Regarding the concept of a game, I would like to describe a less-common model of using 3D content in AR. As a rule, 3D objects are shown on or above some physical surfaces; figuratively speaking, such AR is convex. However, it is interesting to try to create a concave reality by creating an illusion of a hole in a surface. Fiducials create a boundary for such a virtual recess and players have the chance to look inside the rabbit hole. The idea is manifested in the best way when markers are placed on a wall, since the game creates an AR window where some virtual scene is shown. The illusion is pretty smooth because the virtual image has a real-time response to change the player's point of view and position is space of a camera; there are parallax, real time linear perspective distortions, and so on.

Game content is perceived as a truly and natural addition to the real world. It is like having a window into new dimension in your own room. Of course, a special step is needed to process the final image because a mask should be used to cut off all the faces of the 3D model that have to be invisible, but that should not be a big problem, as long as the marker can provide the exact coordinates of the window.

The following image represent a schematic image of the AR window:

In an ideal scenario, the image recognition system in the game is smart and capable enough to rotate any object in an AR window, that has contrast cadres; so players may use framed pictures on walls or computer monitors. Since rotating objects can be considered unrealizable for some reasons graphical markers can be used instead. It is more efficient to use something similar to the ARtag system when several fiducials are used at once, as such an approach will add some more reliability. For example, the game will continue operating in conditions where only some of the markers are within camera range.

The game scene looks like a diorama: a window-frustum shaped skybox with some decorations inside and of course, a few characters. It is good to make a shooter game. Several enemies are seen behind the AR window; players need to shoot them using their mobile device as a gun.. In the meantime, the enemies have guns, so they can hit the player as well. This means the player has to use strafing, and hide behind the window ledges in the game scene. The ideal plot for using such game mechanics is a story about a fortification defended against hordes of enemies. The AR window demonstrates some avenues to approach a pillbox, it stands on a hill, but a player is a shooter in this improvised tower. There are dozens of angry enemies downhill; who are trying to reach the pillbox. The player has to defeat those enemy waves.

At harder levels, some additional elements can be introduced; enemies may begin to use tanks or other types of armored protagonists may emerge. It would be spectacular to use particle effects to illustrate hits in the pitfalls; some splinters may shoot out the AR window.

The following are the screenshots of various game situations:

This particular game can also have an alternate mode called Sniper duel, for example. The player is a sniper positioned in a building; he has one window for shooting. In front of him is another building with a lot of windows and several holes in the walls. That is the place where the enemy sniper is hidden (maybe a couple of them); the enemy constantly changes his position. The player should be very attentive to be able to catch the small signs that the opponent is about to shoot them. At the same time, he should not be an easy target, his head sticking out a window in the game scene; it is more sane to protrude a little from the corner of the window in the scene. That is a slow, but very alarming type of a shooter game.