java - How to Draw an BufferedImage to a JPanel

Question

I am trying to use some sort of draw method to draw a sprite image to my subclass of JPanel called AnimationPanel. I have created a Spritesheet class which can generate a BufferedImage[] that contains all of the sprites in the sheet. In my AnimationPanel class, which implements Runnable, I am getting that BufferedImage[] from the spritesheet instantiated in the AnimationPanel constructor. I want to be able to loop through this array and display each sprite to the screen. How would I do this? Here are my AnimationPanel and Spritesheet classes.

AnimationPanel

package com.kahl.animation;

import javax.swing.JPanel;

public class AnimationPanel extends JPanel implements Runnable {

//Instance Variables
private Spritesheet sheet;
private int currentFrame;
private Thread animationThread;
private BufferedImage image;

public AnimationPanel(Spritesheet aSheet) {
    super();
    sheet = aSheet;
    setPreferredSize(new Dimension(128,128));
    setFocusable(true);
    requestFocus();

}

public void run() {
    BufferedImage[] frames = sheet.getAllSprites();
    currentFrame = 0;
    while (true) {
        frames[currentFrame].draw(); //some implementation still necessary here
        currentFrame++;
        if (currentFrame >= frames.length) {
            currentFrame = 0;
        }
    }
}

public void addNotify() {
    super.addNotify();
    if (animationThread == null) {
        animationThread = new Thread(this);
        animationThread.start();
    }
}

}

Spritesheet

package com.kahl.animation;

import java.awt.image.BufferedImage;
import java.imageio.ImageIO;
import java.io.IOException;
import java.io.File;

public class Spritesheet {

//Instance Variables
private String path;
private int frameWidth;
private int frameHeight;
private int framesPerRow;
private int frames;
private BufferedImage sheet = null;

//Constructors
public Spritesheet(String aPath,int width,int height,int fpr, int numOfFrames) {

    path = aPath;
    frameWidth = width;
    frameHeight = height;
    framesPerRow = fpr;
    frames = numOfFrames;

    try {
        sheet = ImageIO.read(getClass().getResourceAsStream());
    } catch (IOException e) {
        e.printStackTrace();
    }

}

//Methods

public int getHeight() {
    return frameWidth;
}

public int getWidth() {
    return frameWidth;
}

public int getFramesPerRow() {
    return framesPerRow;
}

private BufferedImage getSprite(int x, int y, int h, int w) {
    BufferedImage sprite = sheet.getSubimage(x,y,h,w);
}

public BufferedImage[] getAllSprites() {
    BufferedImage[] sprites = new BufferedImage[frames];
    int y = 0;
    for (int i = 0; i < frames; i++) {
        x = i * frameWidth;
        currentSprite = sheet.getSprite(x,y,frameHeight,frameWidth);
        sprites.add(currentSprite);
    }
    return sprites;

}

}

Answer 1

1. I'd encourage the use of a javax.swing.Timer to control the frame rate, rather than an uncontrolled loop
2. Once the timer "ticks", you need to increment the current frame, get the current image to be rendered and call repaint on the JPanel
3. Use Graphics#drawImage to render the image.

See...

• Painting in AWT and Swing
• Performing Custom Painting
• How to use Swing Timers
• Graphics#drawImage(Image, int, int, ImageObserver)

for more details

There is a cascading series of issues with your Spritesheet class, apart from the fact that it won't actually compile, there are issues with you returning the wrong values from some methods and relying on values which are better calculated...

I had to modify your code so much, I can't remember most of them

public int getHeight() {
    return frameWidth;
}

and

public BufferedImage[] getAllSprites() {
    BufferedImage[] sprites = new BufferedImage[frames];
    int y = 0;
    for (int i = 0; i < frames; i++) {
        x = i * frameWidth;
        currentSprite = sheet.getSprite(x,y,frameHeight,frameWidth);
        sprites.add(currentSprite);
    }
    return sprites;

}

Stand out as two main examples...

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestSpriteSheet {

    public static void main(String[] args) {
        new TestSpriteSheet();
    }

    public TestSpriteSheet() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private Spritesheet spritesheet;
        private BufferedImage currentFrame;
        private int frame;

        public TestPane() {
            spritesheet = new Spritesheet("/Sheet02.gif", 240, 220);
            Timer timer = new Timer(100, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    currentFrame = spritesheet.getSprite(frame % spritesheet.getFrameCount());
                    repaint();
                    frame++;
                }
            });
            timer.start();
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(240, 220);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (currentFrame != null) {
                Graphics2D g2d = (Graphics2D) g.create();
                int x = (getWidth() - currentFrame.getWidth()) / 2;
                int y = (getHeight() - currentFrame.getHeight()) / 2;
                g2d.drawImage(currentFrame, x, y, this);
                g2d.dispose();
            }
        }

    }

    public class Spritesheet {

//Instance Variables
        private String path;
        private int frameWidth;
        private int frameHeight;
        private BufferedImage sheet = null;
        private BufferedImage[] frameImages;

//Constructors
        public Spritesheet(String aPath, int width, int height) {

            path = aPath;
            frameWidth = width;
            frameHeight = height;

            try {
                sheet = ImageIO.read(getClass().getResourceAsStream(path));
                frameImages = getAllSprites();
            } catch (IOException e) {
                e.printStackTrace();
            }

        }

        public BufferedImage getSprite(int frame) {
            return frameImages[frame];
        }

//Methods
        public int getHeight() {
            return frameHeight;
        }

        public int getWidth() {
            return frameWidth;
        }

        public int getColumnCount() {
            return sheet.getWidth() / getWidth();
        }

        public int getRowCount() {
            return sheet.getHeight() / getHeight();
        }

        public int getFrameCount() {
            int cols = getColumnCount();
            int rows = getRowCount();
            return cols * rows;
        }

        private BufferedImage getSprite(int x, int y, int h, int w) {
            BufferedImage sprite = sheet.getSubimage(x, y, h, w);
            return sprite;
        }

        public BufferedImage[] getAllSprites() {
            int cols = getColumnCount();
            int rows = getRowCount();
            int frameCount =  getFrameCount();
            BufferedImage[] sprites = new BufferedImage[frameCount];
            int index = 0;
            System.out.println("cols = " + cols);
            System.out.println("rows = " + rows);
            System.out.println("frameCount = " + frameCount);
            for (int row = 0; row < getRowCount(); row++) {
                for (int col = 0; col < getColumnCount(); col++) {
                    int x = col * getWidth();
                    int y = row * getHeight();
                    System.out.println(index + " " + x + "x" + y);
                    BufferedImage currentSprite = getSprite(x, y, getWidth(), getHeight());
                    sprites[index] = currentSprite;
                    index++;
                }
            }
            return sprites;

        }

    }
}

Remember, animation is the illusion of change over time. You need to provide a delay between each frame of the animation, long enough for the user to recognise it, but short enough to make the animation look smooth.

In the above example, I've used 100 milliseconds, simply as an arbitrary value. It could be possible to use something more like 1000 / spritesheet.getFrameCount(), which will allow a full second for the entire animation (all the frames within one second).

You might need to use different values, for longer or short animations, depending on your needs