setting alpha chanel of pixels to zero in java

Question

I want to make a small program that allows you to change the Alpha chanel of selected pixels of an image. think the eraser tool on photoshop. So, I got the program to draw a color on the image, and tried using setRGB(0x00000000,0x00000000,0x00000000) to set the pixel to alpha 0; no dice, just paints black.

I've also tried painting a blue oval (which works great) so I know my mouse listener is working. (thats commented out in the code)

Now I am taking the raster of that image and trying to modify the alpha chanel using setPixels(), or setRect(), but I can't seem to get it to work. I m thinking my problem is with the Array you have to pass. I assume its a 10 x 10 pixel array and 4 channels so the array has to be length 400.

Its my first time trying this, so my code is kinda ugly with alot of stuff commented out, because I've tried about a million ways of doing this; but here it is.

Thanks for any help in advance.

import java.awt.AlphaComposite;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.awt.geom.Rectangle2D;
import java.awt.image.BufferedImage;
import java.awt.image.ColorModel;
import java.awt.image.Raster;
import java.awt.image.WritableRaster;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class MyFrame extends JFrame implements MouseListener,
    MouseMotionListener {

    BufferedImage myImage = new BufferedImage(300, 300,
        BufferedImage.TYPE_INT_ARGB);
    BufferedImage blankImage = new BufferedImage(10, 10,
        BufferedImage.TYPE_INT_ARGB);
    File myPNG = new File("output.png");
    MyPanel mp = new MyPanel();
    File tempPNG = new File("temp.png");
    File initialFile = new File("test.jpg");
    Color blankColor = new Color(0, 0, 0, .0f);
    int diameter = 10;
    int mouseX = 0;
    int mouseY = 0;
    boolean mouseClicked = false;
    boolean fileCreated;
    WritableRaster myRaster;
    WritableRaster blankRaster;
    int[] emptySquare = new int[400];
    ColorModel colorModel = myImage.getColorModel();

    public MyFrame() {
        setLayout(new BorderLayout());
        setSize(800, 800);
        add(mp, BorderLayout.EAST);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
        setFocusable(true);
        addMouseListener(this);
        addMouseMotionListener(this);
        for (int x = 0; x < 10; x++) {
            for (int y = 0; y < 10; y++) {
                for (int b = 0; b < 4; b++) {

                    emptySquare[x * y * b] = 0x00000000;
                }
            }
        }
        myRaster = myImage.getRaster();
        blankRaster = blankImage.getRaster();

        try {

            myImage = ImageIO.read(initialFile);
            fileCreated = ImageIO.write(myImage, "png", tempPNG);
            myImage = ImageIO.read(tempPNG);

        } catch (Exception e) {
        }
    }

    public void paint(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;

        g2d.setColor(Color.BLACK);
        // g2d.fillRect(0, 0, 300, 300);
        g2d.drawImage(myImage, 0, 0, 300, 300, null);

        // g2d.setColor(blankColor);
        // g2d.setPaint(Color.blue);

        // myImage.
    }

    private class MyPanel extends JPanel {

        private MyPanel() {
        }
    }

    @Override
    public void mouseClicked(MouseEvent m) {

        repaint();

    }

    @Override
    public void mouseEntered(MouseEvent m) {
        // TODO Auto-generated method stub
    }

    @Override
    public void mouseExited(MouseEvent m) {
        // TODO Auto-generated method stub
    }

    @Override
    public void mousePressed(MouseEvent m) {

        repaint();

    }

    @Override
    public void mouseReleased(MouseEvent m) {
        // TODO Auto-generated method stub
    }

    @Override
    public void mouseDragged(MouseEvent m) {

        mouseX = m.getX();
        mouseY = m.getY();

        myRaster.setDataElements(mouseX, mouseY, blankRaster);
        myImage = new BufferedImage(colorModel, myRaster, true, null);
        myImage.setData(blankRaster);

        myRaster.setPixel(mouseX, mouseY, emptySquare);
        myImage.setData(blankRaster);
        AlphaComposite.getInstance(AlphaComposite.SRC_OVER, .1f);
        Graphics2D tempG2d = myImage.createGraphics();
        tempG2d.setComposite(AlphaComposite.Clear);
        // tempG2d.setColor(Color.blue);

        tempG2d.fillOval(mouseX, mouseY, diameter, diameter);

        // tempG2d.setColor(blankColor);
        // tempG2d.setComposite(AlphaComposite.Src);

        for (int i = mouseX; i < mouseX + diameter; i++) {
            for (int j = mouseY; j < mouseY + diameter; j++) {
                int white = 0xffffff;

                Color c = new Color(0xccFF0000, true);
                int color = myImage.getRGB(i, j) & 0x00ffffff; // use bitwise &
                // to remove
                // alpha
                // component
                myRaster.setPixel(mouseX, mouseY, emptySquare);
                myImage.setData(myRaster);

                // myImage.setRGB(i,j,color); // transparent white

            }

        }

        // tempG2d.fillOval(mouseX, mouseY, diameter, diameter);
        try {
            // fileCreated=ImageIO.write(myImage, "png", tempPNG);
        } catch (Exception e) {
        }
        repaint();
    }

    @Override
    public void mouseMoved(MouseEvent arg0) {
        // TODO Auto-generated method stub
    }
}

Answer 1

Instead, use getPixel() on the underlying Raster, clear the alpha byte, and update the image using setPixel() and the updated array. See also this related example.