Thread: Need Help with Rendering the screen (with volatile images)

I'm experimenting with image scaling and volatile images, but whatever I try, I can't seem to get my code to actually draw anything.

Code:

package gravitygamesinteractive.scorpioengine.main;
 
import java.awt.*;
 
import javax.swing.JPanel;
 
public class Main extends JPanel implements Runnable{
	public static boolean isRunning=false;
	public static boolean rightPressed, leftPressed, upPressed, downPressed, jumpPressed, actionPressed;
	public Level level;
	private Image screen;
 
	public Main(){
		start();
	}
 
	public void start(){
		level=new Level();
 
		isRunning=true;
		new Thread(this).start();
	}
 
	public void tick(){
		level.tick();
	}
 
	public void render(Graphics g){
		//Graphics g = screen.getGraphics();
 
		g.setColor(new Color(0,0,0));
		g.fillRect(0, 0, 100, 100);
		//level.paintComponent(g);
 
		g = getGraphics();
		g.drawImage(screen, 0, 0, Frame.size.width, Frame.size.height, 0, 0, Frame.realsize.width, Frame.realsize.height, null);
 
		g.dispose();
	}
 
	public void run() {
		screen = createVolatileImage(Frame.realsize.width, Frame.realsize.height);
 
		while(isRunning){
			tick();
			render(screen.getGraphics());
 
			try{
				Thread.sleep(16);
			}catch(Exception e){
 
			}
		}
	}
 
}

The error handling says its on line 46, which is the call to the render method. I can't tell whether its a problem with the screen Image, the Volatile Image, or something else entirely.

The error handling says its on line 46

Can you copy the full text of the error message and paste it here?

Sure, but it's not much:

Originally Posted by error message

Exception in thread "Thread-1" java.lang.NullPointerException
at gravitygamesinteractive.scorpioengine.main.Main.ru n(Main.java:46)
at java.lang.Thread.run(Unknown Source)

Exception in thread "Thread-1" java.lang.NullPointerException

What variable has the null value?

Have you read the API doc for the createVolatileImage() method?

Just read the doc here: VolatileImage (Java Platform SE 6)

After trying to tweak my code, I'm still confused, and I'm still getting errors

Code:

package gravitygamesinteractive.scorpioengine.main;
 
import java.awt.*;
import java.awt.image.VolatileImage;
 
import javax.swing.JPanel;
 
public class Main extends JPanel implements Runnable{
	public static boolean isRunning=false;
	public static boolean rightPressed, leftPressed, upPressed, downPressed, jumpPressed, actionPressed;
	public Level level;
	private VolatileImage screen;
 
	public Main(){
		screen = createVolatileImage(Frame.realsize.width, Frame.realsize.height);
		start();
	}
 
	public void start(){
		level=new Level();
 
		isRunning=true;
		new Thread(this).start();
	}
 
	public void tick(){
		level.tick();
	}
 
	public void renderOffscreen() {
        do {
            if (screen.validate(getGraphicsConfiguration()) ==
                VolatileImage.IMAGE_INCOMPATIBLE)
            {
                // old vImg doesn't work with new GraphicsConfig; re-create it
                screen = createVolatileImage(Frame.realsize.width, Frame.realsize.height);
            }
            Graphics g = screen.createGraphics();
            //
            // miscellaneous rendering commands...
            //
            g.dispose();
        } while (screen.contentsLost());
 
 
 }
 
	public void render(Graphics g){
		 do {
	            int returnCode = screen.validate(getGraphicsConfiguration());
	            if (returnCode == VolatileImage.IMAGE_RESTORED) {
	                // Contents need to be restored
	                renderOffscreen();      // restore contents
	            } else if (returnCode == VolatileImage.IMAGE_INCOMPATIBLE) {
	                // old vImg doesn't work with new GraphicsConfig; re-create it
	                screen = createVolatileImage(Frame.realsize.width, Frame.realsize.height);
	                renderOffscreen();
	            }
	            g.drawImage(screen, 0, 0, this);
	     } while (screen.contentsLost());
	}
 
	public void run() {
		screen = createVolatileImage(Frame.realsize.width, Frame.realsize.height);
 
		while(isRunning){
			tick();
			renderOffscreen();
			render(screen.getGraphics());
 
			try{
				Thread.sleep(16);
			}catch(Exception e){
 
			}
		}
	}
 
}

Errors:

Exception in thread "Thread-1" java.lang.NullPointerException
at gravitygamesinteractive.scorpioengine.main.Main.re nderOffscreen(Main.java:32)
at gravitygamesinteractive.scorpioengine.main.Main.ru n(Main.java:68)
at java.lang.Thread.run(Unknown Source)

So yeah, still a null pointer exception...

What variable has the null value? Why does it have a null value?

Originally Posted by Norm

What variable has the null value? Why does it have a null value?

Well, everything seems to be pointing to the "screen" Image, but oddly enough, I set it to a new Volatile Image twice.

What was its value when you print it out for debugging?

Originally Posted by curmudgeon

You will want to post the actual line of code that is throwing the NPE.

Lines 32 and 33: if (screen.validate(getGraphicsConfiguration()) ==
VolatileImage.IMAGE_INCOMPATIBLE)

Line 68: renderOffscreen();

EDIT: The screen image is returning as null. (Sorry, I didn't see Norm's post right away...)

Why is the value of screen null? Where does it get its value from?

Originally Posted by Norm

Why is the value of screen null? Where does it get its value from?

It should be coming from here:

public Main(){
		screen = createVolatileImage(Frame.realsize.width, Frame.realsize.height);
		start();
	}

...or here...

public void run() {
		screen = createVolatileImage(Frame.realsize.width, Frame.realsize.height);
 
		while(isRunning){
			tick();
			renderOffscreen();
			render(screen.getGraphics());
 
 
			try{
				Thread.sleep(16);
			}catch(Exception e){
 
			}
		}
	}

In post #4 I asked:
Have you read the API doc for the createVolatileImage() method?
When does it return a null value?

Yes, I have read the API doc. The problem still seems to be this line:

if (screen.validate(getGraphicsConfiguration()) == VolatileImage.IMAGE_INCOMPATIBLE){

The screen variable returns as null, even though it is created in the run method (commented out the other time I created it). The problem may also be the "getGraphicsConfiguration", as I honestly have know clue what that method does.

In case of the very small chance that it actually changes anything, I'm compiling against Java 1.6, as I'm not sure if 1.7 is out of beta yet.

Does the screen variable have a value of null? If so any use of screen will cause the error.

You need to find out where the screen variable gets the null value from!!!

Well, I'm completely stumped. I can't seem to figure out why the variable keeps coming up null. Some of the docs I've read say I need to set up a GraphicsConfigurations object, some say I don't. I've tried adding printlns in between every line of code, but I still can't figure it out. I'm honestly thinking I need to go with an alternate drawing method, or the one I was using before. Sorry for both giving up and wasting your guy's time.

Posting an SSCCE helps a lot in providing context to your problem. You are asking us to guess what's null without code to show us what's null. Based upon the posts above, screen is instantiated in the constructor by calling getGraphicsConfiguration or maybe createVolatileImage (I'm confused about which given your code changes from post to post), I can guess you are are receiving the exception because screen is null. To quote the java 6 API, which should be studied before using the methods contain within - for getGraphicsConfiguration():

If the Component has been created, but not yet added to a Container, this method returns null

and for createVolatileImage:

The return value may be null if the component is not displayable

Has the component been added to a container (given the instantiation of screen occurs in the constructor in may of your code snippets...I seriously doubt it)?