A recent article in a prestigious journal has inspired this blog. In it was included a foldout chart classifying stereoscopic moving image systems. The chart was obscure and confusing. I prefer to have people understand stereoscopic imaging, and the chart is of no help. It isn’t as if the classification of stereoscopic imaging systems is at the same level of complexity as that of the Periodic Table. But classifying a technology family, a system created by the human mind, can also be a challenging.

I divide the field into two parts: projection systems and other displays such as flat panels. There are only three ways that a plano-stereoscopic image (a two-view system) can be selected. You can select the image based on color (or wavelength), time, and polarization. One interesting thing about this is that the time selection technique can work with the other two. In fact, for the present generation of stereoscopic projection systems using the Texas Instruments DMD engine, time is combined with the other two methods to provide a shuttering system; so you can’t classify any of the present projection systems under one category, except for shuttering eyewear that use time for selection.

You can have a train of left-right images, variously described as field-sequential or time-multiplexed, in which shuttering eyewear like CrystalEyes are used. XpanD eyewear are used in some theatrical installations and their eyewear shutters open and close in synchrony with the video field rate and, by a well-known principle, when the left image is on the screen the left eye is seeing an image (and the right is blocked), and vice versa.. Provided that the repetition rate is high enough, you see left images with the left eye, right images with the right eye, and a good-quality stereoscopic image (if everything else is done correctly) results. This is time-division multiplexing or temporal multiplexing, and it uses a shutter. There are two other systems that I alluded to. One uses color and the other uses polarization. As noted, both are combined with the temporal multiplexing or shuttering approach for projection.

Color selection has been called the “anaglyph,” and I am not going to depart from that terminology. Generally anaglyphs use broad filtering for two halves of the visual spectrum–one towards the reddish end and one towards the blue end. Such a technique can use two projectors, or the images can be combined on a single file or print and projected using a single projector.

The Dolby system is the modern version of the anaglyph, and they license the technology from INFITEC GmbH. It is a wavelength selection system but it uses very narrow spikes of filtration at three parts of the visual spectrum which are at different locations for the left and right eye. In this way you can get good color images, unlike the traditional anaglyph (which I find to be an abomination for 3-D projection). Dolby uses a spinning filter incorporated into the projector so that the output is a sequence of field-sequential color-encoded images. When combined with proper selection device eyewear, the left eye sees only the left train of images and the right eye sees the right train of images. One advantage is that you don’t need shuttering glasses, which are electronically driven devices that presumably would be more costly than passive devices that employ filtration. Unfortunately, this isn’t the case with regard to the Dolby system in which the selection devices’ lenses, using retarder stacks, cost about as much as the shuttering eyewear.

The next system I’ll describe uses polarization. There are two kinds of polarization: linear and circular. Circular polarization is outputted by my invention, the ZScreen. The ZScreen, when combined with the single DMD projector, produces a selection technique that can be classified as both polarization-selection and temporal-selection. When you’re designing a system like this you have to pay attention to both the polarization and shuttering aspects of the design. That means you need a polarization-conserving screen, which both the Dolby and the XpanD shuttering system don’t require. The ZScreen alternates the characteristic of polarized light at the frame rate to produce alternate trains of left and right images with polarization encoding. When you put on polarizing glasses using this system you’re actually looking through a shutter and the parts of the shutter are distributed among the eyewear, the screen, and the ZScreen. You can say the same thing about the Dolby system. The Dolby system is a shuttering system as well as a color-selection system, with the parts of the shutter distributed among the eyewear, the screen, and the spinning color wheel incorporated in the projector.

For polarization, what has been done since the late ‘30s (and perhaps even earlier) is to put polarizing filters over the left and right projectors. You need a polarization-conserving screen and everybody wears 3-D glasses with polarization filters which could be circular or linear.

For projection systems the wavelength and polarization techniques are combined with the temporal technique for a single-display-device projector (in other words, a single projector coming out of a single optical path). For flat panels or electronic displays of any type we can do the same thing, and we can also use a dot- or line-sequential (spatial) approach. If you have a single display, somehow or other on the surface of the screen, either in time or in space, you need to share the image. This sharing is then combined with the other selection techniques by a means that is analogous to what has been described with regard to single projector systems.

Let’s take the case of a liquid crystal display, because that is the dominant display and will be the dominant display for years to come. If you could make liquid crystal displays run fast enough, you could view the display stereoscopically using shuttering eyewear, for example. The only viable means art this time is line-sequential selection combined with microscopic polarizer. The dominant player in this field is Arisawa Corporation, and they manufacture a micro-retarding device that is applied to the surface of a liquid crystal display. Because the display already uses a linear polarizer for image formation the combination produces areas of left and right handed circularly polarized light. This produces a line-sequential display that alternates, in the case of their embodiment, left- and right-handed circular polarization states in horizontal lines.

The CRT display is history for the most part but for years it dominated desktop stereo displays using the field-sequential technique. Flat panel displays can, without equivocation, locate each pixel, but you can’t do that with a CRT. However, CRTs are fast enough to use either shuttering eyewear or a polarization modulator placed in front of the screen. CRTs have been supplanted almost entirely by liquid crystal displays. That’s too bad, because they work very well for stereoscopic desktop applications for scientific imaging and visualization.

The other type of display that has characteristics similar to a CRT display is the rear-projection television (RPTV) sets made by a number of companies who license the technology from Texas Instruments. To get a high resolution of 1920 pixels, and because of the physics of the DMD engine, they use the diagonal interlace technique, also called the checkerboard technique. Because of the rapid refresh capability of the DMD they are able to use a time-sequential technique, so shuttering eyewear can be used with these kinds of monitors and produce an excellent image (albeit half resolution in each eye).

After I completed this blog article I hit upon the classification system I will present in the next blog (how I hate the sound of that word) article.

Every field has its mythology. Myths can build a collective spirit, and help people reach for a goal. But sometimes myths are destructive because they are based on fiction and can prevent people from making properly informed decisions. This is especially true in a nascent field like stereoscopic filmmaking. Although stereoscopic filmmaking has been around for a long time, people haven’t had a chance to practice their skills, so from a craft point of view, it’s still in a relatively early stage. Until lately there has been a lack of proper technology so that theatrical filmmakers could learn from experience. You learn from experience, which often means your mistakes. Although the maxim is learning from mistakes, you also learn from what you do right and what works. Who’s to know which is more important–doing it right or doing it wrong?  Obviously, nobody wants to do it wrong, so people tend to get conservative and cautious, especially in an undeveloped field like stereoscopic filmmaking.

One of the vexing issues has to do with the perception of projected 3-D images. We’re not used to looking at stereoscopic images on a big screen. We see stereoscopically in the real world every day, but looking at 3-D movies is not the same as looking at the real world, just as looking at motion in the real world is not the same as looking at movies–that is, photographed images that move. With that in mind, I am going to rail against some of the things I have read and heard recently that annoyed me. I don’t know the answers to all 3-D issues, but many people have made assumptions that they could test if they took the trouble to do so.

