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The Sentinel (computer game)
Encyclopedia
The Sentinel is a computer game created by Geoff Crammond
, published by Firebird in 1986 for the BBC Micro
and converted to the C64
(by Crammond himself), Amstrad CPC
(with a cross-compiler written by Crammond), ZX Spectrum
(by Mike Follin
), Atari ST
, Amiga
(both by Steve Bak) and PC
(by Mark Roll). It was released in the US as The Sentry. It was among the first games to feature solid-filled 3D computer graphics
on home computers. While it ran acceptably fast on 16-bit computers, it was slow on 8-bit machines such as the C64, where the next view took up to three seconds to be precomputed. Despite this, the game retained a dedicated base of fans, some of whom were able to modify their computers to enjoy it better (for example, by using a CMD
SuperCPU
in a standard 1-MHz 6502
Commodore 64 to achieve CPU clock speeds of 20 MHz).
The game itself can be best described as an "energy management
" game. It has a first person point of view and features ten thousand playfields. Its uniqueness caused it to be labelled "the first virtual reality
game".
The PC port supported VGA graphics and incremental lighting. This version, however, has very poor audio capabilities, since it can only use the PC speaker
. The Amiga port features a sampled soundtrack by David Whittaker.
A preview of a nonexistent sequel called Monolith appeared in 1995 in the Italian video game magazine The Games Machine
as an April Fool's Day prank. In 1998, the real sequel called Sentinel Returns
was released for the PC and PlayStation
; a freeware
unofficial Sentinel clone called Sentry was also made available for PC the same year. In 2006, two more unofficial clones (Zenith and Sentinel) were released.
In The Sentinel, the player takes the role of a Synthoid (called just "robot" in the US version), a telepathic
robot
who has to take control of a number of surreal, checkered landscapes of hills and valleys, by climbing from the lowest spot, where the hunt begins, to the highest platform, over which the Sentinel looms.
The Synthoid itself cannot move across the level; instead it can look around, accumulate energy by absorbing the objects that are scattered across the landscape, create stacks of boulder
s, generate inert Synthoid shells and transfer its consciousness
from one of these clones to another.
List of executable actions:
Controlling Synthoids that are standing at a higher level is fundamental to the game, because only the objects which occupy a visible square can be interacted with (the player may absorb or create objects on a boulder if the sides can be seen). While doing so, the player must watch for the rotation of the Sentinel and be careful not to stand in an area which the Sentinel can see, or else it will start absorbing energy from the Synthoid, and when the energy is gone, the game is over.
Height is gained by placing a boulder on any visible square, and putting a Synthoid on the boulder. The player may then transfer consciousness to the new Synthoid, and absorb the old one. Stacks of boulders of any height may be created, if the player has enough energy. In order to absorb the Sentinel, the player must create a stack of boulders of sufficient height that the Synthoid on top can look down on the Sentinel's platform. When the Sentinel has been absorbed, the player may no longer absorb any energy from the landscape, although objects may be created as normal.
In later levels, the Sentinel is assisted by a number of Sentries. They behave exactly like the Sentinel, but absorbing them is not necessary to complete the level. Unlike the Sentinel, the Sentries do not stand on a platform but on ordinary squares. Attention must also be paid to nearby trees: if the Sentinel or Sentry cannot see the square the Synthoid is standing on, but its head is visible and there are trees in the vicinity, it may transform one of them into a Meanie, which will force the Synthoid to hyperspace and lose 3 units of energy. If the Meanie itself cannot see the player's square after a full rotation, it will turn back into a tree and the Sentinel or Sentry will resume rotation.
The rotation of the Sentinel and the Sentries is slow and predictable. However, if there are many Sentries, there will be few safe locations anywhere on the landscape. If either the Sentinel or the Sentries come across a source of energy (boulders or a synthoid), their rotation stops while they absorb the energy, one unit at a time. Meanwhile, to keep the total energy of the landscape constant, a tree is created randomly on the landscape for each absorbed unit of energy.
