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How to fit IMG size into JPanel

I'm kinda doing a GUI where when you press the "NEXT" button, it shows one by one the images at some directory.
My question is: How can I fit the size of the IMGs with the JPanles dimensions, I'm working with 6 or more MP images and I need to see the entire image.
Here is the code that gives me the imageIcon and where I add it to the JPanel.
JButton btnNextImg = new JButton("Next IMG");
btnNextImg.setBounds(96, 179, 110, 23);
btnNextImg.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
if (i == nImg)
i = 0;
try {
selectedImage = ImageIO.read(new File("C:\\IMAGES\\"+ String.valueOf(i+1) + ".jpg"));
iSelect = new ImageIcon(selectedImage);
originalImage.setIcon(iSelect);
} catch (IOException e1) {
e1.printStackTrace();
}
i++;
}
});
contentPane.add(btnNextImg);
And where I add it.
JPanel panel = new JPanel();
originalImage = new JLabel();
panel.add(originalImage);
panel.setBounds(5, 226, 309, 280);
contentPane.add(panel);
Thank you so much.

here's an image panel i found somewhere on the internet when i had the same problem:
public class ImagePanel extends JPanel {
private java.awt.Image image;
private boolean stretched = true;
private int xCoordinate;
private int yCoordinate;
public ImagePanel() {
}
public ImagePanel(Image image) {
this.image = image;
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
if (image != null) {
if (isStretched()) {
g.drawImage(image, xCoordinate, yCoordinate, getWidth(), getHeight(), this);
} else {
g.drawImage(image, xCoordinate, yCoordinate, this);
}
}
}
public java.awt.Image getImage() {
return image;
}
public void setImage(java.awt.Image image) {
this.image = image;
repaint();
}
public boolean isStretched() {
return stretched;
}
public void setStretched(boolean stretched) {
this.stretched = stretched;
repaint();
}
public int getXCoordinate() {
return xCoordinate;
}
public void setXCoodinate(int xCoordinate) {
this.xCoordinate = xCoordinate;
}
public int getYCoordinate() {
return xCoordinate;
}
public void setYCoordinate(int yCoordinate) {
this.yCoordinate = yCoordinate;
repaint();
}
}
to add an image to the panel use something like this:
ImagePanel imagePanel = new ImagePanel();
BufferedImage image = ImageIO.read(new File("C:\\IMAGES\\"+ String.valueOf(i+1) + ".jpg"));
imagePanel.setImage(image);

Here is the code that gives me the imageIcon and where I add it to the JPanel.
Check out Darryl's Stretch Icon. It will allow the Icon to fill the entire space available to the JLabel.

java - find h, s, and b of hsb color

I am making an HSB color picker. You can enter in the hue, saturation, brightness of a color, and it will make a small square the color you entered. Now I am allowing the user to save the color, which works, but I want it so that when you press the 'Open Color' button the h, s, and b text input boxes display the h, s, and b values of the saved color. Here is my code:
#SuppressWarnings("serial")
public class Main extends Applet implements ActionListener, java.io.Serializable {
private Rectangle color;
Random randomColor = new Random();
TextField one;
TextField two;
TextField three;
Button enter;
Button random;
Button save;
Button open;
String hvalue="255";
String svalue="0";
String bvalue="255";
Color hsb = Color.getHSBColor(Integer.parseInt(hvalue), Integer.parseInt(bvalue), Integer.parseInt(svalue));
int width = 320;
int height = 260;
String version = "0.0.1";
String name = "Color Picker";
public void init() {
setSize(width, height);
Frame c = (Frame)this.getParent().getParent();
c.setTitle(name + " - Version " + version);
setLayout(null);
color = new Rectangle(width/2-32, 20, 64, 64);
one = new TextField("", 15);
one.setBounds(width/2-60, height-120, 40, 20);
add(one);
two = new TextField("", 15);
two.setBounds(width/2-20, height-120, 40, 20);
add(two);
three = new TextField("", 15);
three.setBounds(width/2+20, height-120, 40, 20);
add(three);
enter = new Button("Enter");
enter.setBounds(width/2-50, height-80, 100, 20);
add(enter);
enter.addActionListener(this);
random = new Button("Random Color");
random.setBounds(width/2-50, height-60, 100, 20);
add(random);
random.addActionListener(this);
save = new Button("Save Color");
save.setBounds(width/2-50, height-40, 100, 20);
add(save);
save.addActionListener(this);
open = new Button("Open Color");
open.setBounds(width/2-50, height-20, 100, 20);
add(open);
open.addActionListener(this);
}
public void paint(Graphics g) {
Graphics2D g2 = (Graphics2D)g;
g2.setColor(hsb);
g2.fill(color);
}
#Override
public void actionPerformed(ActionEvent evt) {
if (evt.getSource() == enter) {
hvalue = one.getText();
svalue = two.getText();
bvalue = three.getText();
try {
hsb = Color.getHSBColor(Integer.parseInt(hvalue), Integer.parseInt(bvalue), Integer.parseInt(svalue));
repaint();
}
catch (Exception e) {}
}
else if (evt.getSource() == random) {
hvalue = String.valueOf(randomColor.nextInt(200));
svalue = String.valueOf(randomColor.nextInt(200));
bvalue = String.valueOf(randomColor.nextInt(200));
one.setText(hvalue);
two.setText(svalue);
three.setText(bvalue);
try {
hsb = Color.getHSBColor(Integer.parseInt(hvalue), Integer.parseInt(bvalue), Integer.parseInt(svalue));
repaint();
}
catch (Exception e) {}
}
else if (evt.getSource() == save) {
try {
FileOutputStream fileOut = new FileOutputStream("/C:/Users/Barbara/Downloads/color.color");
ObjectOutputStream out = new ObjectOutputStream(fileOut);
out.writeObject(hsb);
out.close();
fileOut.close();
} catch(IOException i) {
i.printStackTrace();
}
}
else if (evt.getSource() == open) {
Color openhsb = null;
try {
FileInputStream fileIn = new FileInputStream("/C:/Users/Barbara/Downloads/color.color");
ObjectInputStream in = new ObjectInputStream(fileIn);
openhsb = (Color) in.readObject();
in.close();
fileIn.close();
try {
hsb = openhsb;
repaint();
}
catch (Exception e) {}
} catch(IOException i) {
i.printStackTrace();
return;
} catch(ClassNotFoundException c) {
c.printStackTrace();
return;
}
}
}
}
Should I save the h, s, and b values to the color1.color file? I don't know how, if that is the case.

