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6-2 Raster Graphics EN
There is a direct connection between the resolution and the size of an
image. The image size is the number of pixels in the image multiplied
by the number of bits used to represent a pixel. The smaller the dpi,
the smaller the size of the image; the larger the dpi, the larger the size
of the image. There is a tradeoff in image size versus print quality.
Larger pixels give poorer print quality since diagonal lines can start
looking ragged or pixelated, but the image size is small. Smaller
pixels give good print quality but larger image size. As a rule, 150 dpi
provides a happy medium of good print quality at a relatively small
image size.
A printer has a printing resolution, 600 dpi for example. An image with
an image resolution smaller than the printer's resolution will undergo
a process of scaling as the image is converted to the printer's
resolution, also known as device resolution. For example, a 150 dpi
image is converted to a 600 dpi image at device resolution by
replicating each pixel four times in the horizontal direction and four
times in the vertical direction. Therefore, a single pixel of a 150 dpi
image is replicated by creating 15 new pixels. A 75 dpi pixel is
replicated by creating 63 new pixels.
The large scale replication of pixels for low resolution images is one of
the main reasons for poor print quality: first the image detail may be
missing from the image because of the large pixel size, and second a
blocky, pixelated effect occurs when the pixel is reproduced over and
over again. Various techniques have been devised to compensate for
this pixelated effect. For example, rather than copy or replicate the
pixel, a bi-linear or bi-cubic interpolation algorithm could be used to
smooth out the transitions by considering the surrounding pixels when
making new pixels. However, this tends to blur edges or sharp
transitions that are in the original image. As a rule, better print quality
results from higher resolution images, such as 150 or 300 dpi, that
use smaller pixels and cause less pixel replication.
Pixels represent specific spots within the picture. In a black and white
image, where a pixel is either black or transparent (white), a single
binary digit or bit can represent a pixel. This has been the standard
raster format for many years. See Figure 6-2 for an example.
However, in color images, or grayscale images, those with varying
black dot sizes which yield a range of grays when viewed by the
human eye, a single bit is not sufficient. Therefore, the general form of
a pixel is:
• The number of components in the pixel, which is generally
either one or three, and
• The number of bits per component.