An action game is not strictly necessary to utilize a 3D engine. There are a lot of wonderful games based on 2D graphics or isometric perspective. Astronaut is Gone is designed with flat graphics interpretation in mind as well. All elements such as the Unit, enemies, rocks, and meteors can be based on sprites. The movement mechanics in basic gaming modes do not contradict with that. The player does not need to turn the virtual camera because the object may have only one side. More importantly, the game world itself is not flat; each element features three coordinates. The game's universe looks like a long corridor where different objects are suspended on invisible strings. The corridor's walls, floor, and ceiling frame boundaries of the space that the player may reach. If the player gets close to them, he gets an alert, You are leaving the path, and the Unit stops. The protagonist can move in two planes: in the longitudinal one by traveling forward and backward and in the corridor's cross-section one by maneuvering in any direction within its limits. When he reaches an object inside the game world, he gets a chance to interact with it. The simple rules of the linear perspective help to determine sizes and coordinates of elements depending on their distance to the viewer. This lets him turn a 3D-described scenery into a natural flat image. Such a concept is very close to classical one-screen arcades; the only exception is the format of levels that are three-dimensional. Nevertheless, the final image that you would work with is flat. Therefore, it is not hard to actualize the idea in 2D graphics, especially if your favorite SDK is not supported by 3D.

Schematic image of a level's corridor structures

However, an additional dimension may offer many new opportunities as creative as the gameplay ones. First of all, the action genre is generally associated exactly with 3D graphics. This is like a technical vanguard that requires only advanced solutions since 3D engine gives the game the correct touch needed. Then, it lets us organize more spectacular and natural scenes on the screen. The objects may rotate and interact in an interesting manner, the virtual camera can be used for more dramatic angles, and so on. Some moments can be simply impossible or very hard to recreate via 2D graphics. There is a notable story about the animation studio Pixar, which tried to impress Disney with their approach on 3D graphics animation. They simply showed dynamic shadows using venetian blinds falling on an environment across a scene, a job which could not be done by traditional animation. That demonstration was worth a thousand words. You do not need shadow tricks to impress the audience; a character that can be seen from different angles dynamically in real time is already a good start. In Astronaut is Gone, a rock rotating in three-dimensions and demonstrating all elements of its geometry looks much better than a sprite rotating only clockwise or counter-clockwise.

The technology rewards you with more flexibility and freedom, thus letting you control any scene more boldly. In some cases, it also makes the process of porting the game to another platform, such as computers, more comfortable. 3D graphics are vector-based; so, they can be easily scaled by solving the problems about different screen size, aspect ratios, and so on. There would be some issues with textures and the number of polygons, but they can be managed.

Of course, on the other hand, 3D engines may require some additional knowledge. Most importantly, the assets for them are more complex. They need special efforts on modeling, animation, texture creation, and so on. You must be ready for such a challenge. It is also a question of appropriate optimization. Modern mobile devices are pretty powerful, and nevertheless, they are not as talented as desktop computers or gaming consoles yet. You can prove it yourself by looking at compatible tables of GPU tests on special sites such as GFXBench (http://gfxbench.com), which shows that the average desktop video cards have better results by several digits than video chips of popular smartphones. So, the visual complexity can be an issue (keep in mind that a game should be oriented on the average models of mobile devices that the majority of consumers have now rather than on fresh, new devices that are more powerful but not common yet). Usually, it is not a problem of a single object made up of a large number of polygons. But the situation is more complex; materials, textures with alpha-channels, post-processing operations, and the overall number of objects in the scene may overload the GPU, thus decreasing the FPS dramatically.

Additional calculations such as AI or other complex game logics can take their portion of resources. So, it is always about appropriate equilibrium, and usually, the situation is different for each project because the conditions and number of variables can be different in each particular case. It also depends on the gaming context. Sometimes, it is a task to find out the right harmony. For example, most of your scenes are calm. There are few objects on the screen. The high frame rate tempts you to think about extra tris for the models, but then, there is a battle. Suddenly, a big combat begins. A lot of enemies are around; fragments and particles are flying in the air. The system may be stuck. To prevent such bottlenecks, objects need to have less complex visual multiplicity, providing some fund for the new elements on the screen, or some story tricks can be used. The battle plays its part; all irrelevant elements as props can be removed from the scene, but the level design by itself may be simplified for performance sake. In addition, the structure of the battles can be thought up in a way in which it would require fewer resources. The optimization is a very complex process involving not only the geometry, but also all the aspects of the game. Besides the obvious ways, always try to invent a creative way by looking at the problem from another angle. For example, you can turn the disadvantages into a special graphic style by turning the simpler and optimized type of graphics into a visual language that a game can use when talking with its players.