The first wrong, wrong, wrong myth is that you can’t do fast cuts in stereo. A recent test done by DreamWorks Animation proved the wrong-headedness of this notion. The test was supervised by our own “Captain 3-D,” Phil McNally. He took a section of Kung Fu Panda, about five minutes long, that has rapid cuts. It’s an action sequence of a tiger being released from imprisonment, and it’s spectacular. It leaves the audience gasping and cheering. Why people think you can’t do rapid cutting in 3-D is probably related to the fact that prior 3-D systems of photography and projection were so terrible that fast cuts got blamed for the pain. But what Phil has supervised proves that it’s not the rate of cutting that’s responsible for discomfort.

The next myth is that the breakdown of convergence and accommodation applies to the stereoscopic cinema. Wrong, wrong, wrong. The breakdown of convergence and accommodation is the habituated response we have between the neurological pathway for eye muscles that control vergence (fixation on a point in space) and a separate set of eye muscles and their neurological pathway controlling focusing. These are coordinated in the visual field. When you look at something in the visual world what you are focused on is also converged on. But for projected stereoscopic images that’s not the case for much of what’s going on on the screen. Only action that’s at the plane of the screen with at or near zero parallax involves no breakdown of convergence and accommodation. The very word “breakdown” is a scary term. It sounds like something is falling apart, and the whole ball of wax is going to crumble. That’s the accepted nomenclature and it refers to the fact that this habituated response doesn’t happen for plano-stereoscopic stereoscopic displays. It is a habituated response but the two neurological systems are independent.

When you’re viewing stereoscopic movies, typically you’re sitting tens of feet from a big screen. That’s good, because at such distances there is no breakdown of convergence and accommodation. It’s possible that people sitting in the first row or so of a small theater will have some problem. But the way it works is this:  Past a certain distance A/C breakdown doesn’t happen. I’m tired of reading ill-informed articles saying that the breakdown of convergence and accommodation is a big deal for projected 3-D movies. It of concern for small screens like TVs and desktop monitors, because people sit close to them.

Here’s another wrong, wrong, wrong:  Films that are composed for IMAX won’t work in Real D. But two shows that I consulted on from National Geographic–Sea Monsters: A Prehistoric Adventure and Lions 3D: Roar of the Kalahari–work perfectly fine when translated from IMAX to Real D. The major concern was aspect ratio–cropping from 1.4 to 1.85. Another concern was to make sure that the zero-parallax plane was properly adjusted. IMAX doesn’t care about making the zero-parallax condition correspond with the plane of the screen, because the screen surround is almost beyond the periphery of the visual field. Real D cinemas have the equivalent effect by using floating windows. I’ve been looking at a clip of The Polar Express for three years on a Real D screen. It looks great and it was prepared for IMAX. You can translate from one to the other. You can translate from Real D to IMAX and vice versa, and the results look good.

Another wrong, wrong, wrong is that everything in a 3-D movie needs to be sharp. For photographers that means lots of depth of field. Perversely, the nomenclature that has grown up for people in CG animation is to “turn off the depth of field.”  If photographers say they want a lot of depth of field, the people in CG say, “Let’s turn off the depth of field.”  Maddeningly enough, they mean the same thing. For a long time people have thought you need to have everything in a 3-D movie in focus.

The out-of-focus or depth-of-field cue that occurs in cinematography is a unique depth cue because it was created by photography. In the real world you have so much depth of field that nothing is out of focus so there is no resultant depth cue. But we have learned to associate out-of-focus images with images that are in the background. Depth of field is a depth cue but an invented imaging depth cue. It is a depth cue that you won’t see in paintings prior to the invention of photography, but it’s one that we’ve learned to appreciate because of photography. The history of stereoscopy and the history of photography are linked. Wheatstone enunciated stereoscopic imaging in 1838, and a year later he made the first stereoscopic photographs. His initial work was with drawings.

It turns out that, based on tests I’ve done and seen, there’s nothing wrong with out-of-focus backgrounds for 3-D. The question that comes up is: do you need this cue when you’re doing stereoscopic imaging?  You probably don’t need the background focus cue that much anymore, because you’ve got the stereoscopic cue. But the background cue doesn’t hurt, and you can use it. However, the foreground out-of-focus thing–like the tip of a finger coming off-screen–might make sense in a 2-D show, but in a 3-D show you’re better off having off-screen objects sharp.

Another wrong, wrong, wrong myth of stereoscopic filmmaking that people have been rapidly learning is wrong in the past year or so, is that stereoscopic photography should be done with the interaxial (distance between camera lens axes) and the interocular or interpupillary (distance between the eyes) with the same value. It turns out that a lot of good stereoscopic cinematography can be done–and, especially on a set, must be done–with the interaxial less than the interpupillary separation. If you don’t do this you can blow out background points. That is, you will have strongly divergent background points if you insist upon having the interaxial equal to the interpupillary. You can also wind up with exaggerated stereoscopic depth effects in which objects look elongated. The point of stereoscopic filmmaking is to make images that are enjoyable to look at, and that leads me to my next point.

Wrong, wrong, wrong:  Stereoscopic movies must be orthostereoscopic. There’s a concept in photography called orthoscopy in which certain geometric constraints have to be taken into account in order for an image to have the same appearance that it would have in the visual world. Given a certain focal length of a lens you have to be a certain distance from a projected image, given its magnification, for the image to be orthoscopic. In other words, it needs to subtend the same height on your retina as it would have in the visual field from a given distance. Human beings can’t change the focal lengths of their eyes (unless you’re looking through a telescope or binoculars). But filmmakers can. Typically, for orthoscopy to work, images shot with wide angle lenses have to be viewed close, and images shot with telephoto lenses have be viewed from far. To pull it off, everybody in a theater would have to keep moving around to different seats or you’d have to change the size of the projected image for on a shot-by-shot basis. But this is not done because we have learned how to look at projected images shot with different lenses.

The planar orthoscopy conditions apply to orthostereoscopic conditions. The orthostereoscopic condition also includes the condition that the interaxial equals the interpupillary. This is a condition that can only be fulfilled for some of the people in the audience. That’s because the mean interaxial separation for human beings is 65 millimeters. I don’t know the value of the standard deviation, but from kids to adults (with big heads) it’s anyplace from 45 to 74 millimeters. So that means that if you shot for one set of eyes the majority of people would not be seeing an orthostereoscopic image. For an orthostereoscopic image to be seen, in addition, you need to be sitting in the dead nuts middle of a row, at exactly the right distance given the focal length and magnification. And there’s no good reason to do any of that, because the point of stereoscopic filmmaking is to create an enjoyable image, not a scientifically correct one.

Another wrong, wrong, wrong, myth, has to do with divergence–that you’re going to tear people’s eyeballs out of their heads if you have more than 2-1/2 inches (the nominal interpupillary separation) of background parallax. It’s a good thing to avoid divergence, but think about it this way:  For people who are sitting an average viewing distance from the screen, the difference between 2-1/2 inches and 3 or 4 inches of background parallax is mice nuts. That’s because the best measure of parallax is angular measure, not that which you can measure by laying a ruler on a screen. Large values of parallax, either off-screen or divergent parallax, have the biggest effect on people who are sitting up close. But for the vast majority of people in a theater, a little divergence is probably not going to hurt them very much. Discomfort also depends on how long the shot is on the screen, how sharp it is, and a whole bunch of other things. Stereoscopic filmmaking is an art. But for CG animation there’s no excuse, if you’re shooting for a certain size screen, to have divergence. Cinematography (with cameras) is a more difficult art.

