I’m taking a short break from building the sim. Work is hectic and the next few weekends will be spoken for. To fill in the gaping void I thought I’d do a couple of posts on the more theoretical aspects of building a flight simulator – insofar as I know about them. One thing I do know a reasonable amount about is projectors, so I thought I’d write down a few basic details about how they’re used for flight simulators, what sorts to get and what sorts to avoid, and so on. I was asked by Chris Hoffman at MyCockpit (which, once again, I encourage you to go check out if you’re not already a member) to do something similar as a tutorial over there, so this is my prototype for that.
This will be longer than my usual posts, so be warned.
Update 25/09/10: this post needs a hefty re-edit to correct numerous stylistic, grammatical and presentational issues, and a fair number of typos. I will get around to this in the near future. In the meantime I’ll leave it up, mistakes and all. Reader beware!
Projector fundamentals
The notion of a projector has existed for centuries. The camera obscura, basically a room with no windows and a tiny hole in one wall, usually an outward-facing wall, through which light enters and projects onto the opposite wall – essentially a giant pinhole camera – has existed since the time of the ancient Greeks. The first projector you might recognise in the modern sense was probably the Magic Lantern, invented in the 1660s, and used to project the image of a painting or illustration. While the effect was still little known, magicians used it in their shows to create the impression of spooky apparitions and ghosts. Over time, magic lantern shows came to be popular entertainment.
The modern projector first came into wide use with the advent of motion pictures. It was this use, and the need to make projectors smaller and more reliable, that would eventually lead to the PC projectors we use today. On the way we got overhead projectors (still used in schools and offices today), slide projectors, and home cine projectors. All operate on the same basic principle. A bright light source, these days usually a metal-halide halogen bulb, shines a strong beam of light through a piece of film or other semi-transparent object which carries an image. The image is projected through an adjustable lens used to both magnify the image size and focus it onto a projection surface. A particular variant called the opaque projector is capable of projecting the image of a non-transparent source, using a system of mirrors. This allows images on paper or other opaque backgrounds – and later, video on a TV screen – to be projected.
The fundamental engineering involved in building a projector has changed little since those early days. They have gotten smaller and lighter as the technology for the light source and lens assembly has improved, but the little projector on your desktop shares a common ancestor with the massive movie projectors used in cinemas. All that has really changed is the image source, which has gone from being a painting, to a still image on film, to a moving strip of film, and then through analogue video to digital video, and the emergence of projectors which could be combined with a PC to create projected images on-the-fly and thus make the projector so useful for business presentations and flight simulators alike.
Modern projector types
There are three major types of projector still available to the home and business markets, excluding the overhead projector which, from now on, we’ll discount. They are:
CRT video projectors
Effectively extinct now, apart from very specialist applications, CRT projectors were the first to be able to project video directly, instead of from film. The image source is usually three separate monochrome cathode ray tubes, each showing a portion of the video signal corresponding to either the red, green or blue components of the image. The image from each goes through its own lens, coloured appropriately for the component, and the three images overlap to create a full-colour image on the projection surface. As you can imagine, calibrating and focusing these projectors is a major effort that has to be repeated frequently, and it has all the drawbacks of analogue vs digital video, especially for PC use. CRT projectors are also physically huge. They do generate the largest image you can get from a non-commercial system, however, and unlike digital systems they are not confined to a single ‘native’ resolution.
DLP / LCOS video projectors
An all-digital system, DLP (Digital Light Projection) is a technology you may have seen in the cinema if you’ve watched a film in ‘digital projection’. Home /office versions began to appear a few years ago. DLP is actually a variant of the opaque projector, in that the image source is not placed directly into the light beam. Instead, light reflects from a digital micromirror device (DMD), an array of tiny mirrors which physically move up and down to represent the pixel make-up of the source image. If the mirror is flat, no light is reflected; otherwise, the degree to which it is raised determines the amount of light reflected and thus the brightness of that pixel. This light is then used to produce the projected image.
Like CRT projectors, the DLP image source is monochrome. To produce a colour image, DLP projectors use one of two methods. In single-DLP solutions there is one DMD, and this displays the red, green and blue component images in sequence at a rate 3 times the actual refresh rate of the input. A colour wheel – literally a transparent plastic wheel with three colour segments – spins behind the lens in sync with the sub-frame refresh and tints each sub-frame accordingly. Persistence of vision makes the three monochrome sub-frames merge into one colour frame. In triple-DLP solutions there are 3 DMDs, one for each colour, and their three images are combined optically. This technique tends to be used in very high-end AV projectors and cinema projectors only, leaving most AV users – and thus flight simmers – stuck with single-DLP solutions. The problem with these is something called the rainbow effect, caused by the rapid colour-cycling produced by the colour wheel technique. Some people see very annoying colour edging and banding effects in pictures from these sorts of projectors. Some people can’t see the effect at all, though, so you may wish to try before you buy.
