Discussion of the Term Resolution The term Resolution has
various meanings depending upon the context in which it is used.
This article will use scanners as input devices to illustrate
different types of resolution. First, I will discuss raster images
and resolution. A scanner takes an "electronic picture" of an
image. The image can be reflective (paper) or transparent (film).
Since this is a type of picture, it captures the image in bitmap or
raster form. This form means that the output image is made up of
pixels the same pixels viewed in Photoshop or Paint Shop Pro.
Each pixel has a color depth. Usually one decides on how the image
will be used, and then, on what output resolution is necessary.
There used to be different output numbers such as 300, 200, etc. in
manuals. Now, they have been replaced with terms such as good,
better, best. These terms are ambiguous. Normally, if one is
scanning for print and will use the whole image, 300 dots per inch
(dpi) is a good choice. If one is scanning for the web, 72 dpi,
also called screen resolution, is a good choice. Scanners are
constructed employing two types of scanning resolution, hardware
and software. Hardware resolution is the optical resolution. This
means that the resolution is set through the optics of the scanner.
One could call this a "pure" resolution. The other type of
resolution that comes with a scanner or can be bought separately as
a scanning package is software resolution. This starts where
optical resolution leaves off. This type of resolution can be
called interpolation, resampling, or upsampling. I will discuss
this later. Various terms are also thrown around having to do with
image resolution. These are ppi, spi, dpi, etc. ppi and spi - When
scanners first became available to the home user, one could choose
to scan through and specifying an input resolution. The term ppi
means pixels per inch and spi means samples per inch. If one had a
4" x 5" picture, and one wanted an 8" x 10" image at 300 dots per
inch, the input would have had to be 600 ppi or 600 spi, because
the size of the output image would have been doubled. Now we rarely
use that term in that context, and think of output resolution or
dpi, which means dots per inch from printer terminology. Presently,
the size of the input image is not as relevant. On most scanning
devices, one can just put the output resolution desired and the
computer software does the work. If you take a bitmap or raster
image and magnify it, you will see a series of squares. These are
pixels. They contain RGB information. The amount of the information
is called bit-depth.
When one starts a new image in Photoshop, or any other image
editing or painting program, one can open a blank canvas and set
the output image resolution and either the linear size in inches,
millimeters, or the pixel linear dimensions. Depending on the pixel
dimensions, the color depth, and a few other factors, the size of
the image will be determined. Notice in the screen capture below
the words: Image Size: 4.29M.
One can then open this blank image in Photoshop and see all the
information.
The other alternatives: input an image through inputting devices
such as scanners or digital cameras; or through exporting an image
from a vector based program and changing that vector image to a
raster image. However since many other factors are involved when
formats are changed, I will not discuss the latter operation in
this article.
Let's assume that one has either created [or input an image] and
its dimensions are 1/4" x 1/4" with a dpi of 72. The image is
finished, but one wants to increase its linear dimensions to 1/2" x
1/2" and keep the dpi at 72. What does one do? One has to add
pixels. Photoshop gives a choice on how one will either
interpolate, upsample, or resample the image. All terms are used
interchangeably. While interpolating is accomplished very well now, the
software still has to add pixels. The best way to understand this
is to look at real images such as the bead on the upper left. That
is why I am using such absurdly small sized images. The original
size of the bead above was a little more than 1/4" or 20 pixels.
And the resolution was 72 dpi. Look at the colors of the bead.
In order to enlarge the image of the bead, not magnify it, but
virtually enlarge it, I had to use interpolation. It was enlarged
to 40 pixels wide, keeping the dimensions constant which turned
out to be a little over 1/2" x 1/2". Had I enlarged it to exactly
1/2", the pixel linear dimensions would have been different.
Again, notice the colors in the bead. And remember that the
software had to figure out the exact colors to add. What was the
point of this whole exercise? Just look at the two pictures of the
beads and ask yourself how the colors for the additional pixels
were chosen. Are they positively accurate? Basically, using good
programs like Photoshop, for most work, one can resample upwards or
sample up carefully as long as one keeps in mind that software is
adding pixels. Where it can become difficult is with color
separation printing; where one is working with a 72 dpi image; and
it has to be made into a 300 dpi image; and a lot of pixels have to
be added. A program such as Genuine Fractals 4 Pro uses a totally different
method of increasing the number of pixels and is safer to use than
trying to do it oneself if the increase is a lot. However, if an
image is going to be printed using color separation, and requires a
lot of interpolation, the best bet is to speak to whomever is doing
the final output and let them advise you on what to do.
All supporting images are copyright, and
cannot be
copied, printed, or reproduced in any manner without written
permission from the artist.
- The Paula Sander's Report is a regular
Renderosity Front Page featured column, where Paula investigates
and comments on graphic software, techniques, and other relevant
material through her reviews, tutorials, and general articles.
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