List of objects that can exist in the Sentinel world:
A level is won by absorbing the Sentinel off its platform, creating a new Synthoid in the place of the Sentinel, transferring the consciousness to it and hyperspacing to a new level. When entering a new level, its number and an 8-digit code are displayed on the screen: these should be noted by the player, as entering them correctly is the only way to access that level again.
The number of levels that are skipped between two that are played depends on the amount of energy the player has accumulated when he jumps into hyperspace: the more energy he has, the more levels he will skip. Sometimes (depending on the individual skill of the player) it is necessary to replay a level in order to win it with less energy than the last time, because the difficulty of the levels is not incremental and one of them may be just too hard to complete.
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. What looks like an artistic effect, is actually a critical feature of the rendering engine.
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, presumably the landscape's 4-digit number (since there are 10 unique digits and 10^4 = 10,000).
magazine, describing it as an exceptional piece of software in a class of its own, and refusing to give it a numbered rating as a result.
The ZX Spectrum version received a CRASH
Smash award, the magazine praising its originality, atmosphere and tension. It was also placed at number 7 in the Your Sinclair
official top 100.
Geoff Crammond
Geoff Crammond is a computer game designer and programmer who specialises in motor racing games. A former defense industry systems engineer, he claims to have had little interest in motor racing before programming his first racing game back in 1984, but he holds a physics degree, which may explain...
, published by Firebird in 1986 for the BBC Micro
BBC Micro
The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation...
and converted to the C64
C64
C64 or C-64 may refer to :* Commodore 64, the most successful home computer of the 1980s* Epson Stylus C64, an Inkjet Printer from Epson* C-64 , a road in the United States of America* C-64 Norseman, an aircraft...
(by Crammond himself), Amstrad CPC
Amstrad CPC
The Amstrad CPC is a series of 8-bit home computers produced by Amstrad between 1984 and 1990. It was designed to compete in the mid-1980s home computer market dominated by the Commodore 64 and the Sinclair ZX Spectrum, where it successfully established itself primarily in the United Kingdom,...
(with a cross-compiler written by Crammond), ZX Spectrum
ZX Spectrum
The ZX Spectrum is an 8-bit personal home computer released in the United Kingdom in 1982 by Sinclair Research Ltd...
(by Mike Follin
Mike Follin
The Revd. Michael Stuart Follin was, until the late 1990s, a computer game programmer. Amongst the companies at which he worked was Software Creations, where he worked on games for the ZX Spectrum, including highly rated arcade conversions of Bubble Bobble and Ghouls 'n Ghosts, as well as the...
), Atari ST
Atari ST
The Atari ST is a home/personal computer that was released by Atari Corporation in 1985 and commercially available from that summer into the early 1990s. The "ST" officially stands for "Sixteen/Thirty-two", which referred to the Motorola 68000's 16-bit external bus and 32-bit internals...
, Amiga
Amiga
The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...
(both by Steve Bak) and PC
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
(by Mark Roll). It was released in the US as The Sentry. It was among the first games to feature solid-filled 3D computer graphics
3D computer graphics
3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...
on home computers. While it ran acceptably fast on 16-bit computers, it was slow on 8-bit machines such as the C64, where the next view took up to three seconds to be precomputed. Despite this, the game retained a dedicated base of fans, some of whom were able to modify their computers to enjoy it better (for example, by using a CMD
Creative Micro Designs
Creative Micro Designs is a computer technologies company which today sells PCs and related equipment, but which started out in 1987 selling self-designed firmware updates and hardware for the Commodore 64 and C128 8-bit home/personal computers....
SuperCPU
SuperCPU
The SuperCPU was developed by Creative Micro Designs, Inc. It is a processor upgrade for the Commodore 64 and Commodore 128 personal computer platforms. The SuperCPU uses the W65C816S 8/16 bit microprocessor. The unit can have up to 16 MB RAM installed. The unit sported a "Turbo" switch which, when...
in a standard 1-MHz 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
Commodore 64 to achieve CPU clock speeds of 20 MHz).
