DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 5.8 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,
to be described in the
following chapter, the situation
is more complex.
jaggies”) will intrude. Limited performance of projection
lenses mitigates aliasing somewhat; however,
aliasing can be quite noticeable, as in the examples of
Figures 5.3 and 5.4 on page 45.
In a typical direct-view digital display, such as an LCD or
a PDP, each pixel comprises three color components
that occupy distinct regions of the area corresponding
to each pixel. Ordinarily, these components are side-byside.
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 colored 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 5.8.
• 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 scanline
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 serious
disadvantage to making the spot larger than necessary:
Sharpness is reduced.
CHAPTER 5 IMAGE STRUCTURE 49