The Sentinel (video Game) - Limitations of The Game Engine

Limitations of The Game Engine

The game engine ingeniously gives the player the illusion of playing in a completely solid 3D environment with polygon-based graphics, even on very basic platforms. This is achieved by having the player character immobile in one place. The player may look around in all directions and see a scrolling rendered view from the current location. As the player transfers consciousness to another Synthoid, the game pauses momentarily as the CPU renders another complete view from the player's next location. This then becomes the view the player sees. Objects such as boulders, trees, Synthoids and even the Sentinel are then rendered on top of this background. This means that the player may "move" freely around the landscape, but the landscape need not be rendered in realtime.

Another point to note is that 3D views of landscapes cannot usually be rendered once and then rotated by means of scrolling. Simply expressed, the perspective transformation prevents this by scaling objects proportionally to (1/z), where z is the distance from the eye point to the object along an axis perpendicular to the screen. The problem with this is that as the viewer rotates, objects that are a constant distance from the eye-point will not maintain a constant distance along the z axis. The effect is that -using a mathematically correct perspective transformation- objects will get larger and smaller as the viewer rotates. Obviously this cannot be allowed to occur for a rendered-once scene, and so it can be deduced that a modified perspective transformation was used in the game. It is highly probable that the transformation used in the game scaled proportionally to (1/distance), where distance is measured in 3-dimensional space, thus eliminating the change in size of objects due to rotation, and thus facilitating rotation-by-scrolling. One down-side of using such a modified transformation is that the geometry of the scene becomes warped slightly, giving it something of a fish-eye appearance. This problem was clearly apparent during gameplay, as nominally straight lines became curved. Indeed, the image on the front of the game packaging itself show this warping effect in action.

It is clear that the memory limitations of the 8-bit microcomputers would preclude 10,000 landscapes being stored individually in the computer's memory. Instead, a procedural generation algorithm is used which generates each landscape from a small data packet: the 8-digit number given at the completion of a previous landscape. The number of landscapes was quite arbitrary given the generation algorithm, and was chosen as a balance between giving the player good value while not overwhelming them with an unreachable goal. Not all the landscapes were actually tested, but it was always possible to skip a difficult level by completing an earlier one with a different amount of energy.