Drawing Pixels in Java

Optional anecdote

I find the Java library documentation very difficult to read and so I present here a similar discovery. Most of what I report I did actually get from the manual but only with very critical help from a few Java gurus.

How to do pixels from a Java Program

Many graphics environments try to keep programmers away from pixels for good reason: such as they may change their nature in the future. Indeed pixels are gradually getting smaller. If you want to draw a fractal, however, imposing intermediate graphic abstraction degrades the power of the pixels to show the truth. Even when you compute a smooth function, in order to plot it, it is logical to compute coordinates for just the points that you can plot—the pixels!

I provide a Java program that illustrates four ways of drawing from a Java application, of which three provide pixel access. We use the method setRGB to draw the background. Alternatively we use setPixels or setPixel on the WritableRaster, access to which has been extracted from the BufferedImage. Change (1>0) to (1>1) to use the 2nd alternative.

With Java 1.5 or 1.6 on my Mac I get:
pix.jpeg
pix.png (This should look like the image above.)

On my Mac (OS X 10.8.2, Java version 1.8.0-ea) I get such a picture on my screen with

javac JavaPix.java
java -cp . Main pix.png pix.jpeg

The first command produces files MainCanvas.class and Main.class which the second command uses. The second command forms an image on the screen and produces files pix.png and pix.jpeg as shown above. The screen image includes an ellipse which is drawn when paint is called—the ellipse but not the shading, remains tangent to the window edges if you manually resize the window. The two files do not have the ellipse because it is only computed as the pixels are put on the screen. The size of the png file produced by the program is 115KB. If you open and export the png file using the Apple’s Preview program that comes with OS X, the new size is 1028 bytes. Beware, sometimes Preview expands .png files.

I just now (2013 April) found Java 1.8 EA here. It would seem that Apple does not trust Oracle’s code and so you must perform the shell command:

xattr -d ~/Downloads/com.apple.quarantine jdk*

if you wish to trust this Oracle code on your machine.

Code Explanation

X and Y are the constant image sizes. I is a BufferedImage which in this case is a wrapper for an array of ints, one per pixel, which we can modify by one of two schemes:

I.setRGB sets an addressed pixel to a color described in the right 24 bits of the int. With this method we set a tan background and scratch it just to prove that we were there. (This takes 180 ms to set the 71K pixels. (400MHz PowerPC)) (It takes 4.8 ms on a 1.8 GHz Intel duo.)
For better performance we extract a WritableRaster from our BufferedImage which is merely a different wrapper for the stored pixels. The technique I show here requires an array of words, three per pixel, that we fill with a graduated color. In this context “a[z] =” sets the red intensity, “a[z+1] =” the green and “a[z+2] =” the blue. Then, with the wr.setPixels invocation, we send that array, a, to the WritableRaster which slurps up the color data. (It takes 28 ms to copy these 71K pixels thus. DVD players must find something faster!)
Changing the if(true) to if(false) causes the program to produce the same result using setPixel instead of setPixels. (This takes 7.5 ms.)

Now the picture, except for the oval, is in the shared pixels behind the BufferedImage and WritableRaster.

Having computed the picture once, the paint method of our MainCanvas class refers to the BufferedImage as Main.I, and then draws the oval. Thus we access the constant image each time the window is resized.

Here is the program that created my favicon. To produce the file I did:

javac favicon.java
java -cp . Main

Putting the resulting file favicon.ico in my top level web site directory sufficed except for Safari which also worked when I added the line “AddType "image/jpg" .ico” to the file .htaccess in the same directory. It seems that older Explorer browsers still act as if a favicon must be a .gif file.

More insights from Sun

Other uses of Java Pixels

3 body problem
Hydrodynamics
Really bent beam
This program produces an image that changes with time. It is paced by the computation in the main loop.