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Digital images are made up of bits of
a scene packed together to crate a representation of the original
much like the dots of ink in the pictures in a magazine. The
descriptions below are intended to briefly explain image creation
and how image resolution varies with various imaging methods. |
 ► Raster
graphics, also known as bit-mapped graphics, are created by
scanners and digital cameras. |
 ► Raster
images are "painted" across the computer screen in an array of
square elements called pixels. Pixel is short for picture element. |
 ► Each
pixel is stored in an area of memory called a bit-map. Each pixel
has a numbered address. |
 ► The
quality of a raster image is determined at capture by two factors:
spatial resolution and brightness resolution. |
 ► Pixel
size is determined by the rate at which the camera or scanner
sample the image. A long sampling interval produces an image low
in spatial resolution. A shorter interval produces higher spatial
resolution. |
 ► The
brightness or color value of each pixel is defined by one bit or
by a group of bits. The more bits used, the higher the brightness
resolution. |
 ► A 1-bit
image can have only 2 values, black or white. 1-bit images
simulate grays by grouping black and white pixels. This process is
called dithering or halftoning. An 8-bit gray-scale image displays
256 levels of brightness. Each pixel is black, white or one of 254
shades of gray. |
 ► A higher
resolution 12-bit medical image provides more than 4000 brightness
levels. |
 ► In a
24-bit image, each pixel is described by three 8-bit sets of
numbers representing the brightness values for red, green and
blue. High resolution 24-bit images display 16.7 million colors. |
 ► Each
pixel in a 24-bit image has one of 256 brightness values for red,
green and blue. |
A Short Answer To Resolution
All digital images are made up of tiny squares called pixels,
and each pixel has a specific location, size and a very specific
color value. Digital cameras capture images at 72 “pixels per
inch” (ppi) multiplied by the number of pixels present on the
image sensor. Scanned images and image editing software allow
you to choose your resolution. The pixel size is determined by
the resolution of your image and is actually inversely
proportional to resolution. In a 24-bit (RGB) color image, there
are 256 possible shades of red, green, and blue mixed to
represent the color of each individual pixel. 256 shades of red,
green, and blue can be combined to produce a total of 16.7
million different colors. 16.7 million colors are sufficient for
photo-realistic output.
Printer resolution is given as the number of “dots per inch”
(dpi). Dpi is the maximum number of ink dots that the printer is
capable of placing on your paper per inch. Photo-quality ink-jet
printers spray ink through multiple nozzles to produce various
sized droplets with variable droplet spacing. When the droplets
are sprayed onto paper the drops overlap, combine, or are spaced
so as to reproduce the color of the individual pixel being
printed. The printer may have to spray a drop of all six colors
on top of each other to generate a dark color. In other words,
it takes multiple drops of ink from the printer to create the
color in each square pixel. So even if the printer resolution is
given as 1440 dpi it does not mean that there are 1440 dots
lined up in a row in each inch of the print. Much to the
surprise of most people, the printer’s resolution numbers have
nothing to do with the size of, or the number of pixels in your
image. Pixels per inch (ppi) and dots per inch (dpi) refer to
totally different things.
It would not be advisable to send your printer a file with the
camera or input resolution being the same as the stated printer
resolution. In fact, you only need to send a file closer to 240
- 300 pixels per inch. This solution is ideal, as this allows
the |
printer maximum flexibility in
combining the ink colors to generate the color of each pixel. If
you were to send an image sized at 1440 ppi to your printer it
would simply throw out most of your data. The worst part is that
in the printer driver interpolation, you have no idea which
pixels the printer (and its software) will decide to get rid of,
and your printed results would be totally unpredictable.
Images that will be viewed on a computer monitor such as those
destined for e-mail or display on the worldwide web should be
sized to a resolution of 72 ppi. This is because image pixels
will be translated directly into monitor pixels. The size at
which the image will actually be displayed is dependent on the
size of the viewer’s monitor, the monitor’s resolution setting,
and the actual pixel resolution of the monitor. When the image
resolution is higher than the monitor resolution, the image will
be displayed on screen at a larger size than the print size.
Therefore viewer would have to scroll all around to view the
image, which would be inconvenient. For example, if you want to
e-mail an image and be certain that it does not exceed the size
of the recipient’s monitor you would size it for 4x6 inches and
set the resolution to 72 ppi. If the recipient’s monitor is a
72-ppi monitor (Mac) the image will display at 4 x 6 inches, and
if they have a 96-ppi monitor (PC) the image will display at 3 x
4.5 inches. Both image sizes would easily fit on a small monitor
without having to scroll to see the whole image.
Image resolution may be set, changed, or viewed in any editing
software program, such as Adobe’s Photoshop. Photoshop is
expensive and it requires a long learning curve to operate, but
it is the worlds most often used editor. Editing software is
where all color, cropping and repair decisions are made. It is
also where resolution is determined for web use, e-mailing, or
printing.
Remember to keep your “ppi” separate from your “dpi” and imaging
will be much smoother. |
|
Some information presented here courtesy
Eastman Kodak Company. © Eastman Kodak Company, 1994-2003 |
|
WEB Links for
Online Digital Photography Info & Prints |
1.
Kodak Picture Center Online
2. Photo Center-Epson
3. SnapFish.com
4. PictureTrail.com
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5.
Eframes.com
6. Seemesmile.com
7. ClubPhoto.com
8. PhotoAccess.com
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9. PhotoWorks.com
10. Shutterfly.com
11.
Ofoto.com |
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