LCOS (Liquid Crystal on Silicon) projectors are similar to DLP projectors in most respects, but instead of a micromirror device they use a type of LCD panel that is constructed on a reflective mirrored substrate. Like DLP, they come in three-panel and single-panel-plus-colour-wheel types, but unlike DLP, the majority of LCOS devices available are three-panel systems, so the rainbow effect is rarely an issue.
LCD video projectors
These projectors use a transmissive LCD panel as an image source. This is the same technology used in your flat-panel monitor or LCD TV at home, but on a much smaller scale. The LCD panel within the projector is comparatively tiny, yet will still have a comparable number of pixels to the giant panel in your TV. The difficulty inherent in reliably producing panels of such high pixel density is what kept LCD projector resolutions low and costs high for so many years. Even today, it’s relatively costly to buy a projector that will display more than 800×600 pixels, which is a resolution you’d laugh at on a PC monitor.
In case you’re thinking ‘my LCD monitor isn’t transparent’, the short answer is that it is; it just has an opaque reflective background panel placed behind it, together with a series of light tubes that act as a light source. An LCD projector simply replaces the tubes with a single, ultra-bright bulb, and removes the opaque background. This is the closest analogue to the old film / slide projectors, using a semi-transparent image source and avoiding the need to use complex arrays of beam splitters. Indeed, some intredpid DIYers have made their own LCD projectors on the cheap from panels removed from LCD monitors and stuffed into a box with a light source and a lens assembly. I wouldn’t recommend this for most sim-builders, unless you really must build absolutely every element of your sim yourself!
LCD projectors suffer from what’s called the screen door effect. This is where the individual pixels, and the microscopic gaps between them, can be seen on the screen because of the huge magnification factor. The larger the image, and the lower the resolution, the worse it gets. For flight sim use, where images can often be pretty large, a higher resolution is thus much better than the 800×600 or even 640×480 offered by many cheap projectors.
Common factors
Whichever projector technology is in use, there are common factors that must be taken into account. First is the lamp. Like any light bulb, this has a finite lifespan. Projector lamps are designed to work hard over many duty cycles, but at a conservative estimate you’ll need to change yours at least every couple of years, less if you use it for several hours a day. Unlike common-or-garden light bulbs, the lamp in a projector is an expensive component to replace, as you’re replacing not just the bulb – which is a highly specialised thing in and of itself – but also the lamp assembly, which is designed to prevent the bulb from exploding, or to contain the fragments if it does explode, which – like regular bulbs – they occasionally do. Some newer projectors are becoming available using LEDs as a light source, which in theory could last the entire lifetime of the projector and longer, but the inherent low brightness of LEDs is a problem, because to get enough lumens (the standard measurement of light intensity) to rival a lamp, you need many dozens of LEDs. At the moment, LED projectors are typically low-cost, low-resolution, low-quality jobs. New, ultra-bright LEDs are starting to solve this problem, and it’s likely the projector lamps for ordinary home AV projectors will disappear in favour of LEDs sometime in the next few years. This will make owing a projector rather more economical, though you can expect to pay through the nose for the early models.
Then there’s the heat problem. The lamp gets hot – very hot indeed – and so all projectors need some form of cooling. This comes in the form of one or more fans. Fans make noise. Noise can spoil your experience. For a quieter experience you need to position your projector where it can suck in plenty of cool air and get rid of the warm air inside without bathing in it. You also need to make sure you clean the air filters, which most modern projectors will remind you to do after so many hours of operation. If dust and dirt clogs the filter, less air gets in and the projector runs hotter. Heat not only causes the fans to go faster and make more noise, it also reduces the operational life of the projector and the bulb.
You also need to keep the lens assembly clean, and this is harder than you might think. At all costs, you want to avoid touching your projector lens. Greasy fingerprints are really hard to get off and tend to leave a greasy patina across the whole lens even if it looks clean to your eye. The projection process will amplify any imperfection to maddening proportions. Use the dust blower included in your projector kit (or go out and buy one) for removing specks of dust, and wipe the lens only if you absolutely have to, with the cloth provided.