You can use the converters provided by the Color class:
Color color = Color.red;
float [] hsb = Color.RGBtoHSB(color.getRed(), color.getGreen(), color.getBlue(), null);
or
Color color = Color.red;
float [] hsb = new float[3];
Color.RGBtoHSB(color.getRed(), color.getGreen(), color.getBlue(), hsb);
the values retured are between 0 and 1. So you would probably want to multiply them by 360 to allow them to fit your programs model.
Javadoc for RGBtoHSB

Graphics component won't initialize

I need to make a program that will open an image and later be able to draw over it. However I got stuck because it returns my graphics as null. Here is the chunk of code where the problem should be. If anything I can add the entire thing.
public void paintComponent(Graphics gd) {
// super.paintComponent(g);
super.paintComponent(gd);
//gd.drawString("This is my custom Panel!",10,20);
pencil(gd);
}
public void pencil(Graphics gd){
Graphics2D g2 = (Graphics2D)gd;
System.out.println("g2 " + g2 + "method was called");
//g2.setColor(Color.yellow);
//Line2D line = new Line2D();
g2.drawLine(x1,y1,x2,y2);
//System.out.println("pencil");
}
public void actionPerformed(ActionEvent e) {
if (e.getSource() == open){
int returnVal = fc.showOpenDialog(Lab3.this);
if (returnVal == JFileChooser.APPROVE_OPTION) {
File file = fc.getSelectedFile();
try {
img=ImageIO.read(file);
ImageIcon icon=new ImageIcon(img); // ADDED
image.setIcon(icon); // ADDED
Dimension imageSize = new Dimension(icon.getIconWidth(),icon.getIconHeight()); // ADDED
image.setPreferredSize(imageSize); // ADDED
image.add(hbar, BorderLayout.SOUTH);
image.add(vbar, BorderLayout.EAST);
image.revalidate(); // ADDED
image.repaint(); // ADDED
Graphics gd = image.getGraphics();
}
catch(IOException e1) {}
}
}
if (e.getSource() == pencil){
try{
System.out.println("g2" + g2);
//pencil();
pencil(g2);
//g2.draw(new Ellipse2D.Double(0, 100, 30, 30));
//g2.drawRectanlge(100,100,200,200);
}
catch ( Exception err1 ) {
System.out.println( err1.getMessage( ) );
}
}

I'm guessing you can't get Graphics to work
so what you need to do is get your canvas/panel/frame from swing
Canvas canvas = new Canvas();
JFrame frame = new JFrame();
frame.add(canvas);
Graphics g = canvas.getGraphics();