The game itself can be best described as an "energy management
Resource management
In organizational studies, resource management is the efficient and effective deployment of an organization's resources when they are needed. Such resources may include financial resources, inventory, human skills, production resources, or information technology...
" game. It has a first person point of view and features ten thousand playfields. Its uniqueness caused it to be labelled "the first virtual reality
Virtual reality
Virtual reality , also known as virtuality, is a term that applies to computer-simulated environments that can simulate physical presence in places in the real world, as well as in imaginary worlds...
game".
The PC port supported VGA graphics and incremental lighting. This version, however, has very poor audio capabilities, since it can only use the PC speaker
PC speaker
A PC speaker is a loudspeaker, built into some IBM PC compatible computers. The first IBM Personal Computer, model 5150, employed a standard 2.25 inch magnetic driven speaker. More recent computers use a piezoelectric speaker instead. The speaker allows software and firmware to provide...
. The Amiga port features a sampled soundtrack by David Whittaker.
A preview of a nonexistent sequel called Monolith appeared in 1995 in the Italian video game magazine The Games Machine
The Games Machine
The Games Machine was a video game magazine that was published from 1987 until 1990 in the United Kingdom by Newsfield, which also published CRASH, Zzap!64, Amtix! and other magazines.-History:...
as an April Fool's Day prank. In 1998, the real sequel called Sentinel Returns
Sentinel Returns
Sentinel Returns is a video game developed by Hookstone, produced by No-Name Games and published by Sony in 1998, for PC and PlayStation...
was released for the PC and PlayStation
PlayStation
The is a 32-bit fifth-generation video game console first released by Sony Computer Entertainment in Japan on December 3, .The PlayStation was the first of the PlayStation series of consoles and handheld game devices. The PlayStation 2 was the console's successor in 2000...
; a freeware
Freeware
Freeware is computer software that is available for use at no cost or for an optional fee, but usually with one or more restricted usage rights. Freeware is in contrast to commercial software, which is typically sold for profit, but might be distributed for a business or commercial purpose in the...
unofficial Sentinel clone called Sentry was also made available for PC the same year. In 2006, two more unofficial clones (Zenith and Sentinel) were released.
Game mechanics
Telepathy
Telepathy , is the induction of mental states from one mind to another. The term was coined in 1882 by the classical scholar Fredric W. H. Myers, a founder of the Society for Psychical Research, and has remained more popular than the more-correct expression thought-transference...
robot
Robot
A robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...
who has to take control of a number of surreal, checkered landscapes of hills and valleys, by climbing from the lowest spot, where the hunt begins, to the highest platform, over which the Sentinel looms.
The Synthoid itself cannot move across the level; instead it can look around, accumulate energy by absorbing the objects that are scattered across the landscape, create stacks of boulder
Boulder
In geology, a boulder is a rock with grain size of usually no less than 256 mm diameter. While a boulder may be small enough to move or roll manually, others are extremely massive....
s, generate inert Synthoid shells and transfer its consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...
from one of these clones to another.
List of executable actions:
- Look around by moving the pointerCursor (computers)In computing, a cursor is an indicator used to show the position on a computer monitor or other display device that will respond to input from a text input or pointing device. The flashing text cursor may be referred to as a caret in some cases...
on the screen (press S=Left, D=Right, L=Up, <=Down) - Toggle cursor on|off. You turn faster with it off (press SPACE)
- Absorb an object to gain its energy (point a square where an object is present and press A)
- Create trees in empty squares (point the desired square and press T)
- Create one or more boulders in empty squares (point the desired square and press B)
- Absorb one or more boulders (point the boulder on the bottom of the stack and press A)
- Create a new Synthoid shell in an empty square or on a boulder (point the desired place and press R)
- Transfer consciousness to another Synthoid (point the Synthoid and press Q)
- Hyperspace to a random part of the level at the expense of 3 units of energy (press H) (note that the player may not hyperspace to a higher square; only one of equal or lower height). Also used to hyperspace to the next level when the player has reached the Sentinel's platform.
