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Relatively smooth motion

When I compile and run this program, I see the sprite moving with a relatively smooth motion. Unless your computer is very slow, you should probablysee the same thing.

Beginning of the class named Slick0150a

Listing 1 shows the beginning of the class definition for the class named Slick0150a .

Listing 1 . Beginning of the class named Slick0150a.
public class Slick0150a extends BasicGame{Image bug = null; Image background = null;float backgroundWidth;float backgroundHeight;float bugX = 100; float bugY = 100;float bugWidth; float bugHeight;float bugXDirection = 1.0f;//initial direction to rightfloat bugYDirection = 1.0f;//initial direction is downfloat xStep = 4.0f;//horizontal step size float yStep = 3.0f;//vertical step sizefloat bugScale = 0.75f;//drawing scale factor//Used to compute and display the time required for the// bug to make each round trip across the game window // and back.long oldTime = 0; long traversalTime = 0;//Frame rate we would like to see and maximum frame// rate we will allow. int targetFPS = 60;

Listing 1 consists entirely of instance variable declarations. The purpose of each of these variables shouldbecome clear as they are used later in the code. No explanation beyond the embedded commentsshould be needed at this point.

The constructor and the main method

There is nothing new in the constructor and the main method. You can view them both in Listing 9 .

The init method

The init method is shown in Listing 2 . I will explain the new material in this method.

Listing 2 . The init method for Slick0150a.
public void init(GameContainer gc) throws SlickException {oldTime = gc.getTime(); bug = new Image("ladybug.png");background = new Image("background.jpg");backgroundWidth = background.getWidth(); backgroundHeight = background.getHeight();bugWidth = bug.getWidth()*bugScale;bugHeight = bug.getHeight()*bugScale;System.out.println( "backgroundWidth: " + backgroundWidth);System.out.println( "backgroundHeight: " + backgroundHeight);System.out.println("bugWidth: " + bugWidth); System.out.println("bugHeight: " + bugHeight);gc.setTargetFrameRate(targetFPS);//set frame rate}//end init

Get the time

The GameContainer class provides a method named getTime , which is described simply as:

"Get the accurate system time."

I am interpreting this to mean that the method will return the system time good to one millisecond relative to a well-defined time origin.

(Standard Java uses January 1, 1970 as the origin or epoch of time but Slick2D may use a different origin. Since we will be working with changes intime and not absolute time, the time origin doesn't matter.)

The init method in Listing 2 calls the getTime method to get and save the time in an instance variable named oldTime . The values in this variable will be used later to compute the round-trip timerequired for the sprite to move across the game window and back to the starting point at the right edge of the window.

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Source:  OpenStax, Anatomy of a game engine. OpenStax CNX. Feb 07, 2013 Download for free at https://legacy.cnx.org/content/col11489/1.13
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