Displaying and manipulating updated image

This program should first open an image, and then allow it to be manipulated via grayscale, scale, and rotate methods (not functional at the moment; disregard it). However, I'm not sure how I can call upon the updated image every time a method is performed. For example, if I grayscale an image, it goes to grayscale. But if I then scale it to a different size, the resulting image is a scaled version of the original image, not a scaled grayscale image. I tried putting in "image2 = image;" to no avail. How can I fix this?
Thanks.
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.io.*;
import javax.swing.*;
import java.awt.image.*;
import javax.imageio.ImageIO;
public class Picture{
JFileChooser fileChooser = new JFileChooser();
final JFrame frame = new JFrame("Edit Image");
Container content;
static BufferedImage image;
BufferedImage image2;
JLabel imageLabel;
public Picture() {
//asks for image file as input
fileChooser.setDialogTitle("Choose an image file to begin:");
fileChooser.showOpenDialog(frame);
File selectedFile = fileChooser.getSelectedFile();
if (fileChooser.getSelectedFile() != null) {
try {
//reads File as image
image = ImageIO.read(selectedFile);
}
catch (IOException e) {
System.out.println("Invalid image file: " + selectedFile);
System.exit(0);
}
}
else {
System.out.println("No File Selected!");
}
}
public int width() {
//returns width of present image
int width = image.getWidth();
return width;
}
public int height() {
//returns height of present image
int height = image.getHeight();
return height;
}
public void getImage() {
this.image2 = image;
}
public void saveImage() {
//saves current image as JPEG
fileChooser.setDialogTitle("Save this image?");
fileChooser.showSaveDialog(frame);
try {
//writes new file
ImageIO.write(this.image, "JPG", fileChooser.getSelectedFile());
}
catch (IOException f) {
System.out.println("Saving failed! Could not save image.");
}
}
public void show() {
//set frame title, set it visible, etc
content = frame.getContentPane();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setResizable(false);
//add the image to the frame
ImageIcon icon = new ImageIcon(image);
imageLabel = new JLabel(icon);
frame.setContentPane(imageLabel);
//add a menubar on the frame with a single option: saving the image
JMenuBar menuBar = new JMenuBar();
frame.setJMenuBar(menuBar);
JMenu progName = new JMenu("Edit Image");
progName.setBackground(Color.RED);
menuBar.add(progName);
JMenu fileMenu = new JMenu("File");
menuBar.add(fileMenu);
JMenu editMenu = new JMenu("Edit");
menuBar.add(editMenu);
ImageIcon exitIcon = new ImageIcon("app-exit.png");
JMenuItem exitAction = new JMenuItem("Exit", exitIcon);
progName.add(exitAction);
exitAction.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
ImageIcon saveIcon = new ImageIcon("save-icon.png");
int askSave = JOptionPane.showConfirmDialog(null,"Save current image?", "", JOptionPane.YES_NO_OPTION, JOptionPane.QUESTION_MESSAGE, saveIcon);
if (askSave == JOptionPane.YES_OPTION) {
//opens save image method, then exits
saveImage();
System.exit(0);
}
else {
//exits without saving
System.exit(0);
}
}
});
ImageIcon newIcon = new ImageIcon("new-image.png");
JMenuItem newAction = new JMenuItem("Open Image", newIcon);
fileMenu.add(newAction);
newAction.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
ImageIcon saveIcon = new ImageIcon("save-icon.png");
int askSave = JOptionPane.showConfirmDialog(null,"Save current image?", "", JOptionPane.YES_NO_OPTION, JOptionPane.QUESTION_MESSAGE, saveIcon);
if (askSave == JOptionPane.YES_OPTION) {
//opens save image method, then asks asks for new image file
saveImage();
Picture p = new Picture();
imageLabel.setIcon(new ImageIcon(image));
//resizes canvas to fit new image
frame.setSize(width(), height());
}
else {
//asks for new image file since user did not want to save original
Picture p = new Picture();
imageLabel.setIcon(new ImageIcon(image));
//resizes canvas to fit new image
frame.setSize(width(), height());
}
}
});
ImageIcon saveIcon = new ImageIcon("save-image.png");
JMenuItem saveAction = new JMenuItem("Save Image As...", saveIcon);
fileMenu.add(saveAction);
saveAction.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//opens save image method
saveImage();
}
});
ImageIcon gsIcon = new ImageIcon("grayscale-image.png");
JMenuItem grayScale = new JMenuItem("Grayscale", gsIcon);
editMenu.add(grayScale);
grayScale.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//grabs height and width of image, then grayscales it
grayscale(width(), height());
}
});
ImageIcon scaleIcon = new ImageIcon("scale-image.png");
JMenuItem scaleImg = new JMenuItem("Scale Image", scaleIcon);
editMenu.add(scaleImg);
scaleImg.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//asks for height and width to create new image
ImageIcon widthIcon = new ImageIcon("LR-arrows.png");
String scaleWidth = (String)JOptionPane.showInputDialog(null,"What should the new width be?", "", JOptionPane.QUESTION_MESSAGE, widthIcon, null, null);
ImageIcon heightIcon = new ImageIcon("UD-arrows.png");
String scaleHeight = (String)JOptionPane.showInputDialog(null,"What should the new height be?", "", JOptionPane.QUESTION_MESSAGE, widthIcon, null, null);
//turns user input strings into doubles
double x = Double.parseDouble(scaleWidth);
double y = Double.parseDouble(scaleHeight);
//casts doubles as ints
int newWidth = (int)x;
int newHeight = (int)y;
//resizes frame to fit new image dimensions
frame.setSize(newWidth, newHeight);
//calls scale method to resize image using given dimensions
scale(newWidth, newHeight);
}
});
ImageIcon rotateIcon = new ImageIcon("rotate-image.png");
JMenuItem rotateImg = new JMenuItem("Rotate Image", rotateIcon);
editMenu.add(rotateImg);
rotateImg.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
}
});
//paint the frame
frame.pack();
frame.repaint();
frame.setVisible(true);
}
// convert to grayscale
public void grayscale(int width, int height) {
// create a grayscale image with original dimensions
image2 = new BufferedImage(width, height, BufferedImage.TYPE_BYTE_GRAY);
// convert colored image to grayscale
ColorConvertOp grayScale = new ColorConvertOp(image.getColorModel().getColorSpace(),image2.getColorModel().getColorSpace(),null);
grayScale.filter(image,image2);
imageLabel.setIcon(new ImageIcon(image2));
getImage();
}
//scales image by a given factor
public void scale(int width, int height){
//uses user-input dimensions to create new image
image2 = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
Graphics g = image2.createGraphics();
//gets new dimensions and resizes image
g.drawImage(image, 0, 0, image2.getWidth(), image2.getHeight(), 0, 0, width(), height(), null);
imageLabel.setIcon(new ImageIcon(image2));
getImage();
}
//rotates the image
public void rotate(int width, int height, int theta) {
}
public static void main(String[] args) {
Picture p = new Picture();
p.show();
}
}