- Rotate 180 degrees (press U)
Controlling Synthoids that are standing at a higher level is fundamental to the game, because only the objects which occupy a visible square can be interacted with (the player may absorb or create objects on a boulder if the sides can be seen). While doing so, the player must watch for the rotation of the Sentinel and be careful not to stand in an area which the Sentinel can see, or else it will start absorbing energy from the Synthoid, and when the energy is gone, the game is over.
Height is gained by placing a boulder on any visible square, and putting a Synthoid on the boulder. The player may then transfer consciousness to the new Synthoid, and absorb the old one. Stacks of boulders of any height may be created, if the player has enough energy. In order to absorb the Sentinel, the player must create a stack of boulders of sufficient height that the Synthoid on top can look down on the Sentinel's platform. When the Sentinel has been absorbed, the player may no longer absorb any energy from the landscape, although objects may be created as normal.
In later levels, the Sentinel is assisted by a number of Sentries. They behave exactly like the Sentinel, but absorbing them is not necessary to complete the level. Unlike the Sentinel, the Sentries do not stand on a platform but on ordinary squares. Attention must also be paid to nearby trees: if the Sentinel or Sentry cannot see the square the Synthoid is standing on, but its head is visible and there are trees in the vicinity, it may transform one of them into a Meanie, which will force the Synthoid to hyperspace and lose 3 units of energy. If the Meanie itself cannot see the player's square after a full rotation, it will turn back into a tree and the Sentinel or Sentry will resume rotation.
The rotation of the Sentinel and the Sentries is slow and predictable. However, if there are many Sentries, there will be few safe locations anywhere on the landscape. If either the Sentinel or the Sentries come across a source of energy (boulders or a synthoid), their rotation stops while they absorb the energy, one unit at a time. Meanwhile, to keep the total energy of the landscape constant, a tree is created randomly on the landscape for each absorbed unit of energy.
List of objects that can exist in the Sentinel world:
- Tree (1 unit of energy)
- Meanie (1 unit of energy)
- Boulder (2 units of energy)
- Synthoid (3 units of energy)
- Sentry (3 units of energy)
- Sentinel (4 units of energy)
A level is won by absorbing the Sentinel off its platform, creating a new Synthoid in the place of the Sentinel, transferring the consciousness to it and hyperspacing to a new level. When entering a new level, its number and an 8-digit code are displayed on the screen: these should be noted by the player, as entering them correctly is the only way to access that level again.
The number of levels that are skipped between two that are played depends on the amount of energy the player has accumulated when he jumps into hyperspace: the more energy he has, the more levels he will skip. Sometimes (depending on the individual skill of the player) it is necessary to replay a level in order to win it with less energy than the last time, because the difficulty of the levels is not incremental and one of them may be just too hard to complete.
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. What looks like an artistic effect, is actually a critical feature of the rendering engine.
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
Procedural generation
Procedural generation is a widely used term in the production of media; it refers to content generated algorithmically rather than manually. Often, this means creating content on the fly rather than prior to distribution...
algorithm is used which generates each landscape from a small data packet, presumably the landscape's 4-digit number (since there are 10 unique digits and 10^4 = 10,000).
Critical reaction
The C64 version received a Gold Medal award by Zzap!64Zzap!64
Zzap!64 was a computer games magazine covering games on the Commodore International series of computers, especially the Commodore 64 . It was published in the UK by Newsfield Publications Ltd and later by Europress Impact....
magazine, describing it as an exceptional piece of software in a class of its own, and refusing to give it a numbered rating as a result.
The ZX Spectrum version received a CRASH
CRASH (magazine)
Crash was a magazine dedicated to the ZX Spectrum home computer. It was published from 1984 to 1991 by Newsfield Publications Ltd until their liquidation, and then until 1992 by Europress.-Development:...
Smash award, the magazine praising its originality, atmosphere and tension. It was also placed at number 7 in the Your Sinclair
Your Sinclair
Your Sinclair or YS as it was commonly abbreviated, was a British computer magazine for the Sinclair range of computers, mainly the ZX Spectrum.-History:...
official top 100.