All of which means that running projectors is not a zero-friction experience. They need more maintenance than a CRT or a flat-screen TV, and so you need to invest well and be proactive about your maintenance. No-one wants a projector suddenly out of action due to owner negligence.
Projection in flight simulation – a potted history
Projectors have been used in flight simulators almost since the start. The very earliest flight sims had no external visuals – they were instrument trainers, designed to teach pilots how to fly under Instrument Flight Rules (IFR) in the dark or bad weather. Probably the earliest sim was a seat mounted on half a barrel which was pulled from one side to the other by fellow trainees to simulate the experience of banking or pitching. The earliest meaningful sim was the Link Trainer, created in 1929 by Edwin Link, who could reasonably be said to be the first simpit builder. The Link looks rather like one of the child’s rides you used to see outside supermarkets and in parks, a blue box you sit in with stubby wings and an instrument panel which bobs around on a universal joint powered pneumatically by recycled organ bellows. As odd as it looks, though, it was an immense aid to the nascent US Army Air Corps, whose pilots could be trained no other way than on-the-job, leading to no end of nasty prangs and fatal accidents. Later versions of the trainer enclosed the pilot completely, though without an external view at all, to simulate full IFR flight.
By the 1950s, simulators were commonly using a model board for external displays. Computers in these days had no visual display to speak of, and while some simulators used specialist purpose-built electronics which could be compared to computers, simulators were still generally fairly analog. The model board system used a 3-dimensional terrain mockup of the area to be simulated, mounted on a board. A camera was suspended over the board on rails such that it could track across the board in any direction and also move up and down. The view from this camera was then used as the visual display for the simulator. Sometimes this would be projected in front of the sim, thus marking the first use of projection systems in flight simulation.
Early computer-based simulators
Sims powered by computers with a graphical output began to appear in the 1960s. These were top-of-the range dedicated mainframe machines, often racks full of hardware for a single simulator, with dedicated graphics processing machines alongside those used to calculate the flight parameters. The graphics were nothing special to begin with – little better than wireframe – but soon advanced well beyond what the home user of the 80’s could expect to see on their microcomputers. Ironically, today’s most modern sims tend to have a visual system which produces less detail than a high-end PC running FSX, because their power is focused not on generating a realistic environment but on replicating the subjective ‘feel’ of flying. Airports look spectacular, but the land inbetween is generally featureless green. Almost no-one does a full hours-long ‘flight’ on a commercial flight simulator.
Along with computers came much better motion systems. The so-called Stewart Platform, a system of six hydraulic or pneumatic cylinders arranged in a circular configuration, was able to produce motion in all six degrees of freedom, and scientists soon discovered that the way the human balance and acceleration sensing system works was amenable to being fooled by what’s called ‘Accleration Onset Cueing’. This uses abrupt movements to trigger the sensation of motion, followed by a slowdown and reverse of the motion which is kept slow enough that the body doesn’t notice it. This allows for the sensation of sustained motion well beyond the extent that the motion platform is capable of producing otherwise.
Now that large enclosures could be hefted about on hydraulic platforms, the simulator designers had lots of room to work on visual systems, and many of those that they chose incorporated projectors in some way.
Civilian airline simulators
The airline industry went down the full-motion platform route enthusiastically. Computing power and graphical sophistication improved to the point where it was possible to create a simulator that so realistically simulated the real aircraft that it could be used by pilots for substantial portions of their refresher or conversion training, as well as their initial training. Interestingly, despite their wrap-around visual displays, commercial flight simulators of the 20th century usually used CRT monitors rather than projectors, in a scheme called a ‘collimated display’. In this system, extra-bright CRT screens were positioned above the flight-deck ceiling, where they reflected down onto a mirror formed from a section of a sphere. The images from multiple screens were carefully aligned to blend into each other to give a seamless view. The spherical mirror caused the light reflected back to the sim pilot to appear to be focused at infinity – as it would be in the real world – to correct for parallax errors that would otherwise be introduced by having a flat plane as a display. Because the surface of the CRT itself was a section of a sphere, the mirror would actually correct the distortion introduced into the image by the CRT screen. If you can remember the days of multiple corrective settings on your bulging CRT monitor, setting up for pincushion and bowing, you’ll know what I mean.
Now that CRTs are dying, commercial sims more often use projectors (finally) which rear-project their images onto a specially curved screen which replaces the bank of CRTs. The image on the screen is then reflected in the collimated mirror to produce the final image. An example of this set-up can be seen here.