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.io.*;
import javax.swing.*;
import java.awt.image.*;
import javax.imageio.ImageIO;
public class Picture{
JFileChooser fileChooser = new JFileChooser();
final JFrame frame = new JFrame("Edit Image");
Container content;
static BufferedImage image;
BufferedImage image2;
JLabel imageLabel;
public Picture() {
//asks for image file as input
fileChooser.setDialogTitle("Choose an image file to begin:");
fileChooser.showOpenDialog(frame);
File selectedFile = fileChooser.getSelectedFile();
if (fileChooser.getSelectedFile() != null) {
try {
//reads File as image
image = ImageIO.read(selectedFile);
}
catch (IOException e) {
System.out.println("Invalid image file: " + selectedFile);
System.exit(0);
}
}
else {
System.out.println("No File Selected!");
}
}
public int width() {
//returns width of present image
int width = image.getWidth();
return width;
}
public int height() {
//returns height of present image
int height = image.getHeight();
return height;
}
/*
public void getImage() {
this.image2 = image;
}
*/
public void saveImage() {
//saves current image as JPEG
fileChooser.setDialogTitle("Save this image?");
fileChooser.showSaveDialog(frame);
try {
//writes new file
ImageIO.write(this.image, "JPG", fileChooser.getSelectedFile());
}
catch (IOException f) {
System.out.println("Saving failed! Could not save image.");
}
}
public void show() {
//set frame title, set it visible, etc
content = frame.getContentPane();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setResizable(false);
//add the image to the frame
ImageIcon icon = new ImageIcon(image);
imageLabel = new JLabel(icon);
frame.setContentPane(imageLabel);
//add a menubar on the frame with a single option: saving the image
JMenuBar menuBar = new JMenuBar();
frame.setJMenuBar(menuBar);
JMenu progName = new JMenu("Edit Image");
progName.setBackground(Color.RED);
menuBar.add(progName);
JMenu fileMenu = new JMenu("File");
menuBar.add(fileMenu);
JMenu editMenu = new JMenu("Edit");
menuBar.add(editMenu);
ImageIcon exitIcon = new ImageIcon("app-exit.png");
JMenuItem exitAction = new JMenuItem("Exit", exitIcon);
progName.add(exitAction);
exitAction.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
ImageIcon saveIcon = new ImageIcon("save-icon.png");
int askSave = JOptionPane.showConfirmDialog(null,"Save current image?", "", JOptionPane.YES_NO_OPTION, JOptionPane.QUESTION_MESSAGE, saveIcon);
if (askSave == JOptionPane.YES_OPTION) {
//opens save image method, then exits
saveImage();
System.exit(0);
}
else {
//exits without saving
System.exit(0);
}
}
});
ImageIcon newIcon = new ImageIcon("new-image.png");
JMenuItem newAction = new JMenuItem("Open Image", newIcon);
fileMenu.add(newAction);
newAction.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
ImageIcon saveIcon = new ImageIcon("save-icon.png");
int askSave = JOptionPane.showConfirmDialog(null,"Save current image?", "", JOptionPane.YES_NO_OPTION, JOptionPane.QUESTION_MESSAGE, saveIcon);
if (askSave == JOptionPane.YES_OPTION) {
//opens save image method, then asks asks for new image file
saveImage();
Picture p = new Picture();
imageLabel.setIcon(new ImageIcon(image));
//resizes canvas to fit new image
frame.setSize(width(), height());
}
else {
//asks for new image file since user did not want to save original
Picture p = new Picture();
imageLabel.setIcon(new ImageIcon(image));
//resizes canvas to fit new image
frame.setSize(width(), height());
}
}
});
ImageIcon saveIcon = new ImageIcon("save-image.png");
JMenuItem saveAction = new JMenuItem("Save Image As...", saveIcon);
fileMenu.add(saveAction);
saveAction.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//opens save image method
saveImage();
}
});
ImageIcon gsIcon = new ImageIcon("grayscale-image.png");
JMenuItem grayScale = new JMenuItem("Grayscale", gsIcon);
editMenu.add(grayScale);
grayScale.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//grabs height and width of image, then grayscales it
grayscale(width(), height());
}
});
ImageIcon scaleIcon = new ImageIcon("scale-image.png");
JMenuItem scaleImg = new JMenuItem("Scale Image", scaleIcon);
editMenu.add(scaleImg);
scaleImg.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//asks for height and width to create new image
ImageIcon widthIcon = new ImageIcon("LR-arrows.png");
String scaleWidth = (String)JOptionPane.showInputDialog(null,"What should the new width be?", "", JOptionPane.QUESTION_MESSAGE, widthIcon, null, null);
ImageIcon heightIcon = new ImageIcon("UD-arrows.png");
String scaleHeight = (String)JOptionPane.showInputDialog(null,"What should the new height be?", "", JOptionPane.QUESTION_MESSAGE, widthIcon, null, null);
//turns user input strings into doubles
double x = Double.parseDouble(scaleWidth);
double y = Double.parseDouble(scaleHeight);
//casts doubles as ints
int newWidth = (int)x;
int newHeight = (int)y;
//resizes frame to fit new image dimensions
frame.setSize(newWidth, newHeight);
//calls scale method to resize image using given dimensions
scale(newWidth, newHeight);
}
});
ImageIcon rotateIcon = new ImageIcon("rotate-image.png");
JMenuItem rotateImg = new JMenuItem("Rotate Image", rotateIcon);
editMenu.add(rotateImg);
rotateImg.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
}
});
//paint the frame
frame.pack();
frame.repaint();
frame.setVisible(true);
}
// convert to grayscale
public void grayscale(int width, int height) {
// create a grayscale image with original dimensions
image2 = new BufferedImage(width, height, BufferedImage.TYPE_BYTE_GRAY);
// convert colored image to grayscale
ColorConvertOp grayScale = new ColorConvertOp(image.getColorModel().getColorSpace(),image2.getColorModel().getColorSpace(),null);
grayScale.filter(image,image2);
imageLabel.setIcon(new ImageIcon(image2));
//getImage();
image = image2;
}
//scales image by a given factor
public void scale(int width, int height){
//uses user-input dimensions to create new image
image2 = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
Graphics g = image2.createGraphics();
//gets new dimensions and resizes image
g.drawImage(image, 0, 0, image2.getWidth(), image2.getHeight(), 0, 0, width(), height(), null);
imageLabel.setIcon(new ImageIcon(image2));
//getImage();
image = image2;
}
//rotates the image
public void rotate(int width, int height, int theta) {
}
public static void main(String[] args) {
Picture p = new Picture();
p.show();
}
}

