One of the chief goals of UHD was to increase the field of view, providing a more immersive experience. The optimal distance for standard definition is 6 picture heights (PH), for HD, it’s 3 PH and for 4K UHD it’s 1.5 PH. Edward Reuss, in a paper published in the SMPTE Motion Imaging Journal, discusses another potential benefit of displays that exceed the limits of human visual acuity: eliminating artifacts caused by sharp edges in the image, namely, aliasing and moiré patterns. These annoying artifacts, which can be seen in everything from striped shirts and herringbone jackets to picket fences, are the result of sampling theorem. Reuss explains:
“Spatial artifacts, such as stair-step edges and moiré patterns, occur as a consequence of the sampling theorem. To properly reconstruct an image from a rectangular array of uniform samples, the sample rate, meaning the inverse of the image sample interval, must meet the sample-rate criterion, which states that the sample rate must be at least twice as high as the bandwidth of the spatial energy encountered within the image. Sharp edges within an image may cause the high frequency spatial energy to exceed the Nyquist limit of half the sampling rate, causing spatial alias artifacts in the reconstructed image.” According to the author, applying an anti-aliasing filter before sampling, or resampling the image, is the sole means of eradicating spatial artifacts, but the remedy isn’t entirely free of compromises. Anti-aliasing filters soften the image, and some may prefer to deal with the artifacts rather than seeing a somewhat blurrier picture. Are spatial artifacts even a real concern? Reuss writes:
“In part, that depends on your audience. Spatial artifacts will occur from time to time unless they are filtered out somewhere in the workflow. The content creator or network engineer has to ask how sensitive is the audience to these spatial artifacts versus their sensitivity to the loss of edge sharpness. Will they object more to a fuzzy picture or to an occasional moiré effect in the on-screen talent’s necktie? A naïve audience may not notice the spatial aliasing artifacts at all, having been conditioned to seeing these effects as far back as the early days of color television. However, an audience that is more sophisticated may notice every time they see a jagged edge or a moiré pattern.” Which is where 4K television comes to the rescue:
“One way to “have one’s cake and eat it too” is to push the image resolution well beyond the limit of human acuity and then apply an anti-alias filter on the image in this “hyper-acute image space” that reduces the artifacts but keeps the blurring of the edges below the limit of the viewer’s visual acuity. UHD (4K) workflows may provide such a solution. In short, if we accept the proposition that the resolution of a typical size (40 in. to 65 in. diagonal) UHD (4K) television is finer than human visual acuity at normal living room viewing distances, then filtering the edges in the 4K images to meet the sample-rate criterion should result in edges that are still sharper than the visual acuity of the viewer.” As with most image artifacts, be it noise in the shadows or motion judder, aliasing is exacerbated by HDR WCG:
“The introduction of high-dynamic-range (HDR) and wide-color-gamut (WCG) formats will only enhance the impact of the spatial artifacts. Stair-step edges and moiré patterns that viewers might ignore at standard image intensities will leap out in HDR and WCG images. Properly shot and produced HDR and WCG images can greatly enhance the sense of realism, but the spatial artifacts can only distract from that realism. So HDR/WCG workflows will benefit the most from the use of anti-alias filters with 4K and 8K resolutions to maintain the sharpness of the edges on filtered images.”
Beyond the Limits of Visual Acuity: The Real Reason For 4K And 8K Image Resolution. Edward Reuss, SMPTE Motion Imaging Journal, March/April 2018
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