Military jet simulators
The military tends to use civilian-type simulators for large aircraft – cargo planes etc – and for helicopter simulation. But for fighters and other fast jets, the motion system of a conventional sim just won’t work. There have been some developments of full-motion sims for fast jets which use a platform that can move some distance at high acceleration on rails or some kind of articularted arm, but these are uncommon. Most military jet sims are fixed-based, and employ projection systems. Since fighter and bomber jets generally have either a canopy that gives 360 degree vision or at the least a very wide field of view, it’s common for military jet sims to use a hemispherical or dome projection system, with the image projected from several projectors onto the dome and corrected to provide an all-around view. This is rather like your local planetarium, in a way.
Projection in home cockpit environments
Since home cockpit builds first became practical, projectors have been the mainstay of most simpit display systems. No other technology offers the key benefits of a large image seen at a distance along with the ability to generate it from a computer. Some semi-commercial systems exist that use large CRT monitors directly bolted into place where the windows of the shell go, and some home builders have adopted this approach – notably in some F16 sims where a trio of monitors produces a good 220 degree + outside view. Some projects use several flat-screen TFT panels rather than a projector. This works best where the sim environment is constrained or there is no shell, just a console. Newer, large-size LCD panels of 40″ and over are starting to show up in some builds. Nonetheless, the majority of projects seem to use a projector, or graduate to one later.
Single screen
There are several projector configurations commonly used by cockpit builders. The simplest uses a single projector, displaying an image on a wall or screen outside the shell windows. This gives reasonably good immersion, at least when looking straight ahead. It also requires the least configuration and does not need any form of correction. The problem is that the edge of the image is usually visible from the cockpit, and as soon as you turn your head it’s obvious that it’s a flat plane in front of you, not a wrap-around environment.
3 screens
The necessity for the image to wrap around you leads to another common configuration, which uses 3 projectors and 3 screens. One screen stands in front of the windows, with the other two set at 45 degrees to it and thus wrapping around the window section. If the screens are big enough then this can truly fill your field of view with display no matter where you look in the sim. The problems with this kind of configuration are several, though. First, you generally need to run more than one PC generating visuals to make it smooth enough; alternatively a product like the Matrox Triple-Head-2-Go allows you to plug the 3 projectors into a single graphical output from your PC, with the video card seeing a single, very wide display surface. But in this case you have to have 3 outside-view windows set up in FSX, which becomes a major drain on frame rates, and aligning the edges of the windows so that they exactly match the edges of the screens and blend nicely is a major pain. Another problem is that the horizon line will not appear perfectly straight – the sudden 45-degree change in angle causes the horizon to look bent and the transition of scenery from one screen to the other to look somewhat odd. This can be corrected for to some extent by a piece of software like NTHUSIM – of which more later shortly.
Curved screen
A slightly better option is to have a curved screen, which wraps around the cockpit shell in a circular fashion. This means that at any angle of view, the distance from your eyes to the screen is the same, eliminating the wonky-horizon effect. The problem is that projectors are designed to display images on flat surfaces, not curved ones. The image is distorted by the curve of the screen, ending up with a rectangular image looking like a barrel. The solution to this problem is correction – the image from the projector needs to be warped so that it cancels out the barrelling effect precisely, leaving what looks like a curved image that fits the surface. Some expensive projectors have this capability built in, but those that will be likely to buy do not; the solution is a piece of software called Sol7, or NTHUSIM (which is a cut-down version of Sol7 designed for simbuilders). This applies corrective algorithms to the video output of your PC at the display driver level, thus being transparent to software like Flight Simulator running on the PC. With NTHUSIM you can correct and blend the output of multiple projectors to create a wrap-around display not too dissimilar from commercial flight sims, albeit without the collimation that corrects for parallax effects. A good example of an NTHUSIM powered 225-degree display can be seen here.
The issue with NTHUSIM is that you can only stretch the display so far before individual pixels become magnified out of all proportion and the resolution of the image is effectively lost. In practice, about the biggest slice of display you can expect to achieve with a single projector and NTHUSIM is about 75 degrees field of view. Hence the need for three or more projectors for a true wrap-around display.
Picking a projector for your project
So, you’re building a simpit and you are asking yourself – which projector should I buy? And how many? The answer is that there’s no one-size-fits-all approach – all projects are different. You need to know what you want to achieve, and then what’s possible, and if necessary find an appropriate compromise.
Here are some factors to consider.
Do you need a wrap-around view?