public void getImage() {
this.image2 = image;
}
//..............................................
// convert to grayscale
public void grayscale(int width, int height) {
// create a grayscale image with original dimensions
image2 = new BufferedImage(width, height, BufferedImage.TYPE_BYTE_GRAY);
// convert colored image to grayscale
ColorConvertOp grayScale = new ColorConvertOp(image.getColorModel().getColorSpace(),image2.getColorModel().getColorSpace(),null);
grayScale.filter(image,image2);
imageLabel.setIcon(new ImageIcon(image2));
getImage();
}
ColorConvertOp.filter(BufferedImage src, BufferedImage dest)
Your grayscale method has image2 as the dest, when you call getImage() it replaces image2 with the original not settieng the original to your new image2.
Either have grayscale(int, int) return a BuffereImage or change getImage to:
public void getImage() {
this.image = image2;
}

How do I read pixels from a PNG file?

I know how to capture a screenshot by using Robot, Windowtester or FEST. I also know how to read a pixel from the screen by using robot.
int x = 10;
int y = 10;
Color px = getPixelColor(int x, int y);
However, I don't know how to read a pixel from an image that is already captured. I'm planning to compare a current image, with an image from file. Lets say both are PNG. Are there any frameworks that I can use to compare images pixel by pixel?

Is this in Java? If so, you can use ImageIO.read( "yourImage.png" ) to get a BufferedImage. That will have a getData() method which will give you a Raster object, on which you can call getPixel. See link

This should work:
javax.imageio.ImageIO.read(new File("filename.png"))
Then you can walk through the pixels and compare with the images pixel by pixel with this:
java.awt.image.BufferedImage.getRGB(int x, int y).

You can read image file: Reading/Loading an Image.
And then get color using getRGB method.

