The Projection Dilemma

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.

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