SubscribeModern training simulators offer breathtaking realism thanks to the latest projection technology. But advances create problems too. Robert M. Clodfelter, David Sadler and Johan Blondelle of Barco Simulation describe the dilemmas of tiling multiple projectors.
As image generators (IG) become more powerful and affordable, simulation imagery reaches even higher degrees of realism. But large fields-of-view and high image resolutions demand pixel counts that far exceed the ability of a single projection display. A hemisphere filled to eye limiting resolution requires 95M pixels.
It is therefore necessary to devise techniques to seamlessly tile images from multiple projectors. However, large field-of-view images formed by tiling multiple projectors may exhibit geometric distortions caused by system optics, projector misalignment, imperfect lenses and the displays themselves. Uncorrected tiled images may also have luminance and color variations both between and within each projector channel. By careful consideration and attention to the system design these effects can be controlled and a seamless tiled image created.
Figure 1: Three-channel tiled display system used for air traffic control (ATC) simulation. Copyright 2002, National Aerospace Laboratory, Amsterdam, The Netherlands
Tiling
Requirements for large-field-of view, high-resolution displays are diverse. Applications require a number of different system configurations such as front or rear projection and screen shapes, such as spherical, cylindrical, conic, or flat. Screen sizes range from as small as 1m x 1m to a huge 3m x 20m flat, or even 30m diameter hemispherical domes. Users may require high brightness, high contrast or both. The system may be for a single user or for multiple simultaneous users. Whatever the variation, the tiling problem remains substantially the same.
Perfect projectors do not exist; if they did, no one could afford them. Fortunately modern projection devices maintain an exquisite balance between performance, features, and cost. When used singularly, images appear very good and design compromises are virtually unnoticeable to even the trained observer. However, when projectors are used side by side, the most casual observer notices that the images do not properly register, their colors do not match, and they are probably not the same brightness or color. Many of these artifacts are caused by the projectors themselves, not how they are used.
The best approach to eliminating these artifacts is to select projectors that have the appropriate combination of features and design trade-offs to avoid them in the first place. To correct geometry problems, look for projectors that have low distortion lenses. If the projector is to be used off axis, look for a design that allows for horizontal and vertical optical lens shift as needed. Matching image geometry is only part of the solution; colorimetry and luminance between projectors must also match.
Variations in the color spectrum of the lamps used, lamp aging, dichroic mirror tolerances, illumination system design, poor gamma correction, and vignetting in the lens cause color and luminous variations between matrix projectors. Each variation can be controlled usually at additional cost to the projector unit. Projector selection should be made with regard to total system cost and not just projector unit cost as correcting these variations outside of the projector may be more expensive than using a projector that has been designed to minimize these variations.
Possibilities
How many projectors should a tiled system use? As few as possible. Pixels are expensive, even commodity pixels. To properly answer this question the desired resolution, screen size and shape must be taken into account, as well as the tiling configuration.
The projectors should be arranged to optimally fill the screen area with a channel-to-channel overlap of 5% to 20%. This means for every pixel bought, up to 20% are duplicated in the overlap areas to create a smooth transition from one projector to the next. Take care as some "low cost" tiling solutions may use overlaps exceeding 50%. Such an extremely large overlap helps to mask the many luminance and chrominance errors of the "low cost" projectors at the expense of doubling the number of projectors needed and halving the system reliability.
Figure 2: Three-channel tiled display system showing projectors, stands, cylindrical screen and overlap areas.
CRT
Many compatible projection technologies exist to create tiled displays. The oldest is based on the cathode ray tube (CRT). CRT projectors offer extremely high quality images at moderate cost. Their drawbacks are that they are not very bright, may require daily tuning, tend to be large and, if uncorrected, do not have uniform luminance across their display field. Their cost per pixel is very competitive and research efforts have given rise to features like beam astigmatism correction for sharp focus and digital convergence and geometry control for precise convergence and geometry.
Resolutions to 3,200 by 2,560 pixels and video bandwidths up to 180MHz are currently possible. Inherent to CRT technology is that the image is always real-time without any latency; an important concern for man-in-the-loop training. CRT displays also have the advantage that image warping is done without video resampling or scaling.
