Groups of video projectors can be arrayed into electronic displays that offer larger, brighter, and higher resolution images. This paper provides a broad overview of the design considerations and opportunities inherent in arraying projectors in a group. A projector array provides:

Although video walls are the most common example of electronic image arrays, the limiting factor of traditional video walls is the visual segmentation. Minimizing the segregation between arrayed images is highly desirable with the goal being to make the segregation indistinguishable.

Overlapping and seamlessly blending multiple video projectors into a single composite image goes a long way toward eliminating the segregation of projector elements and opens the way to many new practical applications. This is particularly significant in displaying computer graphics.

Computers have the ability to generate multi-channel composite images at resolutions that far exceed traditional electronic media and even the maximum resolution of any single monitor or projector. These images can then only be displayed using an arrayed system.

The challenge is to make the entire projection array behave as a single imaging device. This can seem to be a daunting task with large arrays, but one whose solutions are already clear. Consider that a typical CRT projector is, in fact, an array of three projectors (R, G, & B) engineered into a single plastic housing and arrayed in a superimposed and converged geometry.

Similarly, an array can be managed with integrating electronics to create a "virtual" packaging around multiple projectors, arrayed in adjacent and registered geometries, with the entire package behaving as a single cohesive imaging device.

2.1 Edge matched formats

Edge matched formats rely on very clean projection geometries where the two projected images are immediately adjacent to one another. Defining a perfect single pixel row edge between projectors is improbable because small variations in the vertical geometry cause small overlaps and brightness aberrations between the images. It is typically better to have a definitive mechanical edge, and it is recommended that there be an actual "mullion" or black separation between array elements in such applications.

2.2 Edge blended formats

Edge blended formats rely on an overlap region with redundant picture information from each projector in the overlap. The overlap region is, therefore, double the brightness of the rest of the image and needs to be blended using an edge blending processor capable of fading down each overlapping edge of the projectors in such a way as to compensate for the gamma (video signal reduction vs light output curve) of the phosphor, light valve or LCD. The goal is a uniform brightness level across the overlap region.

2.3 Wide-field arrays

Wide-field arrays use adjacent geometries. These are typically laid out in horizontal patterns to create wide scope images. 

In some instances, the projectors may be stacked vertically creating very tall images.

It is also possible to vary this geometry format by rotating the projectors 90 degrees, then, laying out the format horizontally. This is particularly useful if each array element has a 9:16 aspect ratio format. With a 20% overlap, the resulting display has a very pleasing aspect ratio of 1:1.46. Using HDTV resolution for each array element, the resulting display is 2690 X1920 pixels for a composite resolution of over 5,000,000 pixels.
 

2.4 Matrixed arrays

Matrixed arrays organize the projectors in XY grids. Video walls are an example of a matrixed array. Alternatively, using a soft edge blending processor capable of affecting all four sides of the projector, it is possible to construct huge seamless projections. There are two challenges in this geometry format. The first is the need for separate soft edge blend control in the corner regions where all four projectors overlap, The second is physical placement of the projectors so as not to block the viewer. The obvious answer would be to rear project the image, which is logical but nevertheless challenging since rear projection materials are manufactured in finite sizes, and thus can be the limiting factor.
 

3.1 Increased image size

It is obvious that by adding more array cells, that the aggregate size of a composite image can me made larger and larger.

3.2 Increase image brightness

Given a specific image size for each array element or projector, the overall brightness of the display remains constant as more and more array elements are added. This is in contrast to a single projector whose brightness is diminished as the size of the display is enlarged.

3.3 Increase resolution

By feeding each projector a portion of the image at a specific resolution of video, the "overall" resolution of the display is increased. Each projector provides its own elemental percentage of the overall result providing an "additive resolution" effect. This is especially evident in edge blended applications, where the result is a single composited image. For example, a triple VGA resolution display with a 25% overlap would result in a 1600X480 or 768,000 pixel composite resolution after subtracting the redundant overlapping pixels.

