the blurring to be detrimental, but to avoid spatial aliasing in capturing high-quality continuous-tone imagery, some overlap is necessary in the distribution of sensitivity across neighboring sensor elements.
Having introduced the aliasing artifact that results from poor capture PSFs, we can now return to the display and discuss reconstruction PSFs (spot profiles).
Spot profile
The designer of a display system for continuous-tone images seeks to make a display that allows viewing at a wide picture angle, with minimal intrusion of artifacts such as aliasing or visible scan-line or pixel structure. Picture size, viewing distance, spot profile, and scan-line or pixel visibility all interact. The display system designer cannot exert direct control over viewing distance; spot profile is the parameter available for optimization.
On page 77, I demonstrated the difference between a box profile and a Gaussian profile. Figures 7.3 and 7.4 showed that some overlap between neighboring distributions is desirable, even though blur is evident when the reproduced image is viewed closely.
When the images of Figure 7.3 or 7.4 are viewed from a distance of 10 m (33 feet), a pixel subtends a minute of arc (1⁄60°). At this distance, owing to the
limited acuity of human vision, both pairs of images are apparently identical. Imagine placing beside these images an emissive display having an infinitesimal spot, producing the same total flux for a perfectly white pixel. At 10 m, the pixel structure of the emissive display would be somewhat visible. At a great viewing distance – say at a pixel or scan-line subtense of less than 1⁄180°, corresponding to SD viewed at three times normal distance, or about 20·PH – the limited acuity of the human visual system causes all three displays to appear identical. As the viewer moves closer, different effects become apparent, depending upon spot profile. I’ll discuss two cases: box distribution and Gaussian distribution.
Box distribution
A typical digital projector – such as an LCD or a PDP – has a spot profile resembling a box distribution covering nearly the entire width and nearly the entire height
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Figure 7.8 Gaussian spot size.
Solid lines graph Gaussian distributions of intensity across two adjacent image rows, for three values of spot size. The areas under each curve are identical. The shaded areas indicate their sums. In progressive scanning, adjacent image rows correspond to consecutive scan lines. In interlaced scanning, the situation is more complex.
corresponding to the pixel pitch. There is no significant gap between image rows or image columns. Each pixel has three colour components, but the optics of the projection device are arranged to cause the distribution of light from these components to be overlaid. From
a great distance, pixel structure will not be visible. However, as viewing distance decreases, aliasing (“the jaggies”) will intrude. Limited performance of projection lenses mitigates aliasing somewhat; however, aliasing can be quite noticeable, as in the examples of Figures 7.3 and 7.4 on page 77.
In a typical direct-view digital display, such as an LCD or a PDP, each pixel comprises three colour components that occupy distinct regions of the area corresponding to each pixel. Ordinarily, these components are side-by-side. There is no significant gap between image rows. However, if one component (say green) is turned on and the others are off, there is a gap between columns. These systems rely upon the limited acuity of the viewer to integrate the components into a single coloured area. At a close viewing distance, the gap can be visible, and this can induce aliasing.
The viewing distance of a display using a box distribution, such as a direct-view LCD or PDP, is limited by the intrusion of aliasing.
Gaussian distribution
As I have mentioned, a CRT display has a spot profile resembling a Gaussian. The CRT designer’s choice of spot size involves a compromise illustrated by Figure 7.8.
•For a Gaussian distribution with a very small spot, say a spot width less than 1⁄2 the scan-line pitch, line structure will become evident even at a fairly large viewing
distance.
•For a Gaussian distribution with medium-sized spot, say a spot width approximately equal to the scan-line pitch, the onset of scan-line visibility will occur at
a closer distance than with a small spot.
•As spot size is increased beyond about twice the scan-line pitch, eventually the spot becomes so large that no further improvement in line-structure visibility is achieved by making it larger. However, there is a ser-
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ious disadvantage to making the spot larger than necessary: Sharpness is reduced.
You saw at the beginning of this chapter that in order to avoid visible pixel structure in image display some overlap is necessary in the distributions of light produced by neighboring display elements. Such overlap reduces sharpness, but by how much? How much overlap is necessary? I will discuss these issues in the Chapter Resolution, on page 97. First, though, I will introduce the fundamentals of raster scanning.
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