But there’s a lot of flexibility, and you’ve got to use common sense. Human beings have a flexible visual system.

“Silver screens are crap.”  Wrong, wrong, wrong. There’s a prejudice about silver screens, despite the fact the movie industry itself is described by the term “the silver screen.”  There are some powerful producers and technical people in this town who hate silver screens. It’s not altogether an unreasoned prejudice, because there have been problems with silver screens in the past. There has been blotching, visible seams, and hot spotting. Modern silver screens made in the past couple of years can be good. Well-made screens no longer show seams, no longer have blotching, and no longer have hot spotting. Silver screens are different from matte screens. They have higher gain, and they conserve polarization. But they can be used for showing both 2-D and 3-D. The major disadvantage of silver screens is the same disadvantage you have when you sit in the worst seats in the house and you’re looking at a matte screen. Not only is the image “keystone” distorted because you’re way off on the side, but you get shading. By “shading,” I mean that one side of the picture is going to be brighter than the other. The worst seats in the house for matte screens are even worse for silver screens; there’s no denying it. But the vast majority of seats in a normal theater are good. Silver screens, when you project 2-D images on them, have more contrast. 2-D can look better on a silver screen.

Silver screens sometimes have different colorimetric characteristics. Some silver screens may slightly tint the image so that it’s colder. If they tinted the image so that the image was warmer, or more towards the red, I don’t think people would object. But it’s the coldness that people don’t like, and that can be corrected easily when projecting digitally, which is the major use of silver screens.

The Society for Information Display (SID), the leading global organization dedicated to the advancement of electronic-display technology, today announced the winners of its 13th annual Display of the Year Awards. This year’s honorees represent exciting advances in providing consumers with a superior viewing experience, whether handheld, in the home, or on the big screen. RealD was honored with the Society’s 2008 Silver Award for Display Application of the Year.

The dogs of Hiroshima
barked until the very end.
One slept on the ground, in the shade.
Two pups played and chewed on each other.
One dog came when it was called.
Another sat by the side of her master.

When the great white light came
they were sent to the realm of nothingness,
the realm of atoms ripped apart,
the realm of torrential wind
and flame as hot as had ever been on earth,
even when Moses faced the burning bush.

The dogs of Hiroshima,
until the very moment of the light,
moved as dogs move, and sighed as dogs sigh.
They took upon themselves no cause,
and judged not a living soul,
but they were judged in their innocence,
and the dogs of Hiroshima were no more.

This story appeared in a collection of stories edited by Paul Krassner, “Pot Stories for the Soul.” The collection provides a rich view of the 60’s counter-culture, of which I was a part.

Halloween 1970

By Lenny Lipton

Behind me lay the Sacramento Valley, the A & W Root Beer drive-in in Redding, a hash joint in Weed and the ever-looming Mount Shasta, the Siskiyou, Ashland and the long glide downward into Oregon. Before me, across the road, that Halloween moonlit night, I heard the sounds a rock band coming from the big old house with the Jeffersonian columns. The house sat on a knob of land formed by a bend in the Mohawk River, just a few miles outside of the town of Marcola. They said it had been used in the Jimmy Stewart movie Shenandoah, and true or not, the story lent an air of glamour to the downtrodden manor.

I parked next to the pasture and apple trees where Chief and Apache daily grazed, and after fourteen hours on the road emerged from my fire truck red 544 Volvo. The music grew louder as I walked across the cold hard lawn, opened the door under the columned porch, and feasted my eyes on a mob of laughing, singing, dancing, howling, hooting, and jumping fiends–what we used to call long haired freaks–people with names like Sunshine, Nixy Knox, Belle Donna, Tangerine, Sky, One Eyed Joe, Pink Cloud, Oxygene, and Gentle Waters. They wouldn’t be put off if you called them freaks. They’d like it, because freaks is what they were–hippy freaks.

Zigzagging through the throng I came upon Ken Kesey, Master of the Mystic Arts, who had learned the secret of clouding men’s minds from Dr. Strange et al, sitting at a round table doing five and dime magic tricks. He was fooling with decks of cards, little paddles, shining metal cups and colored balls, amusing a dozen friends. Piled next to the tricks were what I assumed to be uppers and downers sprawled in a colorful heap. At first glance you couldn’t tell the pills from the magic apparatus, and as you will learn, it is this and the Master’s sleight of hand that kept him out of the joint.

I had had only moments to drink in the scene when a hippy jumped into the room raving: “The pigs, the pigs are coming!  We’re sur­rounded by the pigs!”  My first thought, an attempt at denial I admit, was that this was a brother’s paranoid fit, but alas, within moments we got another such report from near naked people who had been steaming in the nearby sweat lodge perched upon the banks of the Mohawk. Bummer!  The police, we were told, surrounded us and sure enough, when I looked out a windowpane frosted with patterns of crystalline lace, I saw three police cars parked on the lawn like panthers ready to pounce. But the music, dancing, and magic tricks continued–the threat taken in stride; for these partying fools were psychedelic commandos–veterans of acid tests, bad acid,  newspapers and television, Jerry Rubin speeches, Timothy Leary declaring victory again and again, police riots, tear gassings, Jerry Lewis telethons, and their parents’ scorn.

In clumped a couple of properly costumed and armed cops; you couldn’t tell them from the real thing. One of them sauntered up to Kesey’s magic roundtable. “What are you doing here officer Doogle?” said Ken. Maybe Doogle was what Ken said, and maybe it wasn’t. If it wasn’t, that’s the only thing I’ve made up.

“We believe there are minors present, in a place where alcohol is being consumed, and we want to look around,” said Doogle.

“Where is your warrant?” said Ken, who without so much as a moment of hesitation continued on with his magic act. He told Doogle that this was police harassment, for this very Doogle was the same officer who had arrested Kesey, a few months before, for the crime of walking a dog without a license through the streets of Eugene. At that instant Kesey proved that he was indeed a Master of the Mystic Arts; his was the greatest magic act I’ve witnessed, dwarfing the disappearance of a stage full of elephants, for right before Doogle’s eyes, Ken hid the dope. The argument between the two of them had so diverted Doogle, that Ken’s manipulation of the pills looked like part of his magic act–he vanished the stash.

Other policemen entered the Marcola House and began to slowly scan each room–looking for crime. I went upstairs and found a scene of panic and chaos, for it was in these quarters that the serious offenders had been medicating themselves. Word of the raid had created a panic, and I saw one man leap out a second story window into the night. Others, like my friend Terry, were frantically attempting to dispose of their dope. He had impul­sively dumped the contents of his baggie into a toilet bowl in order to flush it into the void. Some of those who survived the glorious counter-cultural revolution learned a lesson:  You can’t flush grass down a john.

As the police came up the stairs Terry disappeared leaving me gapping into a toilet bowl. I had a flash born of desperation, and I bent over the toilet making the raucous sounds of vomiting. How much better it would have been had I had something to throw up, I thought, as I stared at the leaves and seeds floating inches from my face. No matter how I tickled my throat with my fingers, I could not barf and by this means conceal the contents floating on the waters below. I made all manner of retching sounds, but it was noise without substance. I sank to my knees to perfect my performance.

“Too much of a good thing,” said a compassionate cop as he watched me through the open bathroom door. He wasn’t getting paid enough to look into that toilet bowl.

After their search the police decided that this was a proper Halloween party; they saw no crimes in progress. Don’t ask me to explain it–nothing is as nutty as the truth. They had had their little Halloween prank; they had come without saying hello, and they left without saying goodbye. The magic had reached a peaked when they were present; it was a more exciting party when they were there, but we didn’t miss them after they’d gone.

*   *   *

Thinking about that Halloween night, after almost thirty years have passed, makes me wonder about what’s happened to the playfulness, the foolishness–the magic in the world. Today the long haired freaks have short hair and the crewcut police have let theirs grow. The hippies have gone straight; they’ve become lawyers, stockbrokers, and college professors. And the police, who after all, are only following orders, are still doing their thing–steadfast guard­ians, with fidelity transcending comprehension.

The history of motion pictures is an interesting one, and I am learning more about it in the context of my present work inventing stereoscopic motion picture systems, and in connection with the work I am doing with studios and filmmakers. I am taking working with filmmakers seriously because the quality of the Real D system is judged by the content projected on our screens. I was recently appointed as the co-chair (Peter Andersen is the other co-chair) of the sub-committee of the ASC Technology Committee tasked to help figure out workflow production pipeline and stereoscopic cinematographic issues. These subjects are tentative and need to be developed and we’re all learning together.

The stereoscopic cinema, in its present incarnation, as manufactured by Real D, is entirely dependent upon digital and computer technology. Digital projection allows for a single projector, while other stereoscopic systems use two projectors. Two projectors work well in IMAX theaters, based on my observations. I cannot say the same for theme parks, whether they use film or digital technology, because there are occasions when the projected image is out of adjustment.  

Replacing multiple machines with a single machine–i.e. a projector–is the way to go, especially in today’s projection booths; because typically there is no projectionist in the booth at the time the film is being projected. There is a technician who will assemble the film reels and make sure everything is going to project well, but then somebody else–maybe the kid at the candy counter–who actually works the projector and makes adjustments. (Interestingly the kid at the candy counter may be well qualified to work the servers and projectors because of his or her PC experience.) 

The product that I invented, the projection ZScreen®, has been used for years for the projection of CAD and similar images for industrial applications. Real D turned the ZScreen into a product that had to work even better for theatrical motion picture applications. It turns out that the film industry has very high standards when it comes to image quality. This is easy to understand, because the industry lives or die by image quality.  

The stereoscopic cinema has had a long gestation. To date, this is the longest gestation of any technology advance in the history of the cinema. For example, within about three decades of the invention of the cinema, sound was added. There were numerous efforts to make sound a part of the cinema and make it a bona fide product. In the three-year period from about 1927 to 1930, rapid advances were made both in sound technology and in aesthetics. If you take a look at movies that were made in 1927, and then you see movies that were made in 1930 or 1931, there’s a gigantic difference. Movies made in the early 1930s look a lot like, and sound like, modern movies. There was a tremendous advance in the technology and in filmmaker know-how in a short period of time. 

It is the creative professionals who will perfect the stereoscopic medium. That’s exactly what they did every time a new technology came along, whether it was sound, color, widescreen, or computer-generated images. In fact, those are the major additions to the cinema, and they all took decades to become an ongoing part of the cinema. Ads for movies never say, “This is a sound movie,” or “This is a color movie,” or “This movie is in the widescreen (or ‘scope) aspect ratio.”  It’s assumed. It’s a rare movie that is in black-and-white. It’s an even rarer movie that is silent. And nobody is going back to shooting 4:3 Edison aspect ratio movies. (Curiously, that’s more or less the aspect ratio used by IMAX for their cinema of immersion.)   

An attempt was made in the early 1980s to use a single projector with the above-and-below format–essentially two Techniscope frames that could be projected through mirrors or prisms or split lenses, optically superimposed on the screen, and polarized. The audience used polarizing glasses to view the images in 3-D. I was the chairman of the SMPTE working group that established the standards for the above-and-below format. But as soon as the standards were established, the above-and-below format was more or less abandoned. A few films like Comin’ At Ya! or Jaws 3-D, and one I worked on, Rottweiler: Dogs of Hell were projected above-and-below, an approach that was technically inadequate. For one thing it was hard to adjust properly and set up the projector to achieve even illumination. I know; I set up a few, and it was tough to do a good job because of the design of the lamp housings and the projectors.

Curiously it was the above-and-below format that led me to the first flicker-free stereoscopic field-sequential computer and television systems. I noticed that the above-and-below format was applicable to video, because that which is juxtaposed spatially can, with the injection of a synchronization pulse between the two frames, become juxtaposed temporally when played back on a CRT monitor; so the first StereoGraphics systems used the above-and-below format.  

The above-and-below video format, which is applicable to video or computer graphics, results in a field-sequential image that can be viewed using shuttering or related polarizing selection techniques. I design the first flicker-free field sequential system in 1980. It used early electro-optics that were clunky, but the flicker free principal was established. Using 60 Hz video, for example, with the above and below format, one achieved a 120 Hz result, that is to say, 60 fields per second per eye. The field sequential system is what is used for the Real D projection system. The electro-optics are different. There’s the ZScreen modulator used in the optical path in front of the projection lens, and audience members wear polarizing eyewear. (The combination of ZScreen and polarizing eyewear actually form a shutter. You can classify the system as either shuttering for selection or polarization, but in fact a proper classification is that it uses both polarization and shuttering.)  But the principal is the same as that used for the early stereo systems I developed. The right eye sees the right image while the left sees nothing and vice versa, ad infinitum, or as long as the machine is turned on. 

The issue I had to solve in 1980 was this:  How to make an innately 60 Hz device work twice as fast. And the above-and-below format did just that. We had to modify the monitors to run fast, but for a CRT monitored it wasn’t that hard. There are two parts to stereoscopic systems’ issues:  The selection device design and content creation. Today we are faced with the same design issue I was faced with in 1980. In addition, content creation has always been a major issue and that’s why I am working with the film industry to work out compositional and workflow issues.

(picture coming here)

Engineer Jim Stewart (left) and I are working on the first electronic stereoscopic field-sequential system that produced flicker free images (Circa 1980). We used two black and white NTSC TV cameras as shown, and combined the signals to play on a Conrac monitor, which, without modification, could run at 120 Hz. The images were half height, but we proved the principal. Stewart is wearing a pair of welder’s goggles in which we mounted PLZT (lead lanthanum zirconate titanate) electro-optical shutters we got from Motorola. The shutters had been designed for flash blindness goggles for pilots who dropped atomic bombs. I kid you not.

It’s an unintended consequence of success: disruption. Real D has demonstrated and is introducing a new version of the ZScreen modulator that is twice as bright as the original product that made possible the current stereoscopic electronic cinema. The ZScreen is an electro-optical modulator that can switch the characteristics of polarized light at video field rate. When used in combination with a Texas Instruments equipped DMD light engine, the result is 144 fields per second projected at the screen–half left-handed and half right-handed circularly polarized. This high field rate is required for eliminating motion and stereoscopic judder.

