World of Goo title screen['The Interactive Palette' is a biweekly GameSetWatch column by Gregory Weir that examines the tools and techniques of the digital games trade with a focus on games as art, using a single game as an example.]

Musical composition has a technique called theme and variation, where a musical theme — a sequence of notes, chords, or rhythms — is repeated throughout a piece with variations, inversions, and embellishments. Variations serve both as a demonstration of the composer's prowess and as an exploration in depth of a specific theme. The most famous example of this technique among layfolk is probably Bach's Goldberg Variations.

We see a similar technique used in video games. Many games, especially those focused on puzzles, start with a simple concept and complicate it by embellishing and expanding that initial idea. Lemmings is a classic example. The game begins with simple introductions of the various "skills" that the player can assign to the lemmings, then gradually adds hazards and obstacles until the player is navigating truly complex levels.

This technique of increasingly complex variations is useful enough to be presented as a design pattern, a specific, repeatable approach to a commonly encountered situation. As Staffan Björk and Jussi Holopainen have pointed out [PDF], a creative task like game design is less-suited to the solution-based approach used in other software design patterns. Instead, game design patterns serve as a template with certain advantages and consequences. By looking at 2D Boy's recent (and excellent) game World of Goo, we can see how this pattern works in practice.

World of Goo tower Going Up

The "theme" for World of Goo's variations comes from an earlier game by the same designer, Tower of Goo. The concept is simple: create a tower from a lattice of "goo." Build it as tall as possible, dealing with realistic physics simulation of the springy, wobbly goo.

World of Goo immediately elaborates on this, and be warned, spoilers are coming. The first variation is the addition of a specific goal: the pipe at the top of the level that the goo is trying to reach. Starting from this basic framework — the player has goo, and must take it to the pipe — the levels begin expanding on it by making the original concept trickier. What if the pipe is across a gap, necessitating a bridge? What if the player doesn't have enough goo to reach the pipe, requiring that she awaken sleeping goo first?

This is the first step to variation: adding complications. By keeping the gameplay mechanics the same, but introducting additional constraints on the player, the depth and versatility of the mechanics are revealed. This is essentially an exercise in making the game more difficult, but if the progression is properly balanced, then players will have mastered the necessary skills for previous levels and won't feel overwhelmed.

World of Goo beauty balls Some Balls Are Prettier Than Other Balls

World of Goo doesn't leave it at that, though. Early on in the game and throughout the chapters, the player is introduced to new types of goo, from the green goo that can be reused to the red goo, which is flammable. These new goos provide the player with new options. Floppy goo bridges can be supported by balloons, and spiny goo lets structures cling to the environment for added stability... or transportation.

By elaborating on the starting theme, Goo's variations let the player incorporate new actions into the existing game world and mechanics. In apparent opposition to adding complications, adding new options typically makes the player's job easier. Insurmountable obstacles can now be surmounted, and previously tricky tasks are made easier. However, the player now must keep more things in her head at once. If new elements are introduced too quickly, the player won't have had time to fully understand and internalize the last addition before the next one comes along.

Complication and elaboration act in tension here. Satisfying gameplay flow can be maintained by slowly ramping up the difficulty with complication until it's quite tricky, then tossing in a new elaboration on the basic concept. Because the new possibility is unfamiliar, the difficulty should be reduced to accomodate the player's learning before beginning the slow ramp up. By progressing in this way, the game can present levels that would have been utterly intimidating at the start of play, and have the player regard them as tricky but approachable challenges.

World of Goo desktop 20% More Infinite in All Directions

World of Goo also occasionally throws a curve ball. In some levels, there's not even an exit pipe. Sometimes the goal is not to build a tower up, but to knock it down. Sometimes goo is only tangentially involved, but the basic control scheme and concepts are the same. In these instances, the basic premise of the game has been subverted, but it's still recognizably the same game.

Inversion or subversion is the most extreme form of variation. It may seem strange to throw away the very basis of the game, but often the fluff surrounding the core gameplay can be just as interesting as the core gameplay itself... at least for a little while. Turning the game upside down like this can give the player a much-needed break from similar levels or give her new insight about the primary gameplay mechanics through their absence.

In World of Goo's structure of variation, the initial premise is alternately complicated by adding new obstacles and elaborated by providing the player with new abilities through goo. This generates a classic stair-step or zig-zag difficulty curve, which keeps the player both challenged and interested. Periodically, to break up the steady climb, a level is thrown in which subverts the theme by replacing the basic premise. The result is a gameplay experience which encourages the player to keep playing to see what new goo or level structure is coming next, and discourages boredom by preventing the player from anticipating the next twist.

World of Goo telescope End of the World

This design pattern, variation on a theme, doesn't automatically work. World of Goo has a simple-yet-deep premise, which makes it possible to generate a wide array of variations. The technique requires a "theme," or core gameplay mechanic, which is both versatile and emergent. That is, it must apply to a range of situations, and it must have interesting properties which are natural consequences of the basic rules.

Myst-style gameplay wouldn't be able to take advantage of this pattern because each puzzle or puzzle group's solution is discrete, not applying to the next obstacle. Diablo's gameplay, as another example, is not emergent; there are few interesting consequences of that game's simple melee/ranged combat system.

World of Goo is perfect for this technique, however. Its premise of building to reach a goal is versatile because it can apply to building up, down, or sideways, to closing distances and to filling spaces. The premise is emergent thanks in part to its origins in mass-and-spring physics. A long bridge will dip, an imbalanced tower will fall, and a dropped lattice will bounce due to the original physical properties behind the basic concept. This results in interesting consequences of the premise that can be exploited to add complications, elaborations, and subversions.

As a point of comparison, Lemmings offers a similar, and similarly versatile, premise: get a certain number of creatures from a starting location to an exit. The emergence here comes from the creatures' mindless and predictable behavior as well as the destructable terrain of the levels. Complication is provided by walls, pits, gaps, and traps, while elaboration is seen in the wide array of skills available to lemmings.

World of Goo, beyond offering gorgeous graphics and inspired sound design, has a deceptively simple premise that lends itself well to the variation design pattern. The competing forces of complication and elaboration give it a fun and engaging gameplay flow. The same design pattern is applicable to any game premise which can be used in a wide range of situations and has interesting consequences. The technique will lead to a gameplay experience with a unified feel and an engaging difficulty progression. Like with the Goldberg Variations, audiences will keep paying attention to see what happens next.

[Gregory Weir is a writer, game developer, and software programmer. He maintains Ludus Novus, a podcast and accompanying blog dedicated to the art of interaction. He can be reached at Gregory.Weir@gmail.com.]