Collision Detection in Java

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Collision Detection
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Sprite collision detection is an essential attribute of any game engine Collision detection lets you know when two sprites are touching Pretty much every game is based on this principal: Bullet hits soldier, monster touches elf, pinball bumps into flipper, Bill Gates grabs money bags, shoes touch lava, and so on In our Micro Racer game, when the player's car collides with an enemy car, we will deduct one unit of energy If the player loses all 100 energy units, the game is over We'll also throw obstacles into the game If the player's car hits an oil slick, for example, it will veer off to the side Finally, we'll include power-ups When the player's car touches a wrench, its damage will be fixed automatically The question is, how can we implement fast but accurate collision detection on small, slow, resource-limited devices
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Basic Collision Detection
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The simplest form of collision detection is generally the fastest form Each sprite and its image are treated as a rectangle If one sprite's rectangle overlaps another sprite's rectangle, as in Figure 152, then we consider the sprites as having collided Notice that in the figure there is a 4x2 section of overlapping rectangles Figure 152 Simple collision between two sprites
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To implement this type of detection, you need to take the x position, y position, width, and height of two sprites and then do some simple math, as shown in Listing 154 Listing 154 Adding Collision Detection to the Sprite
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public boolean collide(Sprite sprite) { if ((x + width) > spritegetX() && x < spritegetX() && (y + height) > spritegetY() && y < spritegetY()) return true; return false; }
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Unfortunately, this is a very primitive form of detection Most sprites are not actually rectangular A racing car, for example, is shaped more like an I Using this form of collision detection, the game might deduct energy from your car, even if the car doesn't actually touch an enemy Additionally, you might want to only detect collision within a certain part of a sprite For example, when a knife hits a monster, you might want to deduct two hit points if the knife stabs the monster's face, and only one hit point if the knife hits the body The next chapter discusses how to implement a more advanced form of collision detection
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Creating Child Sprites
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The Sprite class contains everything we need to draw and move sprites, but it doesn't have a lot of personality To really make a game swing, child classes need to be created for various sprites, each containing their own properties and methods In our Micro Racer game, for example, we will have two types of sprites the player and the enemy
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Building the Player Sprite
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Many games feature a hero or protagonist This is known as a player sprite In our racing game, the red car is the player sprite A player sprite is treated differently than other sprites It represents the one unique object that defines how the game will progress In our racing game, the player sprite contains one additional property energy This property lets us know how often collisions have occurred When energy reaches zero, the game is over If the car can reach the end of a track without losing all its energy, the energy value is reset and the player is given a score A player child class is shown in Listing 155 Listing 155 The Player Sprite Child Class
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import javaxmicroeditionlcdui*; public class Player extends Sprite { protected int energy; public Player(Image image, int width, int height, int numFrames, int energy) throws Exception { super(image, width, height, numFrames); thisenergy = energy; } public int getEnergy() { return energy; } public void setEnergy(int energy) { thisenergy = energy; } }
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The Player class constructor calls its parent constructor, passing up most of the values The only additional value helps set the car's initial energy A more advanced Player class might contain even more information, such as the amount of money the car's owner has made, the damage to various parts of the car, the number of bonus
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points the player has gained, and so on We will flesh out this class in the final chapter of the book, "Micro Racer: Putting it All Together"
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