Configuring Soft- and Hard-Edge Arrays, Part I

An example of hard-edge matching on video cubes at the 2001 Academy Awards show.

Mapping the edges of multiple projectors to make them perform as a single, high-resolution, large-scale display offers a number of direct, application-enabling benefits. Whether the customer is seeking to dramatically increase resolution or brightness on a given screen size, reproduce material that requires unusual screen aspect ratios, create an image that is larger than available throw distance allows from a single projector, or simply punctuate a presentation with stunning visual impact, the tiling of projection systems presents a flexible and executable solution.

Tiling displays in matrix arrays actually started decades ago with multi-image slide presentations. The technique extended to videowall monitors more than 20 years ago, videowall cubes roughly 15 years ago, and in the new century, has progressed to high-performance projection systems of nearly all types. Regardless of the technology employed, if done correctly, displays organized in continuous matrix arrays present imagery in dynamic scale, with extraordinary resolution and presentation flair.

In this column, I'll discuss the differences between a hard-edge match and a soft-edge blend, and I'll also discuss some key performance factors that must be addressed when creating a hard-edge match or soft-edge blend. Next issue, I'll continue this topic with a specific example.

Hard-Edge or Soft-Edge?

When defining an array, the process generally begins by determining whether the use of a hard-edge match or a soft-edge blend is more appropriate for the application. In simple terms, a hard-edge match is the alignment of the last column (or row) of pixels of one projector to the first column (or row) of pixels of another projector. As a point of reference, all videowall cubes employ the hard-edge technique.

In hard-edge applications, the transition from one projector's image to the next literally takes place across just two pixels. Thus, a well-executed hard-edge match requires the projectors to exhibit perfect optical and mechanical geometric performance, as well as extremely flat luminance and color uniformity. Consistent color temperature from unit to unit is also an absolute requirement. Depending on the application, a small mullion can be placed between each projected image, providing a visual break between the displays and rendering the pixel-to-pixel transition a bit less critical.

Hard-edge is most appropriate when resolution cannot be sacrificed in the application or when the image content is conducive to an immediate and somewhat defined transition from one projected image to the next. Generally, the hard-edge technique is employed for data-intensive applications, such as process control, medical, and scientific environments.

By comparison, a soft-edge blend is created by overlapping the edge of one projected image on top of the edge of the adjacent projected image. Given the transition between each projected image is literally blended, the soft-edge technique is more forgiving than a hard edge match to small shortcomings in geometry, luminance uniformity, and color uniformity among projectors.

A soft-edge blend is most appropriately used when content is continuous across the entire screen, such as with motion video, high definition video, or when the transition from one projector to the next must be minimized. Properly executed, soft-edge blend applications provide maximum immersive impact.

Typically each overlap area can represent 5% to 20% of the total horizontal or vertical width of each projected image. The larger the overlap area, the more blended the transition from one projector to the next. The image generator providing the content must be configured to deliver outputs to each projector that compensate for the overlapped portions of the image. Image generators and software with this capability are available from such manufacturers as Dataton, Folsom, Panoram, Vista Controls, and SGI, among others.

To assure the brightness of the overlap area matches the brightness of the non-overlap areas of the screen, the image generators also include the capability to ramp down the contrast of image in the overlapping areas of each projector. Some image generators even provide the ability to ramp up the black level of each projector in the non-overlap areas of the image to match the subtle, dual black level in the overlap. While there are a few projection systems that include the contrast ramp capability, the image generators supplied by the companies referenced above provide greater control over the size, curve, colorimetry, and gamma of the contrast ramp. Consequently, the resulting soft-edge blend is more pleasing and visually believable.

Watching Details

There are a number of key performance factors that, when carefully managed, will vastly improve the result of soft-blend and hard-edge match applications. These factors include:

Individual projector performance: The projection system must employ technology that is inherently adept at producing imagery that exhibits exceptional luminance uniformity, color uniformity, and black level.

Commonality among all projectors: Beyond individual projector performance, as a group, the array projectors must demonstrate similar maximum brightness, native color temperature, and black level before any alignment is performed. Units with vastly different native performance will require significant adjustment of lamp power, contrast or colorimetry. Dramatic adjustments will result in loss of lumens, contrast ratio, and/or color depth.

Long-term stability: Each projector must consistently perform against the individual and common factors listed above to assure the continuous imaging quality of the array stands the test of time. The digital stability inherent in three-chip DLP products makes this the technology of choice for projection arrays.

Lamp performance and adjustability: The lamp technology employed by the projection system must demonstrate predictable lumen maintenance, broad color gamut, color stability, and reliability over time. The projector should also provide the ability to run the lamp at user-adjustable increments, so small lamp brightness adjustments can be made to keep the peak brightness and black level of the array matched across all projectors.

Lens Geometry: This is key for optimizing all array applications, but especially critical for hard-edge match. The geometric performance of the lens should demonstrate extreme accuracy (sub pixel for hard-edge match applications) with respect to horizontal and vertical bow, keystone, and pincushion. Chromatic magnification error must be less than .5 pixels across the entire image.

Screen performance (front screen): Often overlooked is the impact projection screens can have on uniformity. In front projection, short-throw distances (less than two times screen width) combined with screens with gains greater than 1.3 will result in some hot-spotting. The closer the throw distance and the higher the screen gain, the worse the hot spot becomes. Depending on the severity, this visual phenomenon may only be mildly noticeable on a single-unit projection application, but with an array, a series of hot spots destroys the impression that the screen is one, giant, high-resolution window.

Screen performance (rear screen): High gain (or optical) screens and short throw distances are generally not a good match. Digital Projection's experience is that flat, contrast-enhancing rear screen fabrics, such as Stewart's Aeroview 100, transmit uniform luminance with wide vertical and horizontal viewing angles. This is especially important for large arrays, where viewers may be positioned at wildly different viewing angles with respect to the various projectors. Whether front or rear screen, for soft-edge, it is preferable to use a single, seamless sheet for the entire array.

To best understand the differences between and hard-edge match and a soft-edge blend and how key performance factors can either make or break a great application, a real-world example is appropriate. Next issue, I'll describe such an example from conception to finished application.

Mike Levi is president of North American Operations for Digital Projection, Kennesaw, Ga., and is a veteran of the premium, large-screen display market. He can be reached at