DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 5.2 Gaussian reconstruction
is shown for the
same bitmapped image as
Figure 5.1. I will detail the
one-dimensional Gaussian
function on page 150.
I introduced visual acuity on page 8.
For details, see Contrast sensitivity
function (CSF), on page 201.
horizontal “stripes” is part of a window’s titlebar; the
checkerboard background is intended to integrate to
gray. Figure 5.1 shows reconstruction of the image with
a “box” distribution. Each pixel is uniformly shaded
across its extent; there is no overlap between pixels.
This typifies an image as displayed on an LCD.
A CRT’s electron gun produces an electron beam that
illuminates a spot on the phosphor screen. The beam is
deflected to form a raster pattern of scan lines that
traces the entire screen, as I will describe in the
following chapter. The beam is not perfectly focused
when it is emitted from the CRT’s electron gun, and is
dispersed further in transit to the phosphor screen.
Intensity produced for each pixel at the face of the
screen has a “bell-shaped” distribution resembling
a two-dimensional Gaussian function. With a typical
amount of spot overlap, the checkerboard area of this
example will display as a nearly uniform gray as
depicted in Figure 5.2 in the margin. You might think
that the blur caused by overlap between pixels would
diminish image quality. However, for continuous-tone
(“contone”) images, some degree of overlap is not only
desirable but necessary, as you will see from the
following examples.
Figure 5.3 at the top of the facing page shows a 16×20pixel
image of a dark line, slightly more than one pixel
wide, at an angle 7.2° off-vertical. At the left, the image
data is reconstructed using a box distribution. The
jagged and “ropey” nature of the reproduction is
evident. At the right, the image data is reconstructed
using a Gaussian. It is blurry, but less jagged.
Figure 5.4 in the middle of the facing page shows two
ways to reconstruct the same 16×20 pixels (320 bytes)
of continuous-tone grayscale image data. The left-hand
image is reconstructed using a box function, and the
right-hand image with a Gaussian. I constructed this
example so that each image is 4 cm (1.6 inches) wide.
At typical reading distance of 40 cm (16 inches), a pixel
subtends 0.4°, where visual acuity is near its maximum.
At this distance, when reconstructed with a box function,
the pixel structure of each image is highly visible;
44 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES