DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Resolution properly refers to spatial
phenomena. It is confusing to refer
to a sample as having 8-bit “resolution”;
use precision or quantization.
20
40
80
160 C/PH
PH
10
Figure 7.2 Resolution wedge
pattern sweeps various horizontal
frequencies through an
imaging system. This pattern is
calibrated in terms of cycles per
picture height (here signified
PH); however, with the pattern
in the orientation shown, horizontal
resolution is measured.
zontal pixels in the pixel array, without regard for any
overlap at capture, or overlap at display, that may have
reduced the amount of detail in the image. A system
may be described as having “resolution” of 1152×864 –
this system has a total of about one million pixels (one
megapixel, or 1 Mpx). Interpreted this way, “resolution”
doesn’t depend upon whether individual pixels
can be discerned (“resolved”) on the face of the display.
Resolution in a digital image system is bounded by the
count of pixels across the image width and height.
However, as picture detail increases in frequency, electronic
and optical effects cause response to diminish
even within the bounds imposed by sampling. In video,
we are concerned with resolution that is delivered to
the viewer; we are also interested in limitations of
bandwidth in capture, recording, processing, and
display. In video, resolution concerns the maximum
number of line pairs (or cycles) that can be resolved on
the display screen. This is a subjective criterion! Resolution
is related to perceived sharpness.
Resolution is usually expressed in terms of spatial
frequency, whose units are cycles per picture width
(C/PW) horizontally, and cycles per picture height
(C/PH) vertically, or units closely related to these.
Figure 7.2 depicts a resolution test chart. In the orientation
presented, it sweeps across horizontal frequencies,
and can be used to estimate horizontal resolution.
Turned 90°, it can be used to sweep through vertical
frequencies, and thereby estimate vertical resolution.
Resolution in video
Spatial phenomena at an image sensor or at a display
device may limit both vertical and horizontal resolution.
However, analog processing, recording, and transmission
in video limits bandwidth, and thereby affects
only horizontal resolution. Resolution in consumer electronics
refers to horizontal resolution. Vertical resampling
is now common in consumer equipment, and
this potentially affects vertical resolution. In transformbased
compression (such as JPEG, DV, and MPEG),
dispersion comparable to overlap between pixels
occurs; this affects horizontal and vertical resolution.
CHAPTER 7 RESOLUTION 69