predetermined pattern are also implemented using roaming AI.
iYAlien++; else if (iYAlien < iYShip) iYAlien--; This code basically does the opposite of the code used by the chasing algorithm with the only differences being the unary operators (++, --) used to change the alien's position so that it runs away, as opposed to chasing. Similar to chasing, evading can be softened using randomness or patterned movement. A good example of evading is the ghosts in the classic arcade game Pac Man, who run away from the player when you eat a power pellet. Of course, the Pac-Man ghosts also take advantage of chasing when you don't have the ability to eat them. Another good example of using the evading algorithm would be a computer-controlled version of the player's ship in a space game with a computer player. If you think about it, the player is using the evading algorithm to dodge the aliens; it's just implemented by hitting keys rather than in a piece of computer-controlled code. If you want to provide a demo mode in a game like this where the computer plays itself, you would use an evading algorithm to control the player's ship. Hour 23, "Showing Off Your Game with Demo Mode," shows you exactly how to create a demo mode for your games. Patterned Roaming Patterned movement refers to a type of roaming AI that uses a predefined set of movements for a game object. Good examples of patterned movement are the aliens in the classic Galaga arcade game, which perform all kinds of neat aerobatics on their way down the screen. Patterns can include circles, figure eights, zigzags, or even more complex movements. An even simpler example of patterned movement is the Space Invaders game, where a herd of aliens slowly and methodically inch across and down the screen. In truth, the aliens in Galaga use a combined approach of both patterned and chasing movement; although they certainly follow specific patterns, the aliens still make sure to come after the player whenever possible. Additionally, as the player moves into higher levels, the roaming AI starts favoring chasing over patterned movement in order to make the game harder. This is a really neat usage of combined roaming AI. This touches on the concept of behavioral AI, which you learn about in the next section. Patterns are usually stored as an array of velocity or position offsets (or multipliers) that are applied to an object whenever patterned movement is required of it, like this: int iZigZag[2][2] = { {3, 2}, {-3, 2} }; iXAlien += iZigZag[iPatternStep][0]; iYAlien += iZigZag[iPatternStep][1]; This code shows how to implement a very simple vertical zigzag pattern. The integer array iZigZag contains pairs of XY offsets used to apply the pattern to the alien. The iPatternStep variable is an integer representing the current step in the pattern. When this pattern is applied, the alien moves in a vertical direction at a speed of 2 pixels per game cycle, while zigzagging back and forth horizontally at a speed of 3 pixels per game cycle. Behavioral AI Although the types of roaming AI strategies are pretty neat in their own right, a practical gaming scenario often requires a mixture of all three. Behavioral AI is another fundamental type of gaming AI that often uses a mixture of roaming AI algorithms to give game objects specific behaviors. Using the trusted alien example again, what if you want the alien to chase some times, evade other times, follow a pattern still other times, and maybe even act totally randomly every once in a while? Another good reason for using behavioral AI is to alter the difficulty of a game. For example, you could favor a chasing algorithm more than random or patterned movement to make aliens more aggressive in higher levels of a space game. To implement behavioral AI, you would need to establish a set of behaviors for the alien. Giving game objects behaviors isn't too difficult, and usually involves establishing a ranking system for each type of behavior present in the system, and then applying it to each object. For example, in the alien system, you would have the following behaviors: chase, evade, fly in a pattern, and fly randomly. For each different type of alien, you would assign different percentages to the different behaviors, thereby giving them each different personalities. For example, an aggressive alien might have the following behavioral breakdown: chase 50% of the time, evade 10% of the time, fly in a pattern 30% of the time, and fly randomly 10% of the time. On the other hand, a more passive alien might act like this: chase 10% of the time, evade 50% of the time, fly in a pattern 20% of the time, and fly randomly 20% of the time. This behavioral approach works amazingly well and yields surprising results considering how simple it is to implement. A typical implementation simply involves a switch statement or nested if-else statements to select a particular behavior. A sample implementation for the
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. The big section of code in the middle of the function takes care of setting the meteor’s velocity so that it targets one of the four cities positioned along the bottom of the screen (lines 11 27). I realize that this code looks a little tricky, but all that’s going on is some basic trigonometry to figure out what velocity is required to get the meteor from point A to point B, where point A is the meteor’s random position and point B is the position of the city. The AddMeteor() function ends by adding the new meteor sprite to the game engine (line 30). That wraps up the code for the Meteor Defense game, which hopefully doesn’t have your head spinning too much. Now you just need to put the resources together and you can start playing. Testing the Game It’s safe to congratulate yourself at this point because you’ve now worked through the design and development of the Meteor Defense game, and you can now bask in the glory of playing the game. Granted, playing a game for the first time is certainly more of a test than it is a true playing experience, but in this case I can vouch that the game works pretty well. Figure 19.2 shows the game at the start, with a couple of meteors hurtling toward cities. Figure 19.2. The Meteor Defense game gets started with a couple of meteors hurtling at the cities. Saving the cities from the meteors involves targeting the meteors and blasting them with missiles. When you successfully blast a meteor, you’ll see an explosion as the missile and meteor both are destroyed (see Figure 19.3) Figure 19.3. Successfully destroying a meteor results in an explosion being displayed. When the game progresses as more meteors fall, you’ll eventually start losing cities to the meteors. Figure 19.4 shows an example of how the meteors can begin to overwhelm you late in the game. Figure 19.4. As the game progresses, you tend to lose cities as the meteors start to overwhelm you. When you finally lose all four cities to the meteors, the game ends. Figure 19.5 shows the end of the game, which involves displaying the game over image on the screen on top of the other game graphics. Figure 19.5. When all four cities are destroyed, the game ends and the game over image is displayed. Although it’s sad to think that you’ve ultimately failed to save the cities in the Meteor Defense game, you can sleep well knowing that you can always right-click the mouse to start over and take another stab at it.