Using BufferedImage in Java

Objects of the BufferedImage class represent images that we can draw to.

• To create a blank image:

  BufferedImage myImage = new BufferedImage(myWidth, 
                                            myHeight, 
                                            BufferedImage.TYPE_INT_ARGB)
  

  where myWidth is the width in pixels (number of columns), myHeight is the height in pixels (number of rows), and the last argument specifies that each pixel has its own a red, green, blue, and transparency (alpha) value.

• If you have a BufferedImage object, you can ask for its width (number of columns) and height (number of rows):

     int ncols = myImage.getWidth();
     int nrows = myImage.getHeight();
  

• To read the current value of a specific pixel, we can do

     myImage.getRGB(ix,iy)
  

  where ix is the x-coordinate (column number), and iy is the y-coordinate (row number).

  Recall that (0,0) is the upper-left corner, that increasing x values move you rightward, and that increasing y values move you downward.

  The value you get back will be an int. It encodes 8 bits of alpha (0 = completely transparent, 255 = completely opaque), 8 bits of red (0 = no red, 255 = maximum redness), 8 bits of green (0 = no green, 255 = maximum greeness), and 8 bits of blue (0 = no blue, 255 = maximum blueness). You can extract the individual components from the int returned by getRGB as follows:

     int rgb = myImage.getRGB(ix, iy);
     int alpha = (rgb & 0xFF000000)  >>> 24;
     int red   = (rgb & 0x00FF0000)  >>> 16;
     int green = (rgb & 0x0000FF00)  >>> 8;
     int blue  = (rgb & 0x000000FF)  >>> 0;
  

• To overwrite a particular pixel in an image, we can do

     myImage.setRGB(ix, iy, rgb)
  

  where ix and iy are the column and row numbers, and rgb is an integer encoding a particular red-green-blue-alpha value.

  If you want to just copy an existing pixel from one image to another, you can say

     destImage.setRGB(destx, desty,
                      srcImage.getRGB(srcx, srcy));
  

  Alternatively, you can create brand-new rgb values for setRGB by combining individual color components as follows:

     int rgb = (alpha << 24) | (red << 16) | (green << 8) | blue;
  

Putting it all together

Here is a code fragment that takes an existing image myImage, and creates a new image redderImage containing the same picture but with maximal red and maximal opaqueness.

  BufferedImage redderImage = new BufferedImage(myImage.getWidth(),
                                                myImage.getHeight(),
                                                BufferedImage.TYPE_INT_ARGB);
              
  for (int ix = 0; ix < myImage.getWidth(); ++ix) {
    for (int iy = 0; iy < myImage.getHeight(); ++iy) {
 
      int rgb = myImage.getRGB(ix,iy);
    
      int new_alpha = 255;                        // Maximum!
      int new_red = 255;                          // Maximum!
      int new_green = (rgb & 0x0000FF00) >>> 8;   // Copy original greenness
      int new_blue  = (rgb & 0x000000FF) >>> 0;   // Copy original blueness

      int new_rgb = (new_alpha << 24) | (new_red << 16) | (new_green << 8) | new_blue;
      redderImage.setRGB(ix, iy, new_rgb);
    }
  }