Many people find that a simple, single-screen solution suits them very well, and is still very immersive. With all the other nice touches of realism in a good home cockpit – instrument panels, glass displays, buttons to push and lights that light up and flash – you’re only going to give a certain amount of attention to the external display. If you’re OK with not being able to look directly to the left and right, this may be the solution for you. If you’re space-constrained, it may be the only practical solution. Consider starting with a single projector and seeing where can go from there.
Have you get enough room?
Unlike a flat panel or even a CRT, projectors require depth of field to work. You have to place the projector some distance from the screen to get a large image – see the stuff about throw ratios above – and so you need to have that much space available in your sim room. If it’s not possible to front-project because of distance but you have a lot of room around the sim shell, you may be able to rear-project. This requires the use of a transmissive rather than reflective screen. Anecdotal evidence says that a frosted plain white shower curtain makes a good rear projection screen material provided your projector is pretty bright (lots of lumens).
If you have less room that you would like you can look into short-throw projectors. Generally these are much more expensive than the standard types, but you may be able to find a bargain on eBay. One or more of these can give you a large projected image to be proud of even in cramped quarters.
What can you use for a screen?
There’s no need to go out and buy expensive screen material if you have a decent bright projector. A consistent reflective white surface is all you need. Some people advocate light grey or even slightly-blue screen colours, but in my experience this merely detracts from the image brightness and affects the colour balance. You can get decent reflection from a painted wooden board, or various types of fabric. One useful material is black-out cloth, which is designed to be used as curtain liner – it completely blocks light on one side, and reflects it on the other. Cover a board or a frame with this material and use it as a projection surface and you’re going to get good results.
If you need to rear-project then life gets more complicated. You need a material which transmits rather than blocks light, but not one which simply lets the light pass through it unmolested. Some builders have had good results with bed sheets up to a certain thread count; others recommend shower curtain fabric. If you are following this path, test a wide variety of materials which might be suitable and see which one works best. All rear-projection screens are going to leach brightness from your image, so for best results use a projector with lots of lumens.
Projector resolution
Up until recently, anything over 800×600 native resolution would cost you big £££. Unfortunately, 800×600 isn’t really enough resolution to really fool the eye, particularly when blown up to a screen several feet wide. Recently, projectors with resolutions of 1024×768 or 1280×1024 have become more affordable, and with the advent of affordable home AV systems there are increasing numbers of wide-screen projectors with a native aspect ratio of 16:9 or 16:10, sporting resolutions like 1280×720 and right up to full HD, 1920×1080. You can still expect to pay upwards of £1000 for a full HD projector, and you won’t find many that sport short throws – AV projectors are usually used in large home theatre rooms and will require a throw distance of at least a couple of metres to give a big picture.
Inputs
Make sure your projector has a DVI or HDMI input for a direct digital connection to your PC. Older projectors, and cheaper ones with lower resolutions tend to have only an analog VGA connection which reduces picture quality and often requires tedious adjustments to be made to get the best picure. With a digital connection you can connect DVI -> DVI, DVI -> HDMI and HDMI -> HDMI with a simple cable, as the signals are identical. Convertor plugs for either end are available, and you can buy premade cables in all those configurations, depending on what outputs you have on your graphics card and inputs on your projector.
What about A.N.Other Brand cheap Chinese projector I saw on eBay?
There is a rash of unrealistically cheap projectors on the market, often coming directly from China – which is not to say that they make inferior goods in China, just that these particular models are cheap junk – which advertise fantastic specs but always disappoint. The tend to use LCD panels which are not designed for projector use, use LEDs instead of a lamp and thus are not particularly bright, and end up offering a dismal native res like 640×480 – while being marked as ‘compatible’ with full HD. Do yourself a favour and don’t consider these, no matter how shiny and modern they look in the photos. Buy a projector from a known brand and read reviews before you commit yourself.
If you’re buying multiple projectors for a multi-screen display, buy the same model for all if you can. Technically similar projectors from multiple vendors can end up displaying images quite differently and you might not be able to get them to match up properly.
Summing up
There’s lots more than can be said about the fine art of using projectors in home simpits, but hopefully this article covers the basics and will get you set to start thinking about your own projector system. Google is your friend here – there’s lots of information out there. Places like MyCockpit will help you on your way. Just remember – think carefully before splashing out your hard-earned cash, and don’t expect it to be a friction-free experience. Projectors are fiddly and cumbersome. In an ideal world we would not be using them. But nothing else comes close to generating the kind of visuals you need for a convincing cockpit experience. At least, not yet…
Normal service on the project should resume soon. In the meantime, look out for more of these special posts.
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