FUNDAMENTALS OF GAME DESIGN, SECOND EDITION
Challenges
Action games have more different kinds of challenges than just about any other genre, although almost all of these challenges test physical skills—speed and reaction time, steering and shooting, timing and rhythm, and the ability to execute combo moves in fighting games. Other common types of challenges include pattern recognition (recognizing the attack and patrol behaviors of enemies) and exploration (learning your way around a space). A few action games, such as Tetris and Portal, include puzzle-solving elements. The complexity of the puzzles should be inversely proportional to the amount of time pressure the player feels.
This section looks at the way pure action games characteristically present and organize their challenges. Action-adventure hybrid games frequently incorporate challenges from the adventure game genre, such as locked doors and mazes. See the "Challenges" section of Chapter 19 for more information.
In a game that requires navigation through a space, the player's avatar is typically faced with three types of problems: passive obstacles, stationary dangers, and active dangers or enemies.
A passive obstacle impedes movement without actually threatening the avatar. To get past a wall or a chasm, the player climbs over or jumps across. Obstacles can also trap the avatar in a region with other dangers. Obstacles are usually, but not always, indestructible.
A stationary danger attacks the avatar when she gets close to it but does not move around the landscape. Examples of stationary dangers include electric fences, swinging blades on a pendulum, or plants that bite. Some stationary dangers must be attacked or destroyed to allow the avatar to pass by; others are indestructible and must be treated like obstacles, so the avatar has to avoid or surmount them.
Active dangers, or enemies, attack the avatar, moving around in the landscape. In old games, they often moved in a fixed pattern that the player could learn to avoid, but in modern games, artificially intelligent enemies locate and pursue the avatar. See Chapter 6, "Character Development," for more information on designing enemy characters.
When enemies appear or attack in groups, usually groups of the same type or similar types of enemies, they are said to come in waves. The makeup of the wave normally varies, including a selection of enemies appropriate to the current level of the game. As the game progresses, the waves include stronger enemies. At later stages of the game you may want to phase out the earlier weaker enemies, because they will no longer provide an effective challenge. Enemies increase in strength and number, reaching a peak at the end of the level.
Waves of enemies can appear in exactly the same way every time the player plays (a scripted wave) or they may vary according to an algorithm that you design. Scripted waves of enemies have appeared in games ranging from the original Space Invaders to Max Payne and many others. In these types of games, you simply build the size and composition of the wave into the level however you wish.
If you want waves of enemies to arrive according to an algorithm rather than in a fixed progression, choose particular locations in the level (or time intervals in the game) and a pool of enemy types from which to draw the enemies who will appear. Design your algorithm to select a number of enemies consistent with the level of the game and the difficulty setting, if there is one. You can implement a simple form of dynamic difficulty adjustment by having your algorithm check the amount of avatar lives or health points remaining; if the number is low, generate fewer enemies or weaker ones.
In many games, a large enemy, or big boss, significantly harder to fight than any of the previously encountered enemies, guards the end of a group of themed levels. Defeating the boss takes the player to a new set of themed levels. Boss characters
often can't be hurt by normal methods; damaging them may require special weapons, a special attack method, or special timing. For example, the Piranha Plants in Super Mario Sunshine are invincible until they open their mouths.
The boss character's appearance and actions complement the theme of the set of levels it guards. Sometimes the boss character is simply a much larger version of another character that the player already defeated. This enhances the gameplay by allowing the player to predict some of the boss's behavior and gives him a small advantage in knowing what to expect.
Games with a serious storyline aren't suited for such an unsubtle set of themed levels, but even so, the themed level and big boss are mainstays of action games. Virtually every level-based, action-based game today uses a succession of levels increasing in difficulty, culminating in a climactic defeat of a big boss.
To break up the predictability of predefined waves of enemies, many games insert a randomly generated wildcard enemy to provide a fresh challenge. Wildcard enemies, unlike level bosses, normally attack at the same time as normal enemies and behave in unexpected ways.
The Atari game Asteroids, for instance, offers predictable waves of enemies (in this case drifting rocks; although they do not shoot, they are still an active danger), but at random times during the waves, a UFO appears and follows an unpredictable path, shooting at the player. The game awards extra points for shooting the UFO, but trying to do this tends to distract the player and cause her to make mistakes: a perfect example of risk and reward in an action game.
MONSTER GENERATORS AND SPAWN POINTS
Many action games include a feature that causes new enemies to suddenly appear in the game world. If the enemies appear from a visible object that the player can destroy, that object is a monster generator. If the enemies appear seemingly out of thin air at a particular location, that location is a spawn point. The game Doom uses spawn points: Monsters suddenly appear, each in a flash of green light, and there is nothing a player can do to prevent it. In Gauntlet, on the other hand, monster generators are machines visible in the game world. If the player destroys a monster generator, no more monsters come out of it. As a result, the player has a choice of strategies: fight the monsters or destroy the generator. The strategy with the lowest risk involves destroying the monster generators before they can spawn too many monsters, but players aiming for the highest score may delay destroying the monster generator until they boost their scores sufficiently by killing enough monsters. This choice makes the game more interesting.
Monster generators and spawn points may create either a limited or an unlimited number of enemies. If you design a spawn point that produces an unlimited
number of enemies, the player can never defeat them all and must meet some other condition (such as finding the level exit) to make the level end. If you use a monster generator instead, even if it generates enemies indefinitely, the player can ultimately get rid of them all by destroying the generator itself. A monster generator or spawn point might produce only one type of enemy, or it might offer a range of different foes. You should adjust the strength of the enemies that it generates based on the difficulty level of the game.