Load them as BufferedImage instances and it is relatively easy.
Here is part of code that creates images with text, then creates a new image that shows the difference between the image with & without text.
for (int xx=0; xx<width; xx++) {
for (int yy=0; yy<height; yy++) {
Color originalColor = new Color(originalImage.getRGB(xx,yy));
int r1 = originalColor.getRed();
int g1 = originalColor.getGreen();
int b1 = originalColor.getBlue();
Color newColor = new Color(textImage.getRGB(xx,yy));
int r2 = newColor.getRed();
int g2 = newColor.getGreen();
int b2 = newColor.getBlue();
Color bnw = Color.black;
if (r1==r2 && g1==g2 && b1==b2) {
bnw = Color.white;
}
bnwImage.setRGB(xx, yy, bnw.getRGB());
int rD = Math.abs(r1-r2);
int gD = Math.abs(g1-g2);
int bD = Math.abs(b1-b2);
Color differenceColor = new Color(rD,gD,bD);
differenceImage.setRGB(xx, yy, differenceColor.getRGB());
}
}
Screensot
Full code
import java.awt.image.BufferedImage;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.imageio.*;
import java.io.*;
import javax.imageio.stream.ImageOutputStream;
import javax.imageio.plugins.jpeg.JPEGImageWriteParam;
import java.util.Locale;
class ImageWriteTest {
private BufferedImage originalImage;
private BufferedImage textImage;
private BufferedImage differenceImage;
private BufferedImage bnwImage;
private JPanel gui;
private JCheckBox antialiasing;
private JCheckBox rendering;
private JCheckBox fractionalMetrics;
private JCheckBox strokeControl;
private JCheckBox colorRendering;
private JCheckBox dithering;
private JComboBox textAntialiasing;
private JComboBox textLcdContrast;
private JLabel label0_1;
private JLabel label0_4;
private JLabel label0_7;
private JLabel label1_0;
private JTextArea output;
final static Object[] VALUES_TEXT_ANTIALIASING = {
RenderingHints.VALUE_TEXT_ANTIALIAS_OFF,
RenderingHints.VALUE_TEXT_ANTIALIAS_ON,
RenderingHints.VALUE_TEXT_ANTIALIAS_GASP,
RenderingHints.VALUE_TEXT_ANTIALIAS_LCD_HBGR,
RenderingHints.VALUE_TEXT_ANTIALIAS_LCD_HRGB,
RenderingHints.VALUE_TEXT_ANTIALIAS_LCD_VBGR,
RenderingHints.VALUE_TEXT_ANTIALIAS_LCD_VRGB
};
final static Object[] VALUES_TEXT_LCD_CONTRAST = {
new Integer(100),
new Integer(150),
new Integer(200),
new Integer(250)
};
ImageWriteTest() {
int width = 280;
int height = 100;
gui = new JPanel(new BorderLayout(0,4));
originalImage = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
textImage = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
differenceImage = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
bnwImage = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
JPanel controls = new JPanel(new GridLayout(0,2,0,0));
antialiasing = new JCheckBox("Anti-aliasing", false);
rendering = new JCheckBox("Rendering - Quality", true);
fractionalMetrics = new JCheckBox("Fractional Metrics", true);
strokeControl = new JCheckBox("Stroke Control - Pure", false);
colorRendering = new JCheckBox("Color Rendering - Quality", true);
dithering = new JCheckBox("Dithering", false);
controls.add(antialiasing);
controls.add(rendering);
controls.add(fractionalMetrics);
controls.add(colorRendering);
textLcdContrast = new JComboBox(VALUES_TEXT_LCD_CONTRAST);
JPanel lcdContrastPanel = new JPanel(new FlowLayout(FlowLayout.LEADING));
lcdContrastPanel.add(textLcdContrast);
lcdContrastPanel.add(new JLabel("Text LCD Contrast"));
controls.add(lcdContrastPanel);
controls.add(strokeControl);
textAntialiasing = new JComboBox(VALUES_TEXT_ANTIALIASING);
controls.add(textAntialiasing);
controls.add(dithering);
ItemListener itemListener = new ItemListener(){
public void itemStateChanged(ItemEvent e) {
