Moving Objects and Sprite Manager - Part 8

This is part 8 of the series. You are suggested to read the whole series from the beginning. You may find the whole series in the Featured page.

Scrolling Background

We will use a looping background created by ansimuz.

You may find the original collection at :
http://opengameart.org/content/space-ship-shooter-pixel-art-assets

The author release the file under the CC0 license. In summary, the license allow you to use it in your own game, even for commercial purpose.

You may download the file space_shooter_pack.zip

We will use the file desert-backgorund-looped.png inside the zip file.

This is a looping background. That means if we cut the bottom row of the image, and then paste it to the top, we still have a continuous image. Hence the scrolling is as simple as: Note that we are using the same technique in Optimized flood filler to access the pixel array of the image. This is much faster than the getPixel() and setPixel() API.

Reading image from zip file

We will read directly from the zip file so that we don't need to uncompress the file to the harddrive. The source code for reading zip file is defined in readZipImage() in the demonstration. The reason to read directly from zip file is that we may redistribute other people artwork in an unmodified form. This typically fulfill most of the license requirement in http://opengameart.org.

Important
There are many free artworks available in opengameart.org. However, you must read the license of the individual entry before using it. Not every entry is released under CC0. Different authors may submit their work under different licenses. For example, many licenses require you to attribute the original author. Just like what I am doing below :

"Space Ship Shooter Pixel Art Assets" by ansimuz / CC0

You may have a look at Best Practices for attribution

Demonstration

You will need Scroller.java and space_shhoter_pack.zip in the same folder.

Compile and run

javac Scroller.java
java Scroller

/****************************************************************************** * File : Scroller.java * Author : Ivan Macteki @http://java.macteki.com/ * Description : * Scroller demonstration. * required space_shooter_pack.zip * Tested with : OpenJDK 1.8 ******************************************************************************/ import java.awt.image.BufferedImage; import javax.swing.SwingUtilities; class Scroller extends javax.swing.JPanel { // image object for double buffering private BufferedImage drawingBoard; BufferedImage backgroundImage; public Scroller() throws Exception { // you may read the background image in the traditional way: // String jpeg_file="background.jpg"; // backgroundImage = javax.imageio.ImageIO.read(new java.io.File(jpeg_file)); // However, since our background is stored inside a zip file, we will // use our own function readZipImage() // read background image from a zip file backgroundImage = readZipImage("space_shooter_pack.zip", "space_shooter_pack/backgrounds/desert-backgorund-looped.png"); // set panel size to dimension of background image int w=backgroundImage.getWidth(this); int h=backgroundImage.getHeight(this)/2; setPreferredSize(new java.awt.Dimension(w,h)); // create a drawing board with the same size of the panel. // this is used in paintComponent() below drawingBoard=new BufferedImage(w,h,BufferedImage.TYPE_INT_RGB); } // Read a image from the zip file. // e.g. zipFileName="space_shooter_pack.zip"; // imageFileName="space_shooter_pack/backgrounds/desert-backgorund-looped.png" BufferedImage readZipImage(String zipFileName, String imageFileName) throws Exception { java.awt.image.BufferedImage image=null; java.util.zip.ZipInputStream zip = new java.util.zip.ZipInputStream(new java.io.FileInputStream( zipFileName)); java.util.zip.ZipEntry file; while ((file = zip.getNextEntry()) != null) { if (file.getName().equals(imageFileName)) { image = javax.imageio.ImageIO.read(zip); } } // the image in the original zip file is a bit small double scale=2; // hence we scale it up by a factor of 2 int w=image.getWidth(null); int h=image.getHeight(null); int bigW=(int) (scale*w); int bigH=(int) (scale*h); int type=BufferedImage.TYPE_INT_ARGB; BufferedImage bigImage=new BufferedImage(bigW, bigH, type); bigImage.getGraphics().drawImage(image,0,0,bigW, bigH, 0, 0, w, h, null); return bigImage; } @Override // JPanel public void paintComponent(java.awt.Graphics gr) { gr.drawImage(drawingBoard,0,0,this); } public void scrollImage(BufferedImage image) { int w=image.getWidth(null); int h=image.getHeight(null); int[] pixels = ((java.awt.image.DataBufferInt) image.getRaster().getDataBuffer()).getData(); // save the bottomRow of the image before scrolling down int[] bottomRow = new int[w]; System.arraycopy(pixels, (h-1)*w, bottomRow, 0, w); // scrolling down by one row System.arraycopy(pixels, 0, pixels, w, w*(h-1)); // copy the bottom row to the top row System.arraycopy(bottomRow, 0, pixels, 0, w); } private void animateFrame() { java.awt.Graphics2D gr=(java.awt.Graphics2D) drawingBoard.getGraphics(); gr.drawImage(backgroundImage,0,0,this); scrollImage(backgroundImage); repaint(); } void init() { javax.swing.JFrame window=new javax.swing.JFrame(); window.setDefaultCloseOperation(javax.swing.JFrame.EXIT_ON_CLOSE); window.setTitle("Macteki Scroller Demonstration"); Scroller scroller=this; window.add(scroller); window.pack(); window.setVisible(true); // call animateFrame() for every 10ms javax.swing.Timer timer=new javax.swing.Timer(10, (e)->animateFrame()); timer.start(); } public static void main(String[] args) throws Exception { Scroller scroller=new Scroller(); SwingUtilities.invokeLater( ()->scroller.init() ); } } // end Scroller.java