3.4 Extreme Resolution

With image generators or graphics computers capable of multi-pipe high resolution output, it is possible to generate composite images that are three or more channels of 1600X1200 pixels each. With a 25% overlap, this would result in a 4000 X 1200 or 4.8 million pixel resolution image after subtracting the redundant overlapping pixels. There are currently no commercially available single display devices capable of operating at these extreme resolutions. The practical solution is to use a projector array where each projector is working at its own maximum resolution, with the aggregate or composite resolution being the multiple of each element minus the overlap region.

3.5 Projection on complex surfaces

In an example using a coved cylindrical or spherical screen design, a single projector trying to display on the entire screen surface would need to be substantially further back from the screen than would an array. This can be problematic from an architectural point of view.

Perhaps more important, the "chord" described by the curved screen would be deeper than the projector's depth of focus. This, then requires custom optics to maintain edge to edge focus. Custom optics are generally impractical for a three gun CRT type projector but more practical with single lens system. Alternatively, by using a projection array, the "chord" of each projector is dramatically shortened, now falling into the depth of focus operating range of the projector. The combined resolution, bright- ness and depth of focus benefits makes a projector array a good choice for curved screen applications.

The example we will use to examine the components of a typical arrayed display uses 5 projectors with an adjacent, edge blended geometry on a flat screen. The components of the projector array include:

Each components represent another variable consideration in the overall display design.

4.1 Sources

4.1.1 Prepared video

For a number of entertainment applications such as theme parks and museums, it is desirable to have an electronic display system, rather than a film display system, for operational, maintenance and/or architectural reasons. This display nevertheless, needs to be wide-field in scope, or otherwise 'spectacular', bright and high resolution. In these cases, a custom media production is shot on large format film and sectioned into overlapping array elements in post production processes. Each array element is then mastered onto a reliable and synchronizable playback medium such as laser disc, and re-composited into a single image for playback. This provides the size, resolution, brightness, architectural, and operational advantages desirable in such applications.

4.1.2 Image generators

Simulation image generators such as the Lockheed/Martin or Evans and Sutherland systems as well as graphics oriented computers such as the Silicon Graphics systems and even the Macintosh can each be configured to provide multiple simultaneous image channels at a variety of resolutions (depending on the system). This can result in extreme resolution displays useful for simulation, visualization, group VR, group multimedia, as well as full immersive environments. A projected array is an ideal output peripheral for electronic computer displays -- with a myriad of unexplored applications.

4.1.3 Source processors:

When maximum resolution is not required, a 'single-source-input to multiple-output' processor can be used to break out the arrays elements. One example would be HDVS input to multiple NTSC output 'video wall' processors. Another example are systems where a single 1280X1024 computer graphics source is broken out into a four channel, 2 by 2, XY array at VGA resolution, such as with RGB Spectrum's Compuwall system. Neither example takes advantage of the additive resolution capability of arrayed projection. There is another category of processor that will provide for additive resolution. These are typically graphics subsystems developed for command and control displays running under X-Windows such as M3i or Jupiter Systems. Unfortunately, most of these system have not yet allowed the user to break out the image with an overlap region to allow for soft edge blending.

4.2 Array Processor

The geometrically correct images come from the sources and are fed through an array processor designed to reintegrate the array elements into a single composite image. Two of these processing parameters are edge blending and projector matching.

The patented edge blending technology1 which Panoram Technologies, Inc. (PTI) has developed consists of: 1.) a memory lookup table that holds information on luminance, 2.) a method of synchronizing that table to the video coming through, and 3.) a converter for affecting the projected brightness based on the lookup table.

Using a one dimensional lookup table and re-addressing it with each video line, it is possible to provide very detailed adjustment of the image brightness in adjacent overlapping areas. These "smoothing parameters" can be calibrated to the gamma behavior of individual phosphor tubes or light valves as well as a number of other projector aberrations, providing a smooth transfer across the image region.

Having found that the behavior of each R,G & B channel differs, PTI has implemented separately adjustable lookup tables for each gun in the array.

By using a one dimensional lookup table vertically, it is possible to provide edge blending functions for tops and bottoms of the display. A separate horizontal and vertical lookup table provides some measure of control over all four sides, but does not address "4 corner regions" (see Par. 2.4) of matrixed arrays.

In a new implementation of the same principals, PTI has provided a true two dimensional lookup table, able to map "smoothing factors" for the entire raster of each gun in the array. This leads to an additional series of array processing capabilities including automated field flattening, radial anti hot-spotting to compensate for optical vignetting, automated colorimitry adjustment between projectors, automated blending of overlap regions as well as managing trapezoidal and complex blend regions for systems with complex geometries such as spherical screens and domes.

4.3 Array Processor

Control Since the array processor and projectors are typically located in a different places, some form of remote control needs to be accessible at the eye point of the display. We have found that a laptop computer makes the most flexible user interface for the array control. Since most of the components of the system, including sources, array processor and projectors tend to be addressable via serial communication, it is easiest to integrate the various control software programs under one platform. Thus, it becomes possible to provide user setup procedures that address the appropriate components for each setup mode. As manufacturers standardize and publish control protocols, it becomes ever easier to integrate component controls into system controls under one user interface.
 

4.4 Projector Arrays

The actual projector arrays can consist of CRT projectors, Light Valve or LCD projectors. Using CRT projectors provides the most flexibility in terms of geometry control but limits each array element to the maximum size and brightness of CRT projection.

Using bright light valve projectors allows you to expand the overall size of the array, and since several light valve imaging schemes are driven by scanning CRTs, these models still allow for good geometry control of the image.

LCD projectors have the potential of offering a low maintenance and low cost solution for arrayed projection. This is appealing when you start to think of using projectors in larger and larger groups. LCD projectors however, have virtually no geometric controls. However, since they tend to be single lens devices (see Par. 3.5), this limitation may be overcome in pre-integrated arrayed systems where fixed custom optics can be applied.

There is another design consideration with both light valve and LCD projectors.
Somewhere in the crossover region, each projected image must be reduced to zero light output. CRT projectors can be tuned to provide true blacks. Light valves and LCDs however, do not provide a maximum density of true black, even when fed pure black video. We will call this residual light R. In the overlap region, the residual light is equal to 2R. In the four corner region of an XY matrix array, the residual light is equal to 4R. Since this residual light is mechanical and cannot be controlled via the video signal, it imposes a limitation on arrays using these technologies. This is balanced by the ambient light in the environment (which we will call Am) as it will raise the black pedestal of the entire display and thus mask the residual light (R). For adjacent geometries: 2R must be Am and for XY matrix geometries: 4R must be Am.

4.5 Screens

There is much to be said about screens and screen design, much of it beyond the overview scope of this paper. We will, however, address a key issue: Screen gain.

In applications where the projectors are spread horizontally and the viewing area is spread out, it is imperative to use a screen with a gain of 1.0! This design error appears in installation after installation promoted by the desire to deliver brighter images and the natural tendency to think that more is better. In this case, it is not. It is easy to understand when you consider the nature of passive gain achieved from a screen in context to a projector array.

Screen gain is achieved by optically bending the light hitting the screen back toward center. This is often accomplished by narrowing the viewing angle of the screen and redirecting the light toward the viewer - so what's the problem? This scheme only works well when your light source is a single point.

In horizontal arrays, however, your light source is spread horizontally across the entire screen surface. This means there is an extremely complex reflectivity relationship between all these sources and the viewers eye. It is possible to adjust all the colorimitry and edge blend parameters of the array to make a perfectly seamless and integrated image - in ONE location - but the moment the eye point is shifted left or right, all the reflectivity relationships change and brightness related aberrations are experienced in colorimitry and blend regions.

This problem can be minimized with curved screens if the projection design allows for the clustering of the light sources. Cross firing projectors such as in the diagram below are a good example. In such cases, a screen gain of up to 1.3 is acceptable. Higher gains are still not recommended.

In 360 degree applications with viewers in several locations such as with flight tower or ship simulators where the projector lenses are not clustered but arrayed in a circle, again a 1.0 gain screen is essential to creating a single composite image result for all viewers.

Other than the above consideration, arrays can be applied to flat screens, cylindrical curved screen, spherical curved or toroidal screen, front projection or rear projection.