Image Representation in Java

1. Load Image:

• import Java packages:

import java.applet.*; // image will display on Java applet

import java.awt.image.*; //deal with  image object

import java.awt.*; //deal with  Graphic Interface

• load image:

  // Image "ass1.jpg" should be placed in "images" subdirectory

  // getCodeBase() return a URL for the applet

Image image=getImage(getCodeBase(),"images/ass1.jpg");

• track loading image using MediaTracker in package java.awt.MediaTracker:

  MediaTracker tracker = new MediaTracker(this);

  // Register it with media tracker

  tracker.addImage(image, 0);// id=0-- identifier used to track this image.

  // Wait until image has loaded

  try {

      tracker.waitForID(0);

  } catch (InterruptedException e){}

2. Get image information after the image is fully loaded:

int imageWidth=image.getWidth(this);

int imageHeight=image.getHeight(this);

int[] pixels = new int[imageWidth*imageHeight];

// grab pixels

PixelGrabber pg = new PixelGrabber(image, 0, 0, imageWidth, imageHeight, pixels, 0, imageWidth);

try {

    pg.grabPixels();

} catch(InterruptedException e) {}

3. draw the image at (0,0):

public void paint(Graphics g) {

//draw the image at (0,0)

g.drawImage(image, 0, 0, this);

}

4. pixel representation of an color image in Java:

• default color model: RGB
• pixels of image  in 2D array  must transfer to pixels in 1D array: (x,y) =>(y*M+x) where M is imageWidth
• the value of one pixel is represented by 32bits for 4 channels: alpha, R, G , B.
• 8bits for each channel.
• You can get RGB value from ColorModel:
  ColorModel model = ColorModel.getRGBdefault();
  int red = model.getRed(pixels[i]);
  int green = model.getGreen(pixels[i]);
  int blue = model.getBlue(pixels[i]);
• or simply:

int red = (pixels[i]>> 16) & 0xff;
int green = (pixels[i]>> 8) & 0xff;
int blue = (pixels[i]) & 0xff;

5. create Image with given pixels[]:

Image image = createImage(new MemoryImageSource(imageWidth, imageHeight, pixels, 0, imageWidth));// offset=0