updateImages();
}
};
antialiasing.addItemListener(itemListener);
rendering.addItemListener(itemListener);
fractionalMetrics.addItemListener(itemListener);
strokeControl.addItemListener(itemListener);
colorRendering.addItemListener(itemListener);
dithering.addItemListener(itemListener);
textAntialiasing.addItemListener(itemListener);
textLcdContrast.addItemListener(itemListener);
Graphics2D g2d = originalImage.createGraphics();
GradientPaint gp = new GradientPaint(
0f, 0f, Color.red,
(float)width, (float)height, Color.orange);
g2d.setPaint(gp);
g2d.fillRect(0,0, width, height);
g2d.setColor(Color.blue);
for (int ii=0; ii<width; ii+=10) {
g2d.drawLine(ii, 0, ii, height);
}
g2d.setColor(Color.green);
for (int jj=0; jj<height; jj+=10) {
g2d.drawLine(0, jj, width, jj);
}
updateImages();
gui.add(controls, BorderLayout.NORTH);
JPanel images = new JPanel(new GridLayout(0,2,0,0));
images.add(new JLabel(new ImageIcon(originalImage)));
images.add(new JLabel(new ImageIcon(textImage)));
images.add(new JLabel(new ImageIcon(differenceImage)));
images.add(new JLabel(new ImageIcon(bnwImage)));
try {
label0_1 = new JLabel(new ImageIcon(getJpegCompressedImage(0.1f, textImage)));
images.add(label0_1);
label0_4 = new JLabel(new ImageIcon(getJpegCompressedImage(0.4f, textImage)));
images.add(label0_4);
label0_7 = new JLabel(new ImageIcon(getJpegCompressedImage(0.7f, textImage)));
images.add(label0_7);
label1_0 = new JLabel(new ImageIcon(getJpegCompressedImage(1.0f, textImage)));
images.add(label1_0);
} catch(IOException ioe) {
}
gui.add(images, BorderLayout.CENTER);
StringBuilder sb = new StringBuilder();
String[] names = {
"java.vendor",
"java.version",
"java.vm.version",
"os.name",
"os.version"
};
for (String name : names) {
addProperty(sb, name);
}
output = new JTextArea(sb.toString(),6,40);
gui.add(new JScrollPane(output), BorderLayout.SOUTH);
JOptionPane.showMessageDialog(null, gui);
}
private static void addProperty(StringBuilder builder, String name) {
builder.append( name + " \t" + System.getProperty(name) + "\n" );
}
/** Adapted from SO post by x4u. */
private Image getJpegCompressedImage(float quality, BufferedImage image) throws IOException {
ByteArrayOutputStream outStream = new ByteArrayOutputStream();
ImageWriter imgWriter = ImageIO.getImageWritersByFormatName( "jpg" ).next();
ImageOutputStream ioStream = ImageIO.createImageOutputStream( outStream );
imgWriter.setOutput( ioStream );
JPEGImageWriteParam jpegParams = new JPEGImageWriteParam( Locale.getDefault() );
jpegParams.setCompressionMode( ImageWriteParam.MODE_EXPLICIT );
jpegParams.setCompressionQuality( quality );
imgWriter.write( null, new IIOImage( image, null, null ), jpegParams );
ioStream.flush();
ioStream.close();
imgWriter.dispose();
BufferedImage compressedImage = ImageIO.read(new ByteArrayInputStream(outStream.toByteArray()));
JLabel label = new JLabel("Quality: " + quality);
label.setBackground(new Color(255,255,255,192));
label.setOpaque(true);
label.setSize(label.getPreferredSize());
label.paint(compressedImage.getGraphics());
return compressedImage;
}
private void updateImages() {
int width = originalImage.getWidth();
int height = originalImage.getHeight();
Graphics2D g2dText = textImage.createGraphics();
if (antialiasing.isSelected()) {
g2dText.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
} else {
g2dText.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_OFF);
}
if (rendering.isSelected()) {
g2dText.setRenderingHint(RenderingHints.KEY_RENDERING,
RenderingHints.VALUE_RENDER_QUALITY);
} else {
