But Which 2D?
Not all 2D games are the same. Two major styles have developed: “classic” 2D, which is a straight top-down (chess/checkers) or side-on (Sonic games) view, or isometric 2D, which tries to fake 3D with an isometric projection at a pre-set angle. Before making the full jump to 3D, many genres made a move from classic 2D to isometric 2D as an intermediary step.
For example, the original Civilization had a traditional top-down grid view while Civ 2 had a three-quarters isometric view. While this new perspective gave the game world a more life-like appearance, the change did come at a cost to the user's game experience. Namely, distances are much more difficult to judge on an isometric grid as the east-west axis takes up twice as many pixels as the north-south axis.
To solve this problem, for Civ 4, our 3D perspective actually hearkened back to the original game as we showed the game's grid straight ahead and not at an angle. The easier the players perceive the grid through the graphics, the better they can “see” their possible decisions.
It is significant that Advance Wars: Days of Ruin (DS), the latest version in this long-running series, has maintained the traditional chess-board view, keeping the player focused squarely on the core gameplay. The “chunky” unit art familiar to the series is a great example of an artistic style which flows from the limitations of the game's presentation.
In contrast, a game heavily influenced by the Advance Wars series -- Age of Empires: The Age of Kings (DS) -- chose to move the same game mechanics into an isometric 2D world. The transition was not altogether successful.
Not only was the immediacy of the grid harder to follow, but because units extended beyond the edges of their tiles, selecting units and locations became a significant problem when groups of units overlapped one another. Thus, tile-based games tend to be more successful when a top-down view is adopted.
Graphics are not Gameplay
3D graphics are not the same things as 3D gameplay. For example, two sci-fi RTS games -- Homeworld and Sins of a Solar Empire -- use very similar 3D engines to recreate the vast scale and special effects of deep space combat.
However, they do not share core gameplay, as Homeworld is a "true" 3D game, meaning that ships could be moved freely along the z-axis, while Sins actually has 2D gameplay, as the game is played on a single, flat plane, meaning that ships cannot fly above or below each other.
In fact, the game could have been implemented with a 2D engine; using 3D was a secondary choice to enable smooth zooming and to evoke the "feel" of outer space. The team's decision to adopt 2D gameplay saved Sins from the interface complications of Homeworld, which required two or three separate clicks to give units a destination in all three dimensions.
Many other example of hybrids exist, where games use 3D graphics to render essentially flat 2D gameplay. Super Smash Bros. Brawl, for example, is fought on a single, vertical plane that uses the 3D engine for the all-important animations and fluid background environments.
Cliff Bleszinski has described the gameplay of Gears of War as a horizontal version of the classic 2D platform Bionic Commando. Instead of using the grappling hook to ascend from platform to platform, Gears players "jump" from cover point to cover point along a horizontal plane.
Essentially, most games can be divided into three play mechanic categories which are related to but semi-independent from the graphics:
Good rules-of-thumb exist for each of these categories.
Real-world games essentially require 3D graphics. Of course, the term "real" is not meant to be taken literally. The gun from Portal is not real, but the user enjoys playing with it because of the expectation that its unique behavior exists in harmony with the physics and gravity of our own world.
The easiest way to guarantee that the player bring along assumptions from the real world is to immerse them in a 3D environment that looks, behaves, and feels real. These environments are the equivalent of what-you-see-is-what-you-get for games.
On the other hand, tile-based games usually work best as top-down 2D games, with little separating the player from the core game mechanics. For single-plane games, the choice comes down to largely one of aesthetics and technology.
Can the game's platform support 3D graphics smoothly? Does 3D provide an advantage, from either shared animations or dynamic effects or general flexibility, that makes the technology worthwhile?
All in all, 2D is an underrated style that is often unfairly ignored as an old technology. Developers should not underestimate the advantages of avoiding the technical overhead of maintaining a bulky 3D engine and asset pipeline.
Furthermore, well-made 2D graphics never really go obsolete. Sulka Haro, lead designer of Habbo Hotel, likes to point out that their retro 2D style looks just as good today as when the game launched eight years ago. If they had used 3D, Habbo would probably be on its second or third engine by now.
Once a 2D engine is up and running, the artists can focus on simply improving the game's look piece by piece. If 2D helps clarify and communicate the underlying game mechanic, then all the better.
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