Java robot class - getting Pixel color after they're painted

Question

I need to create a colorPicker tool for my little ms paint app. I originally asked how to switch from my Graphics 2D implementation to a Graphics2D-->BufferedImage one, (and then it would be easy to get the pixels) but I have instead been suggested to get the pixel colors thought the robot class.

First of all, here's my MCEVE: NB. It cannot be a single class, it won't save.

import java.awt.*;
import javax.swing.*;

public class Runner {

public static void main(String[] args){

    JFrame Maiframe = new JFrame("Paint");
    Canvas DrawingBoard = new Canvas();
    Maiframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    Maiframe.setSize(700, 500);
    Maiframe.add(DrawingBoard);
    Maiframe.setVisible(true);

  }

}



import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionAdapter;


public class Canvas extends JPanel {

public int xp;  //present x
public int yp;  //present y
public int xo;  //old x
public int yo;  //old y      (Drawing from starting to new point as mouse drags)

public Canvas(){

    super();

    addMouseListener(new MouseAdapter() {   

        @Override
        public void mouseClicked(MouseEvent e) {   
            xo = e.getX();
            yo = e.getY();
            Color col = RGBFinder();
            System.out.println("RGB : Red =" + col.getRed() + "Green" +  col.getGreen() + "Blue" + col.getRed());
        }
    });


    addMouseMotionListener(new MouseMotionAdapter() {  //get coords as mouse drags
        @Override
        public void mouseDragged(MouseEvent e) {

            xp = e.getX();
            yp = e.getY();
            if(SwingUtilities.isLeftMouseButton(e))
            repaint();  //call paintcomponent

        }

        public void mouseMoved(MouseEvent e1){  //keep trak of coords when mouse is not dragging

            xo = e1.getX();
            yo = e1.getY();     
        }
    });

}

public void draw(int x, int y, Graphics g){   //draw the line

    if(xo != 0) 
    g.drawLine(xo, yo, x, y);

    xo = x;    //old x is now present x and so on
    yo = y;     
}

public void paintComponent(Graphics g){
    super.paintComponent(g);
    draw(xp, yp, g);    
   }
}

public Color RGBFinder(){

    try{
        robot = new Robot();
    }
    catch(AWTException e){

        System.out.println("Could not create color picker robot");
    }

    PointerInfo pi = MouseInfo.getPointerInfo();
    Point p = pi.getLocation();
    Color pixelColor = robot.getPixelColor(p.x, p.y);

    //also tried robot.getPixelColor(p.getX(), p.getY());
    //also tried to pass coordinates from listener to RGBFinder, and use those, no luck. (event.getX() ...)

    return pixelColor;

}

And it works just fine.

I needed to implement something to get the color from any pixel, when the mouse clicks.

I did this: (adding this method to Canvas, and calling it from the mouse clicker listener)

public Color RGBFinder(){

    try{
        robot = new Robot();
    }
    catch(AWTException e){

        System.out.println("Could not create color picker robot");
    }

    PointerInfo pi = MouseInfo.getPointerInfo();
    Point p = pi.getLocation();
    Color pixelColor = robot.getPixelColor(p.x, p.y);

    //also tried robot.getPixelColor(p.getX(), p.getY());
    //also tried to pass coordinates from listener to RGBFinder, and use those, no luck. (event.getX() ...)

    return pixelColor;

}

Example of call:

//replace old mouseListener with this

    addMouseListener(new MouseAdapter() {   

        @Override
        public void mouseClicked(MouseEvent e) {   
            xo = e.getX();
            yo = e.getY();
            Color col = RGBFinder();
            System.out.println(" da tela Red =" + col.getRed() + "Green" +  col.getGreen() + "Blue" + col.getRed());
        }
    });

Unfortunately, from this implementation I get undefined behaviour. The color read is always 255, 255, 255. Exept if I color the hole panel, then it gets it right 9 times out of 10, but in some areas it's still missing it.

I have also tried wrapping the whole thing into a bufferedImage with robot#screenCap but that doesn't even remotely work.

What am I doing wrong here?

Thank you very much.

EDIT1:

There are doubts as to how a line can remain on screen after a second one has been drawn. I'll provide a screenshot:

NBB. This works because an instance of Canvas is create inside of a JFrame into Runnable, so the changes are saved, avoiding the need for shapes and arrayLists.

I'll also add a full version of the code which prints wrong RGB results, remember that this does not save the lines as it stands. Please refer to the two separate classes above for testing.

import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionAdapter;


public class Canvas extends JPanel{

public int xp;  //present x
public int yp;  //present y
public int xo;  //old x
public int yo;  //old y      (Drawing from starting to new point as mouse drags)
public Robot robot;

public static void main(String[] args){

    JFrame Maiframe = new JFrame("Paint");
    Canvas DrawingBoard = new Canvas();
    Maiframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    Maiframe.setSize(700, 500);
    Maiframe.add(DrawingBoard);
    Maiframe.setVisible(true);

  }

public Canvas(){

    super();

            addMouseListener(new MouseAdapter() {   

        @Override
        public void mouseClicked(MouseEvent e) {   
            xo = e.getX();
            yo = e.getY();
            Color col = RGBFinder();
            System.out.println("RGB --> Red =" + col.getRed() + "Green" +  col.getGreen() + "Blue" + col.getRed());
        }
    });

    addMouseMotionListener(new MouseMotionAdapter() {  //get coords as mouse drags
        @Override
        public void mouseDragged(MouseEvent e) {

            xp = e.getX();
            yp = e.getY();
            if(SwingUtilities.isLeftMouseButton(e))
            repaint();  //call paintcomponent