g2dText.setRenderingHint(RenderingHints.KEY_RENDERING,
RenderingHints.VALUE_RENDER_SPEED);
}
if (fractionalMetrics.isSelected()) {
g2dText.setRenderingHint(RenderingHints.KEY_FRACTIONALMETRICS,
RenderingHints.VALUE_FRACTIONALMETRICS_ON);
} else {
g2dText.setRenderingHint(RenderingHints.KEY_FRACTIONALMETRICS,
RenderingHints.VALUE_FRACTIONALMETRICS_OFF);
}
if (strokeControl.isSelected()) {
g2dText.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL,
RenderingHints.VALUE_STROKE_NORMALIZE);
} else {
g2dText.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL,
RenderingHints.VALUE_STROKE_PURE);
}
if (dithering.isSelected()) {
g2dText.setRenderingHint(RenderingHints.KEY_DITHERING,
RenderingHints.VALUE_DITHER_ENABLE);
} else {
g2dText.setRenderingHint(RenderingHints.KEY_DITHERING,
RenderingHints.VALUE_DITHER_DISABLE);
}
if (colorRendering.isSelected()) {
g2dText.setRenderingHint(RenderingHints.KEY_COLOR_RENDERING,
RenderingHints.VALUE_COLOR_RENDER_QUALITY);
} else {
g2dText.setRenderingHint(RenderingHints.KEY_COLOR_RENDERING,
RenderingHints.VALUE_COLOR_RENDER_SPEED);
}
g2dText.setRenderingHint(RenderingHints.KEY_TEXT_LCD_CONTRAST,
textLcdContrast.getSelectedItem());
g2dText.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING,
textAntialiasing.getSelectedItem());
g2dText.drawImage(originalImage, 0,0, null);
g2dText.setColor(Color.black);
g2dText.drawString("The quick brown fox jumped over the lazy dog.", 10,50);
Graphics2D g2dDifference = differenceImage.createGraphics();
Graphics2D g2dBnW = bnwImage.createGraphics();
for (int xx=0; xx<width; xx++) {
for (int yy=0; yy<height; yy++) {
Color originalColor = new Color(originalImage.getRGB(xx,yy));
int r1 = originalColor.getRed();
int g1 = originalColor.getGreen();
int b1 = originalColor.getBlue();
Color newColor = new Color(textImage.getRGB(xx,yy));
int r2 = newColor.getRed();
int g2 = newColor.getGreen();
int b2 = newColor.getBlue();
Color bnw = Color.black;
if (r1==r2 && g1==g2 && b1==b2) {
bnw = Color.white;
}
bnwImage.setRGB(xx, yy, bnw.getRGB());
int rD = Math.abs(r1-r2);
int gD = Math.abs(g1-g2);
int bD = Math.abs(b1-b2);
Color differenceColor = new Color(rD,gD,bD);
differenceImage.setRGB(xx, yy, differenceColor.getRGB());
}
}
gui.repaint();
}
public static void main(String[] args) {
SwingUtilities.invokeLater( new Runnable() {
public void run() {
ImageWriteTest iwt = new ImageWriteTest();
}
} );
}
}

In C/C++, if you're comfortable requiring a minimum version of Windows, it's rather easy, you can use GDI+ to load the image and draw it to a memory bitmap, then you can use the returned pointer to get the pixel data.
Use GdiplusStartup() and GdiplusShutdown() to initialise and uninitialise GDI+.
Use a GDI+ Image object, using the overload that takes a filename, to load the image, then use the methods GetWidth() and GetHeight(), a BITMAPINFO structure and the CreateDIBSection() GDI function to create a memory bitmap.
Then use CreateCompatibleDC() to create a device context for the bitmap and SelectObject() to select the bitmap into that device context.
Then you use a GDI+ Graphics object, using the overload that takes a device context, and its DrawImage() method, using the overload that takes x, y, width and height, to draw the image to the bitmap's device context.
After that, you can get/set the pixel data using the pointer returned by CreateDIBSection().
When you're done, use DeleteDC() to get rid of the bitmap's device context BEFORE using DeleteObject() to get rid of the bitmap. GDI+ Image objects can also be used to save images in a supported format, including PNG.