This doubling of light in the soon-to-be-shipping XL (last quarter of 200 product allows for a substantial increase in screen size. Based on the numbers I have in front of me, right now Real D screens have a median size of 37 feet wide for ‘scope, with a standard deviation of plus-or-minus 7 feet. That number is going to change with the introduction of the XL device. Right now the biggest screens we’re running are about 50 feet. The largest screen we are going to be able to do with the new light doubler will be in the range of 60 feet or more depending upon screen gain and the projector used.

There is a new class of projectors based on a 0.98-inch diagonal DMD chip set, and those tend not to be quite as bright as the prior generation of 1.2-inch diagonal chips. But for various technical reasons that I’m not going to burden you with now, the net effect may well be (because of the 144 fps rep rate that we require) that there may not be much of a difference in some cases especially for 1.85:1 aspect ration projection. Improving brightness for what is now an n excellent projection system is a great step in the right direction. To make it absolutely clear, all things being equal (screen, projector, lamp) the XL ZScreen will be just as bright in stereo as a two projector system using equivalent polarizers. Honestly, I can’t imagine why anybody would want to use a double projector system unless they were in the business of selling projectors.

It’s a great step in the right direction because the stereoscopic selection technology that is used by Real D and our competitors all results in the same kind of light loss. There is a duty cycle loss, because in a field-sequential projection system half the light has to go to one eye and half to the other. So that’s a 50% loss right there. And then the various selection techniques that are used–polarization, or the wavelength selection technique, or the shuttering eyewear that are also employed–all lose about another 70% of the light. The net result is that you are down to about 15% of the light that would have been available.

With a high-gain screen you can boost that. For example, with a screen that has a gain of 2 the net loss would be 70% rather than 85%. By doubling the light we have an extremely beneficial result, and that is achieved by means of polarization recovery. I’m not going to go into the details of how this is done. It’s not the time and place to talk about it. But this technique only applies to the polarization image selection system and is not applicable to the competition systems.

The interesting thing about this improvement–which is a great big improvement because, as I’ve said, the stereoscopic cinema really needs a brighter picture, especially if we’re going to go onto bigger screens–is that the parallax values are now going to magnified even further. Let me explain why this is an issue. It’s understood that images that are composed for one size stereoscopic screen may not be optimum for other sizes. As just a rule of thumb I can tell you based on practical experience that images that are prepared for large screens can be beneficially projected on smaller size screens; but images that are prepared for small screens may have problems on large screens. The reason for this is that the basic stereoscopic depth cue in our displays is parallax. Parallax is the distance between corresponding image points. You could measure parallax with a ruler, but a more sensible way to measure parallax is based on angular parallax, which is based on the distance the observer is from the screen. That is because angular parallax relates directly to retinal disparity. So it’s an invariant, whereas linearly measured parallax is not. (See the blog posting Divergence and Projector Alignment for more background on what is to follow below.)

The reason that this is so important is because exceeding certain parallax values will cause discomfort. There are two major reasons why this comes about. One has to do with divergence. When we look at distant objects in the real world–and that could be objects that are really just tens of yards away–the axes of our left and right eyes are parallel. There is no occasion in the visual field in which they are going to be diverging, or have to verge outward in order to fuse corresponding points on the retina. The analog of parallax on the retina is called disparity, or retinal disparity. So that’s one limit. Based on my experience it’s not a hard and fast limit, and there are some ifs and buts, but it’s the key to what I’m talking about here with regard to understanding this additional screen magnification.

Divergence, in some cases even of large values, is of little consequence to people who are sitting in the middle or the back of the theater. That’s because their eyes are going to diverge a lot less–maybe very little, depending on the size of the screen and where they are sitting. With regard to divergence, the rule is that you’ve got to be kind to the people who are sitting in the front of the theater. But how kind?  How much can people tolerate beyond the normal interpupillary separation? 

There is an average interpupillary separation of about 64 millimeters for males and females. That means that half of the population is going to have an interpupillary separation that is larger than that, and half is going to have one that is less. Also, based on my experience, you can get away with a lot in terms of divergence. And I’m not even exactly sure what the factors are after all these years of working in the field. It could be that the image in the background is so out of focus, or what’s happening in the foreground is so interesting, that in some shots divergence in the background doesn’t seem to matter or bother people. On the other hand, there are some people who are more sensitive to these things than other people. Another factor that I can’t account for and don’t completely understand is, are we at a point in the stereoscopic cinema where we’re all learning how to look at stereoscopic images, and a little bit of divergence, or maybe even a lot, isn’t going to hurt most people after awhile?  The most important factor may have already been mentioned–namely that people who are sitting in the middle or back of the house will have less of an issue fusing divergent issues because the best measure of parallax is angular and not linear.

Most people don’t want to sit in the front few rows of the cinema unless they’re forced to. The people who like to sit in the front rows are special people. I’m not saying that there’s something wrong with them. I’m simply saying that they’re people who just have different taste. I like to sit in the middle of the house. (When I’ve had to sit in the front row because I arrived late, I’ve been pleasantly surprised on many occasions at how modern motion picture projection holds up. I think that’s because the digital intermediate process has eliminated a lot of the contrast grain buildup and sharpness reduction that we had in the past when we used film intermediates.)  Be that as it may, people who sit in the front may learn to like the large positive parallax values associated with divergence.

With regard to off screen parallax values, or negative parallax values, there is a lot more latitude. You can take a lot more. People in the field are very sensitive to the concept of the breakdown of accommodation and convergence, which is a habituated response that we have from the time of birth so that the focus of the eyes and the vergence of the eyes are coordinated. In the stereoscopic cinema that is not a major factor despite the fact that it is so frequently cited as a cause for concern. It is well established that for large screen projection–except for the people who might be sitting in the first couple of rows–the breakdown of convergence and accommodation is not a factor. I’m getting tired of people talking about the breakdown of convergence and accommodation.

That being said, you can push parallax values so far that the image is un-fusible. But typically in Real D cinemas (and I can’t give you an absolute number) you can measure off-screen parallax values in many inches–let’s say a foot. But you shouldn’t be measuring parallax values in feet. I can quibble about this, because if you’ve got a spear or something shooting off the screen, in the final frames in which it leaves the screen it could have parallax values of yards, and it will be quite watchable. You might not have time to fuse it, and it will look perfectly fine.

What happens when we go from screens that are maxed out at 50 feet wide to screens that are maxed out at 60 feet or better?  Magnification rules, and the parallax values of background points are going to be linearly proportionately larger. In other words, 1 inch of parallax on a 20-foot screen becomes 3 inches of parallax on a 60-foot screen. This is the same for off screen points and for divergent points. It’s the divergent points that are the biggest problem for the especially (and maybe only) people sitting very close to the screen.

So what to do?  The advice that we gave to people who were shooting movies for the Real D cinema was this:  “Aim for a 40-foot screen. It’s going take of itself.”    If you wound up with 2.5 inches of background parallax for a 40 foot screen then on a smaller screen it’s going to be an inch or so and on a larger screen maybe 4 or 4 inches. So nobody’s going to be bothered by the reduction in parallax on the small screen and experience tells me that the image will still look good. For the large screen the major concern is not appearance or stereo effect but background points diverging.

In this discourse up until now I have been more or less concerned with limiting values of parallax in which we’re in a safety zone of comfort of viewing, rather than how the image looks. How the image looks is an interesting subject that needs to be talked about. The eye-brain, or I should say for a stereo system the eyes-brain, is a very flexible instrument. You can enjoy stereoscopic images that were produced for different size screens. As far as I’m concerned, while the image may not be optimum in terms of the extent of the stereoscopic effects, it will be enjoyable. The major issue is not appearance; it’s going to be comfort. That’s why in this article I’ve been more concerned with comfort than I have been with the aesthetics of the image.

I’m going to discuss about a couple of possible approaches. One involves a solution that’s very similar to that which is employed in IMAX. IMAX limits its background parallax points to +2.5 inches and lets everything play off the screen. You can do that in the Real D cinema too by using floating windows. Refer to these blogs and read at the blog that talks about IMAX and Real D compositional differences. This approach requires resetting the background points either in release print or in the theater. I’m suggesting that there could be two skews of release prints, one for very large screens and one for smaller or average screen sizes. The studios will hate this because they, rightfully, want to reduce the number of release print variations. However, the economic advantage to projecting on large screens means you’ve got more people in the audience and more revenue.

Another answer that would be problematical and require a different set of release prints would also involve developing a new product based on cutting edge technology. This would involve interpolating to reduce the interaxial separation. The first suggestion I made would involve horizontal shifts of the image to keep the background points at approximately the interpupillary separation. Conceptually this is easy to do. But once you’re dealing with an image that has been shot with a particular interaxial separation (that is, the distance between the camera lenses), the range of parallax values is baked in. If you want to reduce the parallax range of the film, which might be the ultimate cure for projecting on large screens, the answer could be to interpolate downward to reduce the effect of interaxial separation. This is a much more sophisticated and difficult approach than that which I have suggested with regard to horizontal shifting of the images to maintain a constant zero-divergence background parallax value. It is also a missing link in post production which can vary every important shooting viable except this one.

There is a lot of prior art on interpolation to reduce interaxial separation. Microsoft and others people have been looking into the problem for reasons that are related to applications such as teleconferencing. I’m not going to go into any details about why this has been done, but there is an extensive body of literature on how to interpolate between two perspective views to produce an intermediate a different view.

So there you have it:  an issue that needs to be addressed. I’ve tried to lay out the background and give you some ideas on where we can go from here. I’m open to comments and criticism. I look forward to hearing from anybody who’s got better ideas.

Historically, there have been different ways to set up stereoscopic projection as can be traced in the literature. Contemplated was dual projection using anaglyphs, or the polarized method for image selection. In one case, the projectors maintain a constant distance apart with the lens axes parallel. In the second case, the lens axes converge at the plane of the screen. In the first method there is no constancy with regard to placement of objects at the plane of the screen and that will change with the size of the screen, but background points can be set to remain at a fixed value no matter what size screen is used. The second method is the one we use when in fact the lens axes are coincidental.

The Real D system was not contemplated in the literature although it bears a close relationship to projection of single projector anaglyphs or Vectographs since the left and right lens axes for the Real D system are coincidental. One basic issue has to do with the constancy of the stereo effect or parallax as related to screen magnification. The amount of parallax is linearly proportional to the magnification.

In stereoscopic projection using the Real D system what can happen is that photography meant for one screen, when projected on a larger screen, can result in background points having divergence. All the parallax points will be increased linearly proportionally with respect to magnification; but may be of greatest concern will be the background points.

Divergence is a condition in which the vergence of the eyes is outward rather than inward. In the real world the condition of divergence does not occur, and eye muscles are not accustomed to fusing such images. Divergent parallax points have parallax greater than the interpupillary separation. Since there is quite a spread of interpupillary distances in the human population, say for children with about a two-inch separation to adults with a maximum of about a three-inch separation, we see that images created for one group may not work for another. In other words, if corresponding background points have a three-inch separation, then kids are going to be experiencing divergence–in fact, one that has an inch and a half of extra parallax–which may cause fatigue or eyestrain. Kids’ muscles may be more supple–anyway, that’s one hypothesis–so they may not have a problem viewing the images.

One approach, probably the one that makes the most sense, is to decide what the largest Real D screen is. Shoot for that, in terms of the background point parallax, and let everything else take care of itself. Let’s take the case of (I’m going to use round numbers here) Real D images that are meant for a 50-foot screen, which would be the outside limit of what we’re presently capable of doing; and let’s suppose that the creators of the images, either CGI or photographic images, decided on maximum positive parallax values of three inches for that 50-foot screen. When projected on a 25-foot screen, the maximum parallax for background points will be an inch and a half. The question is, how would those images look?  When projected on a large screen, using the maximum parallax for background point, say three inches, you might expect that you’d get a deeper effect than on a smaller screen. Practically speaking, that’s not what happens and the image hold up–it looks fine. Even though the maximum background points would only be an inch and a half, and all other parallax values would be halved, on the 25-foot screen my experience is that the projected image still looks perfectly fine.

An important point to remember that sometimes gets overlooked is that, although screen parallax can be measured with a straight-edge ruler, in point of fact the most important way to look at parallax, in terms of view comfort, is its angular extent. Four inches of parallax when viewed from the front row is an entirely different matter for an observer sitting in the rear of the house. In other words, the retinal disparity in the front of the theater is going to be much greater than the resultant retinal disparity from the rear.

A rule of thumb for stereoscopic images is that you have to be kind and do no harm to the people in the front rows. (People who choose to sit in the front row are seeking as special experience for 2D or 3D projection.) You have to be considerate. But most of the people don’t sit in the front row so the photography really needs to take into account the visual effect for people who sit in the middle of the theater, where most people sit. It sounds like it may be a difficult problem to work out, but it really isn’t because it is possible to strike a balance.

The alternative to the recommendation I made above with regard to simply projecting–making no corrections in projection but with the filmmakers cognizant of the range of screens they’re going to use–would be to produce some kind of an offset in projection. It’s a relatively simple calculation because the parallax value is a linear function of magnification.

It’s also possible to laterally shift the images so that we have constant background parallax points, which is the first method described above for use with dual projectors. For Real D the method would be adapted to projection with a single machine and that will be possible with the next generation of TI projectors which will allow for horizontal pixel shifting. In this way we would, no matter what size screen, project and always have background points that are, say, t three inches apart. This is a technique that is used in IMAX. IMAX uses two projectors with an image offset that’s always the interpupillary distance for background points. I believe they recommend two-and-a-half inches for background points. In The Real D system we could also use these horizontal shifts. The problem with this approach is that, while we may be keeping the background points constant, we’re going to affect that which plays in the plane of the screen and the value of off-screen parallax points.

