Drawing Optimizations in Java

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Look closely at the paint() method in Listing 164 The SpriteManager class is drawing sprite images, even if they are located beyond the bounds of the current screen This will slow down the game, because a huge number of unnecessary actions are invoked Your game might have hundreds of sprites in memory, but only one or two drawn on the screen at any given time Instead, painting should be done as in Listing 165 Listing 165 Improved Painting
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import javautil*; import javaxmicroeditionlcdui*; public class SpriteManager { private Vector list; private int width; private int height; public SpriteManager(int width, int height) { thiswidth = width; thisheight = height; list = new Vector(); } public void paint(Graphics g) { for (int i = 0; i < listsize(); i++) { Sprite sprite = (Sprite)listelementAt(i); if ((spritegetX() + spritegetWidth() > 0) && (spritegetX() < width) && (spritegetY() + spritegetHeight() > 0) && (spritegetY() < height)) spritepaint(g); } } }
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The sprite manager stores the values of the screen's width and height Both values are very important, because they represent boundaries The paint() method walks through the manager's sprite list and checks whether each sprite lies inside the screen
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Enhancing Sprite Collision
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To apply the sprite manager to our Micro Racer game, you can create one SpriteManager group for all the enemy cars You can then figure out whether your player sprite has collided with an enemy by creating collision detection within the SpriteManager class, as illustrated in Listing 166 Listing 166 Adding Collision Detection
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public boolean collide(Sprite sprite) { for (int i = 0; i < listsize(); i++) { Sprite comparableSprite = (Sprite)listelementAt(i); if (spritecollide(comparableSprite)) return true;
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} return false; }
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The collide() method checks whether the player sprite (which should be passed in as a parameter) has collided with any of the enemy sprites If there is a collision, the method returns true Moving through the list with a larger number of sprites can significantly slow down the game One possible optimization would be to put the sprites in a correct order within the list, and only check the sprites that are on the screen The ones that are offscreen could never collide with the player sprite
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Summary
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Now we're getting someplace! We have a fully functional game with a hero, a group of enemies, and a means of figuring out whether any fender-benders have occurred However, the game is still pretty simplistic After all, none of our sprites are actually moving yet! The next chapter finally makes sprites spritely, showing you how to actually animate and move them around within a game loop
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17 Sprite Movement
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IN THIS CHAPTER
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Floating-Point in J2ME Game Initialization Movement Piecing It All Together Summary
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So far, we've managed to create a bunch of sprites and draw them on the screen But we're not painting a still life here; we're programming a game! To make a game a game, it must read input events from the keyboard or the screen and move sprites accordingly The game must also set up initial conditions, check controls for game operability, and provide artificial intelligence for computer-controlled characters or game moves
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Before we get too excited, though, there's one thing we're going to have to get out of the way: MIDP does not support floating-point math Why is floating-point math important for games Imagine a game hero that can move in different directions, such as the enemy cars in our Micro Racer game You'll want the cars to move at a constant speed, regardless of which direction the sprite is moving To create this type of smooth movement, you would use a pair of simple trigonometric formulas:
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Vx = V * cos(tau)
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Vy = V * sin(tau)
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where tau is an angle starting at the 3 o'clock position and increasing counterclockwise, as illustrated in Figure 171 Figure 171 Velocity components
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Unfortunately, the results of these formulas are going to involve decimal places If you round things off, movement becomes nonlinear Additionally, 3D games are becoming very popular It's conceivable that 3D engines, or isometric engines (games that simulate depth of scene, such as the view of a maze by peeking over its outer wall), will soon be feasible on smaller devices In order to transform 3D polygons and draw 3D worlds, accurate linear algebra and trigonometric functions are imperative This involves precise floating-point support