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Anyplace Projection

David Colin, May 4, 2004

For years projection technology required the projector to be perpendicular to the screen. Move off the perpendicular and the familiar rectangular image became a trapezoid.

Lens shift was the first to cure off-angle projection using a mechanical shift of the projection lens to "square" the image. The problem with lens shift is that it adds cost and introduces a bit of fragility into the design.

Then along came vertical digital keystone correction with an electronic solution to squaring the image if the projector was too high or too low relative to the screen. This solution was slick, reliable, and economical.

Recognizing that people would like to place a projector left or right of the screen, horizontal digital keystone correction was added and voilà - you could now place the projector anywhere (usually within a 30° angle from the screen).

Freedom at last? Not quite! There is a dark side . . . literally.

Problems with Off-Angle Projection

Three problems emerge with extreme off-angle projection. First and most annoying are the scaling artifacts that are ever present in a scaled image and make the image appear out of focus. You may recall the first commandment of projection - "Thou shalt match the resolution of thy projector and source." Violating the first commandment of projection won't cause you to lose your seat in the hereafter, but it might put the journey out of focus. Digital keystone correction is essentially a modified scaling. It takes the long side of the trapezoid and shrinks it to match the short side.

This brings us to the second point and the darker side of it. When you scale the image within the resolution of the display, you essentially don't use part of the display. This means any light that is delivered to the unused portion is lost. In extreme keystone correction you can be at angles up to 40°. This can cause you to lose access to as much as a third of your usable display after keystone correction, which means 1/3 of your resolution is given up and about 1/3 of your light output is lost.

And finally, another dark side issue is when you're throwing light at extreme angles onto the projection screen. Depending on the type of screen you're using, this will also cause you to lose light as screens return the most light when the projector is perpendicular to the screen.

But fear not, you can manage much of this light loss by your choice of screens, starting with more lumens in the projector, or projecting at shallower angles. And although today's projectors do a better job of distributing light evenly on the screen, the truth is, the perimeter of the image tends to have the least light, which is the first light that you give up when doing digital keystone correction.

Back to Lens Shift

Like digital keystone correction, lens shift will also lose light at off-angle projections to a screen. However, unlike digital keystone correction, lens shift will utilize the entire display and thereby avoid loss of display resolution and light output from the display. The bottom line is lens shift is still the better solution for keystone correction, but you still need a scaling solution when you're handling sources that don't match the resolution of your projector.

New Technology

So where is this heading? In late 2002 NEC introduced 3D Reform and made a measurable improvement in reducing scaling artifacts when using digital keystone correction and projecting resolutions other than the resolution of the projector. They also introduced extreme off-angle support of ±40° vertical and ±35° horizontal. The resulting extreme off-angle projection was now more extreme and more readable giving users greater anywhere freedom than at any time in the past. NEC was now a "scale" ahead of the competition.

Today, a year and a half later, Silicon Optix, a company that specializes in the convergence of silicon and optics, sees an opportunity for a better solution. Their primary focus has been the development of heads-up displays for military applications and the development of the sxW1-LX AnyPlace and eWarp image processor that offered 45° horizontal and 40° vertical keystone correction as well as lens, curved, and domed distortion correction simultaneously. This chip was used by high-end system companies like Folsom Research for rental and staging markets, 3M for the Wall Display, and NexGen for Home Theater projection.

They are now into their second generation of the technology and have repackaged it into an economical system-on-a-chip (SoC) solution known as Reon-GXTM for projection applications. The SoC includes the Silicon Optics AnyPlaceTM technology that provides ±40° horizontal and ±35° vertical digital keystone correction, tilt/rotation correction, as well as, lens throw ratio and distortion correction, thus opening up the possibility of simpler lenses, lower product cost, and even correcting a defective lens. Where were they when the Hubble needed help?

Taking a Closer Look

We had a private viewing of the technology at their facility last month and for this hardened critic of digital scaling, I must admit that it was very good. It isn't the quantum quality leap we got when digital video and data arrived on the scene to replace analog, but it did solve most of the jaggies and the out-of-focus look that was common with scaled text images. The horizontal lines that might have disappeared in some scalers do have some jaggies, but vertical lines are clean and well-defined and most important, text is very readable and no longer the visual distraction it once was.

Below are some images provided by Silicon Optix that show spreadsheet data using 27° horizontal digital keystone correction using the Reon-GXTM and the same data using older scaler technology. We saw comparable results in testing at their facility with live data. We did not get a chance to compare it against the NEC 3D Reform; however, we did look at the NEC 3D Reform after the meeting at our facility. We acknowledge that the only way to compare projectors and certainly esoteric technologies like scaling is to do it side-by-side. With that understanding and based on what we observed, the NEC 3D Reform performance would be very close to the Silicon Optix Reon-GXTM, although we would give the edge to Silicon Optix.

Silicon Optix Reon-GX older scaling technology

The Silicon Optix Reon-GXTM also includes video and image processing for pixel motion detection and adaptive video de-interlacing. It is competitive with Faroujda's DCDiTM, but is not as comprehensive. It also provides 3:2/2:2 pull-down detection and dual independent video processing streams allowing picture-in-picture.

The first projectors using the Silicon Optix technology are expected to begin shipping this year. So if anyplace projection is important to you, things are about to get a lot better. Stay tuned. Perhaps we'll see some surprises at INFOCOMM next month.