I've managed to solve this problem using per-pixel transparency, thanks to someone from emunewz forum :) The trick is to call paintComponent() using repaint() all the time after drawing, but the fillRect() method only need to be called just once so that our lines won't disappear.
Here is my code :
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.Paint;
import java.awt.Point;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class GradientTranslucentWindow extends JFrame implements KeyListener, MouseListener, MouseMotionListener {
private int counter = 0;
private int draw = -1;
private int red[] = {58,71,231,243,255};
private int green[] = {54,224,235,109,40};
private int blue[] = {241,95,61,52,40};
private int R = 240;
private int G = 240;
private int B = 200;
private Point start, end;
private Graphics gd;
private JPanel panel;
public GradientTranslucentWindow() {
setBackground(new Color(0,0,0,0));
setSize(new Dimension(500,500));
setLocationRelativeTo(null);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
panel = new JPanel() {
#Override
protected void paintComponent(Graphics g) {
if (g instanceof Graphics2D) {
Paint p = new GradientPaint(0.0f, 0.0f, new Color(R, G, B, 0), 0.0f, getHeight(), new Color(R, G, B, 150), true);
Graphics2D g2d = (Graphics2D)g;
g2d.setPaint(p);
if(draw==-1) g2d.fillRect(0, 0, getWidth(), getHeight());
}
}
};
setContentPane(panel);
addKeyListener(this);
addMouseListener(this);
addMouseMotionListener(this);
}
public static void main(String[] args) {
JFrame.setDefaultLookAndFeelDecorated(true);
new GradientTranslucentWindow().setVisible(true);
}
public void mousePressed(MouseEvent e) { start = new Point(e.getX(), e.getY()); }
public void mouseClicked(MouseEvent e) {
if(e.getButton() == MouseEvent.BUTTON1) draw = 1;
if(e.getButton() == MouseEvent.BUTTON3) draw = 0;
}
public void mouseEntered(MouseEvent e) {}
public void mouseExited(MouseEvent e) {}
public void mouseReleased(MouseEvent e) {}
public void mouseDragged(MouseEvent e) {}
public void mouseMoved(MouseEvent e) {
gd = this.getGraphics();
if(draw==1){
end = new Point(e.getX(), e.getY());
gd.setColor(new Color( red[counter],green[counter],blue[counter]));
gd.drawLine(start.x, start.y, end.x, end.y);
start = end;
panel.repaint();
System.out.println(start.x + " - " + start.y);
}
}
public void keyPressed(KeyEvent e) {
if(e.getKeyCode() == KeyEvent.VK_SPACE) {
counter++;
if ( counter > 4 ) counter = 0;
}
}
public void keyReleased(KeyEvent e) {}
public void keyTyped(KeyEvent e) {}
}