One of the most important things that needs to be considered at the time of photography or image generation is placement of objects in terms of the plane of the screen. If objects are placed at the plane of the screen they tend to be the easiest images to look at because they have the least crosstalk (the ghost image corresponds to itself) and the breakdown of convergence and accommodation is benign because accommodation and convergence correspond as they do in the visual field. If we make the correction that has been suggested, of image shifting, things may change for the worse. We’re going to be adding or subtracting parallax values to those image elements which would have appeared in the plane of the screen with values at or near zero parallax. In such a case these changes may produce problems in our desire to “fix” the background points. Shifting the images will result in adding to negative parallax (off-screen effects), which could change the ability to view images comfortably. Another matter that needs to be carefully considered are edge effects. If we are increasing negative parallax extent, or changing the parallax values at the vertical edges of the surround, we may be increasing difficulties with regard to viewing the image at the screen edges.

However, and this is a major point, shows shot with floating windows will work just fine with horizontal pixel shifting used to set up initial projection conditions to prevent divergence. That’s because the virtual window itself will simply have its parallax values adjusted automatically when the background points are shift to insure maximum comfort. For floating windows the plane of the screen becomes a virtual plane of the screen. It doesn’t really exist as a perceptual anchor since the physical screen surround is trumped by the virtual surround.

If a shot has background points that are the interpupillary separation for a small screen and we horizontally shift the image to keep that constant for a large screen we are going to move the entire image forward toward the audience adding to all parallax points because of this shift and because of the increased magnification. That might not work well without floating windows because of the conflict of vertical surround cues that might be created.

If a shot has background points that have the interpupillary separation for a large screen and we horizontally shift the image to keep that constant for a small screen we are going to move the entire image backward away from the audience subtracting from all parallax points because of this shift and because of the decreased magnification. That might not work too well either but it will be relatively benign because of the decreased magnification.

My recommendation is that we leave this thing alone until we consider these variables and we’re able to make a recommendation as a filmmaking community. You can’t do a test on one screen. We would need to take material that was specially prepared–worst-case material, let’s say–and project it on big and small screens. For example, we could go to Mann’s or the Bridge with test images and compare them, and see how they looked in the Clarity Theater. This is not necessarily going to be easy testing to do, because if you did a rigorous psychophysical test you’d have a number of test subjects, you’d show them targets, and so forth and so on. So, right now my recommendation is to do nothing and say nothing until we know what to do. We don’t want to create a problem by offering a solution that may not be valid, and may introduce more problems than it solves.

Inventors have been obsessed, and rightfully so, with creating stereoscopic displays that do not require eyewear, or what in the jargon of the field are called “individual selection devices.” I have put some considerable effort into devising variations of this. To avoid humiliation I’m not going to tell you about the wackiest thing I worked on. However, I am going to tell you about the next-wackiest thing and a few that are more sensible.

Those of you who are fans of the anaglyph, like my friend Ray Zone, will immediately recognize the following physiological phenomenon: You’re up late; you’re looking at your anaglyph comic book. Of course you’re close to a bright light; you need a good, bright reading light to look at your anaglyph comic book through those dark red and blue filters. Ray’s published a lot of these comics. So I’m looking at one of his anaglyph comic books, and I’m looking at another one of his anaglyph comic books. In fact, I’ve got many of Ray’s anaglyph comic books. I read one after another. I’ve been wearing the anaglyph glasses now for a long time. I take the anaglyph glasses off, and what do I find? Something that is not entirely unexpected for those of you who are fans of the psychology of perception. What I discover is that the eye that had been looking through the red lens is now seeing the world tinted bluish or greenish, and the other eye that’s been looking through the green lens is seeing the world tinted with its complementary color, red.

Two thoughts occurs to me: Please God, don’t let this be permanent, and now that my eyes are seeing the world with complimentary tints, can I look at this comic book without glasses, and will I see a 3-D image? After all, I’m seeing a reddish world through one eye and a greenish world through the other. Haha! But it occurs to me that if I do this I’m going to see a pseudo-stereoscopic image, because my perception has been, let us say tinted, the complementary colors required for true stereo. Therefore, if I turn the comic book upside down, will I see a stereoscopic image? Do you think this works? Would you like to try it? Let me know how it turns out. Yes, you can try this at home.

And while we’re on the subject, consider this: Could you put red vegetable-color eye drops in one eye and green vegetable-color eye drops in the other eye–eye drops that were safe for ophthalmologic use–and then achieve an anaglyph effect without glasses? Hopefully the eye drops would wear off in time. (Do not try this at home. I’m not kidding.) Certainly the benefit of seeing stereoscopic anaglyphic images without glasses would not outweigh the unfortunate result of having your eyes’ lenses permanently stained. That would be a bad idea; I think we’d all agree.

And then, of course, what about polarizing eye drops?

Finally, another idea–an even better idea. I conceived of this many years ago one night after long hours in the lab. It’s a way to see field-sequential stereoscopic images the same as when you’re wearing CrystalEyes but without eyewear. Here’s the idea: Electrodes are hooked up to your eyelids, and then supplied with the right amount of voltage in synchrony with the video field rate. As the voltage is applied to each eyelid it blinks. When one eye is closed the other is open since it has zero voltage applied to it. There’s a controller hooked up to the video source supplying the current that is then sent alternately to the left and right eye lids. No glasses–only comfortable electrodes. The eyes blink rapidly at video field rate so that you see a field-sequential stereoscopic image completely compatible with Crystal Eyes formatted content, for example.

There are a lot of good ideas. Some make it to the marketplace and some make to this page.

I will now take you, reader, on a journey involving both technology and aesthetics intermingled with projection practices that are a century old and how the stereoscopic electronic cinema evolution impacts this. Stick with me and it hopefully will all make sense by the last sentence. Along the way I’ll inform you about some engineering choices, and how this has impacted not only digital cinema projection, but stereoscopic projection.

Projecting motion pictures is a century-old art. A film projector uses a band of transparent material coated for picture and perforations for an indexing function to properly place each frame of image in the projector’s optical path. The frame is held at rest in what is called the “gate” while the image is projected, one at a time, on the screen. A light source shines through that frame, and the tiny image is projected by the lens onto a screen. The image itself, less than an inch wide, is projected onto a screen that can be 60 feet wide. This is amazing considering that, if done correctly, the results are beautiful despite the enormous area magnification.

As the film rides through the gate, which is the mechanical device holding it in position, the mechanism that drives the film through the projector must accomplish a lot. Each frame has to be held at rest in the gate, and each successive frame has to be indexed in the same relative position and the plane of the film has to be held perpendicular to the lens axis at the same distance from the optical center of the lens. The gate consists of two major positioning components:  a pressure plate and an aperture plate.

The aperture plate is closer to the lens than the pressure plate. It provides a rectangular opening around the frame. The aperture plate, closer to the lens the film itself, combined with the pressure plate, keeps the film running in its proper channel to insure accurate positioning. The film and the aperture plate, whose front surface is a few millimeters closer to the lens, cannot both be brought into sharp focus on the screen. If you bring the aperture plate into sharp focus, the film is out of focus, and vice versa. There’s no choice; you have to focus on the film, because who cares about a sharp aperture plate (not even the guy who made it)? 

However, if you focus on the film, the aperture plate surround or outline is going to be blurry. That is why 35 mm movie projection is set up so that the image bleeds onto the black rectangle that surrounds the screen. If you don’t do it this way, you’ll have blurry edges for the vertical and horizontal portions of the projected edges, and by esthetic convention or consensus that is deemed to be unpleasing. Therefore, some of the image is sacrificed in order to make a nice, crisp, sharp surround.

