Biomedical Engineering Reference
In-Depth Information
conversion to a digital image requires two steps: sampling and quantization. With the pixel array,
sampling and quantization occur in a CCD or CMOS image sensor automatically. Each pixel contains
specific information about intensity and can be described by a specific digital data value in a precise
location.
Each image pixel is represented by the intensity value and a coordinate pair with specific
x
and
y
values arranged in a Cartesian coordinate system. In many cases, the
x
location is referred
to as the pixel number and the
y
location is known as the line number. A digital image is
therefore a two-dimensional array of intensity values. For an image with a size of
M
pixels by
N
pixels, the pixel positioned at coordinates (1,1) is located in the upper left-hand corner of the
array, while a pixel located at (
M
,
N
) would be positioned at the lower right-hand corner of the
array.
Figure. 8.9
shows the schematic representation of the intensity array
I
(
x
,
y
)ofan
M
N
digital image.
The
aspect ratio
of a digital image is the ratio between the width and the height of the image
M
/
N
.
For instance, the recommended NTSC (National Television Systems Committee) commercial
broadcast standard aspect ratio for television and video equipment is
M
/
N
¼
4/3. The standard aspect
ratio for digital high-definition television (HDTV) is
M
/
N
¼
16/9, sometimes referred to as widescreen
format.
The
image resolution
determines the quality of a digital image. The image resolutions are referred
to as spatial resolution and gray-scale resolution. The spatial resolution is determined by the number of
pixels
M
N
. Within a given physical dimension, a digital image with a higher number of pixels will
have a higher spatial resolution. The gray-scale resolution is directly proportional to the bit depth of the
FIGURE 8.9
Schematic representation of the intensity array of an M N digital image.
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