Figure 3: Typical Level D simulator image using both raster and calligraphic scanning techniques at French Navy E-2c program. Photo courtesy CAE.
When used in higher ambient light environments, tiled projection systems are designed with matrix light modulation technologies such as poly-silicon liquid crystal display (LCD), or micro-electro-mechanical systems (MEMS) using mirrors. Poly-silicon modulators are available from many suppliers. All commercially available MEMS based projectors are based on Texas Instruments' digital light processing (DLP) technology. By separating the light source from the modulator, LCD and DLP technologies enable far higher light outputs than CRT. Unlike CRT and dedicated simulation developed LCD projectors, DLP projectors suffer from a one or two field latency that may be a concern for man-in-the-loop simulation applications.
Color matching
In all projectors it is important that the color and luminance of the red, green, and blue light is matched between projectors. A solution is to electronically or optically stabilize the lamp output in each projector. This method also reduces the effect of light output variations due to lamp aging. Further, the red, green, and blue video can be electronically color space corrected for inter-projector variations in source light color. Combined, these two capabilities make tiling possible for projectors using conventional illumination systems.
Warping
Tiled systems using flat screens can be designed and built without image warping (distortion correction) capabilities. By avoiding warping, system cost and latency may be reduced. Systems employing curved screens or off-axis optical designs beyond the capability of lens shift to compensate, necessarily require some form of image warping.
When warping is available, tolerances required to position and optically match projectors are drastically reduced. Images may be warped inside the image generator, by an external video processing box, or by the projector itself. Warping circuits may use bi-linear or bi-cubic interpolation methods to calculate each displayed pixel. Bi-cubic interpolation produces an image with more fidelity and resolution than bi-linear methods.
Blending
Once the projectors are positioned and aligned, their images must be blended together to form one large seamless image. The general technique of edge blending is to overlap the edges of adjacent projected images and blend the overlapped regions to smooth the luminance and chromaticity transition from one image to the other. Blending methods fall into two major categories: optical, and electronic. Optical blending uses either hard edges or gradients to produce the luminance roll off required in the overlap region.
The edges or gradients may be mounted internally to the projector or externally in the light path. Optical blending solutions have the advantage that they work well with matrix displays because they blend both the video image and the minimum leakage light level the projector can produce. As these solutions are manufactured using photographic or lithographic methods, they are generally not easily adjustable and may have lengthy iteration cycles between trials.
Figure 4(top): Optical blend situation showing roll off of white and black levels.
Figure 5(bottom): Electronic blend situation showing roll off of white levels and compensation of black levels.
Alternatively, electrical blending can be applied to the video by circuitry inside the projector, in an external video processing box, or in the image generator itself as an alpha channel. Electrical blending is desirable as it is easily controlled, conducive to automation, and produces generally good results.
However, electrical blending fails miserably for very dark, near black images when used in conjunction with LCD or DLP projectors due to light leakage of these projectors when in the black state. Electrical blending's flexibility and automation possibilities make it compelling; however, the system designer should take care for how the system will be used and under what conditions before proposing its use.
Dedicated Systems
As no single tiled display system for simulators can be considered the best in all circumstances, dedicated solutions are now available for full flight simulators, air traffic control simulators, ship-bridge simulators, and gunnery simulators. Tiled flat wall, cylindrical, and spherical systems are now commonplace. Software efforts that allow applications to transparently use multi-channel tiled display systems without modification further drive this phenomenon.
Projection displays designed specifically for tiling are now available. Features such as luminance stabilization, color matching, warping, edge blending, and luminance and color uniformity correction make high quality tiling possible while eliminating the need for additional video processing boxes.
These projectors when coupled with appropriate application software, human interface devices, and cluster management tools, make simulation, collaboration and visualization systems that are unachievable by any competing technology.
Figure 6: A three-channel front projection cylindrical screen system using single-chip DLP projectors with internal warping and external optical blending.
Figure 7: Helisim, France. Barco Genenis III calligraphic technology for Level D full flight simulation.