        }

        public void mouseMoved(MouseEvent e1){  //keep trak of coords when mouse is not dragging

            xo = e1.getX();
            yo = e1.getY();     
        }
    });

}

public void draw(int x, int y, Graphics g){   //draw the line

    if(xo != 0) 
    g.drawLine(xo, yo, x, y);

    xo = x;    //old x is now present x and so on
    yo = y;     
}

public void paintComponent(Graphics g){
    super.paintComponent(g);
    draw(xp, yp, g);    
   }

 public Color RGBFinder(){

    try{
        robot = new Robot();
    }
    catch(AWTException e){

        System.out.println("Could not create color picker robot");
    }

    PointerInfo pi = MouseInfo.getPointerInfo();
    Point p = pi.getLocation();
    Color pixelColor = robot.getPixelColor(p.x, p.y);

    //also tried robot.getPixelColor(p.getX(), p.getY());
    //also tried to pass coordinates from listener to RGBFinder, and use those, no luck. (event.getX() ...)

    return pixelColor;

 }
}

Answer 1

my little ms paint app

Well its not much of a paint app. All it does is draw a single line. Whenever you attempt to draw the second line the first will be removed.

So the first step you need to do is decide how you want the painting to work. There are two common approaches:

1. Store Objects you want to paint in an ArrayList and then the paintComponent(...) method will paint each Object in the List.

2. Paint directly to a BufferedImage and then the paintComponent(...) method can just paint the BufferedImage.

Check out Custom Painting Approaches for working examples of both of these approaches and give the pros/cons of using each approach.

I have instead been suggested to get the pixel colors thought the robot class

It depends which painting approach you want to use.

If you use the Draw on Component approach then you would use the MouseInfo and Robot to get the pixel color:

PointerInfo pi = MouseInfo.getPointerInfo();
Point p = pi.getLocation();
System.out.println( robot.getPixelColor(p.x, p.y) );

If you use the Draw on Image approach then you would get the pixel color from the BufferedImage:

int rgb = bufferedImage.getRGB(mouseEvent.getX(), mouseEvent.getY());
Color color = new Color( rgb );
System.out.println( color );

Final Update

You still have not posted a SSCCE. The code you posted does NOT draw a line. Even if it did draw a line, how do you expect us to click (with accuracy) a single pixel line.

The point of a SSCCE is to demonstrate the concept you are asking about. You are asking how to get the Color of a pixel on your panel. How the drawing gets on the panel is irrelevant to the question so the painting code should be as simple as possible.

Following is a proper SSCCE. Note:

1. The createAndShowGUI() and main() methods will be the same for all SSCCE. The difference is the `DrawingPanel code/class will change to demonstrate your problem.

2. The custom painting is hardcoded. There is no need for mouseMoved/mouseDragged logic (unless that is the problem) you are trying to solve. All you care about is having different colors to click on.

3. Just use a simple System.out.println(...) to display the value of the Color object.

All I did was copy the relevant pieces of code from you class and remove the irrelevant code to keep the code simple and straight forward. Anybody can do that.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class DrawingCanvas extends JPanel
{
    private Robot robot;

    public DrawingCanvas()
    {
        addMouseListener(new MouseAdapter()
        {
            @Override
            public void mouseClicked(MouseEvent e)
            {
                try
                {
                    robot = new Robot();
                }
                catch(Exception re) { System.out.println(re); }

                PointerInfo pi = MouseInfo.getPointerInfo();
                Point p = pi.getLocation();
                Color pixelColor = robot.getPixelColor(p.x, p.y);
                System.out.println(pixelColor);
            }
        });

    }

    @Override
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);

        g.setColor( Color.RED );
        g.fillRect(0, 0, 40, 40);

        g.setColor( Color.GREEN );
        g.fillRect(40, 40, 40, 40);

        g.setColor( Color.BLUE );
        g.fillRect(80, 80, 40, 40);
    }

    private static void createAndShowGUI()
    {
        JPanel panel = new DrawingCanvas();
        JFrame frame = new JFrame("DrawingCanvas");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(panel);
        frame.setSize(200, 200);
        frame.setLocationByPlatform( true );
        frame.setVisible( true );
    }

    public static void main(String[] args)
    {
        EventQueue.invokeLater( () -> createAndShowGUI() ); // Java 8 only
/*
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                createAndShowGUI();
            }
        });
*/
    }
}

Just copy/paste/compile/execute to see that the code works.

I'll let you figure out why your code doesn't appear to work.

NBB. This works because an instance of Canvas is create inside of a JFrame into Runnable, so the changes are saved, avoiding the need for shapes and arrayLists.

That statement is completely wrong. Painting done with the Graphics Object in the paintComponent() method is only temporary until the next time the paintComponent() method is invoked.

For example add the following code after the frame.setVisible() statement:

SwingUtilities.invokeLater(new Runnable()
{
    public void run()
    {
        Graphics g = panel.getGraphics();
        g.setColor( Color.YELLOW );
        g.fillRect(120, 120, 40, 40);
    }
});

The yellow square will disappear when the frame is resized. For permanent painting you need to use one of the two approaches I suggested in my original answer.

Edit 2:

This is the output I get when I click on the red, green, blue, background respectively:

C:\Java>java DrawingCanvas
java.awt.Color[r=255,g=0,b=0]
java.awt.Color[r=0,g=255,b=0]
java.awt.Color[r=0,g=0,b=255]
java.awt.Color[r=238,g=238,b=238]