About image
size and resolution
In order to produce high-quality images, it
is important to understand how the pixel
data of images is measured and displayed.
Pixel dimensions
The number of pixels along the height and
width of a bitmap image. The display size of
an image on-screen is determined by the
pixel dimensions of the image plus the size
and setting of the monitor.
For example, a 15-inch monitor typically
displays 800 pixels horizontally and 600
vertically. An image with dimensions of 800
pixels by 600 pixels would fill this small
screen. On a larger monitor with an
800-by-600-pixel setting, the same image
(with 800-by-600-pixel dimensions) would
still fill the screen, but each pixel would
appear larger. Changing the setting of this
larger monitor to 1024-by-768 pixels would
display the image at a smaller size,
occupying only part of the screen.
When preparing an image for online display
(for example, a Web page that will be viewed
on a variety of monitors), pixel dimensions
become especially important. Because your
image may be viewed on a 15-inch monitor,
you may want to limit the size of your image
to 800-by-600 pixels to allow room for the
Web browser window controls.
Image resolution
The number of pixels displayed per unit of
printed length in an image, usually measured
in pixels per inch (ppi). In Photoshop, you
can change the resolution of an image; in
ImageReady, the resolution of an image is
always 72 ppi. This is because the
ImageReady application is tailored to
creating images for online media, not print
media.
In Photoshop, image resolution and pixel
dimensions are interdependent. The amount of
detail in an image depends on its pixel
dimensions, while the image resolution
controls how much space the pixels are
printed over. For example, you can modify an
image's resolution without changing the
actual pixel data in the image--all you
change is the printed size of the image.
However, if you want to maintain the same
output dimensions, changing the image's
resolution requires a change in the total
number of pixels
When printed, an image with a high
resolution contains more, and therefore
smaller, pixels than an image with a low
resolution. For example, a 1-by-1-inch image
with a resolution of 72 ppi contains a total
of 5184 pixels (72 pixels wide x 72 pixels
high = 5184). The same 1-by-1-inch image
with a resolution of 300 ppi contains a
total of 90,000 pixels. Higher-resolution
images usually reproduce more detail and
subtler color transitions than
lower-resolution images. However, increasing
the resolution of a low-resolution image
only spreads the original pixel information
across a greater number of pixels; it rarely
improves image quality.
Using too low a resolution for a printed
image results in pixelation--output with
large, coarse-looking pixels. Using too high
a resolution (pixels smaller than the output
device can produce) increases the file size
and slows the printing of the image;
furthermore, the device will be unable to
reproduce the extra detail provided by the
higher resolution image.
Monitor resolution
The number of pixels or dots displayed per
unit of length on the monitor, usually
measured in dots per inch (dpi). Monitor
resolution depends on the size of the
monitor plus its pixel setting. Most new
monitors have a resolution of about 96 dpi,
while older Mac OS monitors have a
resolution of 72 dpi.
Understanding monitor resolution helps
explain why the display size of an image
on-screen often differs from its printed
size. Image pixels are translated directly
into monitor pixels. This means that when
the image resolution is higher than the
monitor resolution, the image appears larger
on-screen than its specified print
dimensions. For example, when you display a
1-by-1 inch, 144-ppi image on a 72-dpi
monitor, it appears in a 2-by-2 inch area
on-screen. Because the monitor can display
only 72 pixels per inch, it needs 2 inches
to display the 144 pixels that make up one
edge of the image.
SOURCE:
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