Here is another thing to think about with regard to motion picture projection. Projectors in most theaters are usually higher than the screen and have to be angled downward. What this means is that, rather than a rectangle frame projected onto the screen, a trapezoidal shaped frame will project onto the screen. It’s typically not a big deal, even if the angle is rather steep, because the material on the edges of the screen can get cropped as described above. Usually the distortion isn’t significant because of the nature of the subject matter (people and monsters) being projected. If the plane of the screen were perpendicular to the lens axis there wouldn’t be any trapezoidal distortion (also called keystone distortion). The theatrical film industry has lived with losing a little bit of picture due to the difference in the distance between the aperture plate and the frame, and because of the trapezoidal distortion noted.

When setting up digital projectors, you don’t have the same concerns. There is no aperture plate in a DMD digital projector. So there is no reason that you should have to lose a single pixel, because there is no conflict between focusing on the aperture plate (which is nonexistent) and the imaging engine.

What Texas Instruments did in the design of their digital projectors was to specify what I call a perspective control lens–because that’s what they are called when used with cameras. Lenses like this have been used in road warrior and other projectors and I remember them being used in some slide projectors years ago. By mechanically moving the lens so that the lens axis remains perpendicular to the plane of the screen but shifts either vertically or horizontally, you can shift the image upward, downward, or to the side without inducing trapezoidal distortion. This means you need to have a lens that “covers” more than it would need to for the frame diagonal. A normal lens designed just to cover the size of the frame (or the image engine) may produce vignetting; that is to say, the corners or edges of the projected image will get dark, in this case unevenly. Designing a lens that has more coverage makes for a more difficult and expensive lens design, but it has benefits because trapezoidal distortion can be eliminated.

When digital projectors are installed, a couple of interesting things happen. In the first place, when the digital projector is added to a facility that has an existing film projector and the film projector remains in place, the digital projector is now placed off-axis. The film projector has the preferred on-axis location, so the digital projector is going to be off to one side. Coupled with the fact that most projectors are going to look downward at the screen, you will get even more trapezoidal distortion from digital projectors.

Since Texas Instruments has specified perspective control lenses in their DMD projectors (these lenses are made mostly by Konica-Minolta and some by Schneider), you would expect that you wouldn’t have any trapezoidal distortion in projection. The preferred method for setting up a digital projector with such   a lens is to maintain the lens axis perpendicular to the plane of the screen. If that condition is fulfilled, then the horizontal and vertical shifting of the lens will result in an image that fills the screen, with no trapezoidal distortion. As long as the effective plane of the image engine is parallel to the plane of the screen there will be no trapezoidal distortion. If you set everything up correctly there is no reason why you need to lose a single pixel, because you don’t need to bleed the projected image onto the black surround to address the aperture plate/film focus issue or trapezoidal distortion effects.

Unfortunately, this is not the way digital projectors are set up. I don’t have the figures, and I haven’t visited every theater but I can tell you that in every projection booth I have been in, and in every theater I know about (which is admittedly a small sample when you consider that there are five thousand digital projectors out there), I have not seen it done correctly. The installers are angling the projector downward like a film projector, and they are bleeding the image over onto the screen surround. That’s because the guys who set up the projectors know how to set up film projectors and don’t seem to know how to set up digital projectors. This means that we are losing pixels that we don’t need to lose and we are getting trapezoidal distortion that we don’t need to get.

What does any of this have to do with the stereoscopic cinema?  Here’s what:  For the floating window that is so effective in increasing the parallax budget or the depth of projected stereoscopic images, trapezoidal distortion will change the shape and spoil the effect because the shape of the floating windows will be altered and because some of it will be cropped. Next, and as important, if you crop the image by bleeding it over onto the screen surround (following conventional film practice) you will be cropping off the floating windows and obviously ruining the effect. Thus the practices in the field of setting up digital projectors have a material impact on the stereoscopic cinema and not a good impact on the planar cinema, for that matter. It’s just foolish not to take advantage of an intrinsic advantage of the digital projector design by losing pixels and by adding distortion where you don’t need to. It is destructive to louse up the projection of a stereoscopic image by ruining the floating window effect that is so effective.

My feeling is that, Texas Instruments, you have a lot of smart guys there, but you made a practical mistake; but I think I would have made the same mistake (I am a smart guy too). By increasing the complexity of the projection lenses, which already have t zoom capability (with a small range for tweaking the magnification), and a relay design because the DMD engine is buried so deep in the projector, you got one complex pile of glass. You’ve added additional complexity to the design by adding lens shifting. This last requirement has to have increased the cost of goods of the projection lens, and as I have stated with such vehemence, the feature is generally not being used.

Is there a cure?  The best answer is to educate the people who set up the projectors. Another way might be to use a software distortion correction, so that the projectionist could use the same old practice of angling the projector downward at the screen. Then using the kind of correction available in Photoshop (for example) with “Transform,” he or she would dial in a rectangle target that would meet the edges of the screen surround. That way one could “pre-distort” the image to eliminate any trapezoidal projection distortion.

There are a couple of arguments against this idea. One is the computational power that would be required for a movie rate result. I am not enough of an expert to be able to tell you if this can work but my gut feeling is that it is not a big deal, and would cost less than what has been required to add extra optical capability to the projection lenses. For all I know, the existing projectors with their built-in computers can handle the job, or maybe the next gen can.

The next argument against my idea is that such corrective pre-distortion is going to move us away from square pixels and change the effective resolution on the screen on a point-by-point basis. A theorist might object to this, because we are changing the image magnification and pixel density across the plane of the screen in order to correct the trapezoidal distortion. But what’s the big deal?  I don’t think anybody could ever see the difference. You’d have a system that would result in an image that had no distortion; I think you would be able to lower the cost of the lens; and you would have something that would work a lot better given the existing practices of projectionists. Plus, the benefits for stereoscopic projection and the floating window would be enormous.

Finally, if there is such a thing for the prolix author short of the big sleep, Eisenstein, the brilliant Russian filmmaker, wrote an essay called the Dynamic Square in which he championed the idea of changing the aspect ratio of a film on a shot-by-shot basis. Sounds like a radical idea but it really is not because that’s what still photographers and painters are able to do in order to best compose their images. Why be stuck with ‘scope for a close-up?  Why be stuck with wide-screen for a panoramic shot?  Why be stuck with the fixed aspect ratios of 2.4:1 or 1.85:1 for that matter? 

A major practical argument against the dynamic square is that when using film, the screen surround masking cannot be sufficiently rapidly moved into place to mask the blurry aperture image, so switching from one aspect ratio to another would be disruptive. But the blurry edges of film projection give way to the crisp edges of electronic projection making Eisenstein’s dream entirely practical. I now await the brilliant filmmaker who will take advantage of this proposal. The practical inhibition may well be that in this interim period in which we are transitioning from film to digital projection movies released on film will have a disadvantage. Oddly the change has already taken place with no fanfare on TV, where content of various aspect ratios is being displayed willy-nilly.

As I said, it would all make sesne b y thelas t sen te n c e.