DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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 45, I demonstrated the difference between
a box profile and a Gaussian profile. Figures 5.3 and 5.4
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 5.3 or 5.4 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 SDTV viewed at three times normal
distance – 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 DMD –
has a spot profile resembling a box distribution covering
nearly the entire width and nearly the entire height
corresponding to the pixel pitch. There is no significant
gap between image rows or image columns. Each pixel
has three color 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
48 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES