Determining Scanner Resolution:
1) Multiply the specified finished
graphic dimension by the desired print
resolution to determine the TOTAL pixels
required for correct sizing.
2) Divide the total pixels by the input
dimensions to determine the scan PPI
(pixels per inch) setting to use on the
scanner.
3) Set the scanner ppi (dpi) resolution.
Use the next higher resolution if the
scanner cannot be set to the exact
resolution needed.
All our images are saved and
manipulated, scanned and scaled to size
using millimetres. Dots per inch is a
common term but not used in Australia.
DPI Dots per inch
Printer ink dots and image pixels are
very different concepts, but both use
the term dpi in their own way.
People sometimes initially assume that
because inkjet printer ratings use dpi,
then therefore image pixel spacing MUST
be called ppi, to distinguish them
differently. These are indeed different
concepts, but while ppi is fine as a
personal preference for resolution if
you wish, to imagine it is a rule
ignores the vast bulk of existing real
world practice.
Some literature does attempt to reserve
use of the term dpi to be used
exclusively for the ink dots made by
inkjet printers, because that's so
different from everything else. Then we
hear scanner resolution called spi
(Samples Per Inch), and that is indeed
what it is (we call these color samples
"pixels"). We hear image resolution
called ppi (Pixels Per Inch), and that
is indeed what it is. The spi and ppi
usage is precisely correct.
But pixels are a kind of colored dot too
(more a concept than a physical colored
dot), and the term dpi is absolutely
correct for "pixels per inch". This is
true because dpi is in fact the printing
industry's long established name for the
spacing of image pixels on paper. Its
origin may be in jargon, but dpi has
always been the proper name for the term
"pixels per inch".
When referring to images, or to printing
images (as opposed to inkjet printer
ratings), we can treat these three
terms, spi, ppi, and dpi, as the same
equivalent concept - all three simply
refer to image pixels per inch - the
spacing of image pixels on paper. You
may use your own preference, dpi or ppi,
but you must understand it either way,
because you are always going to see it
both ways.
The term pixel is a computer
abbreviation for picture element, the
smallest element of a digital image.
Pixels are a very different concept than
the ink dots that inkjet printers can
make. The idea of printing these pixels
in digital images is always about pixels
per inch on paper. All three terms (dpi,
ppi, spi) are the same image pixels per
inch - the spacing of image pixels on
paper. Spi and ppi are excellent tries
in concept, and are good names too,
there is nothing wrong with them, but
the fact remains that common and
historical usage has always called it
dpi.
So yes, inkjet printer rating dpi is
something entirely different, referring
to inkjet printer ink dots instead of
image pixels. The inkjet printer tries
to simulate the color of one 250 dpi
pixel by making several ink dots of four
CMYK ink colors, which are located
perhaps on 1200 or 1440 dpi spacing (See
the Printer Basics section). The printer
is trying the best it possibly can to
reproduce the pixels (pixels is all
there is), but inkjets cannot reproduce
colored pixels directly. Image pixels
and inkjet printer ink dots are NOT the
same thing at all.
However, continuous tone printers
(dye-subs, and Frontier-Noritsu-Lambda-Lightjet
types) don't print discrete ink dots of
three colors like inkjet printers must -
instead they mix the color of the pixel
directly. There are no dithered ink dots
then. But these printer ratings still
refer to the spacing of those image
pixels with the term dpi, simply because
dpi has always been the name for "pixels
per inch". Scanner ratings also always
call it dpi, also referring to pixels of
course (scanners don't use ink dots). I
always say dpi too, simply because that
has always been the name for pixel
resolution.
Ppi is a relatively recent new term, and
we are seeing ppi used more now, and it
is a perfectly fine name too. Recent
photo editor software often says ppi,
while scanner software generally says
dpi (but there are exceptions to both,
preferences allow this). But either term
is correct, and in fact, the long
established name for image resolution
has always been dpi, for many years
before inkjet printers could print photo
images. So you will often see either dpi
or ppi used, and you must understand
both as the same term. It may be a bit
confusing at first, but that's simply
how things are. Think of this as
training to understand what you will see
elsewhere. <grin>
There is really no problem understanding
the two uses of the word dpi if you know
the basics, and realize the context. It
always means the only thing it can mean
in context. This should be no big deal,
the English language thrives on multiple
context definitions.
If the usage context pertains to images
or printing pixels (and it almost always
does), then dpi always means "pixels per
inch". So does ppi, same thing exactly.
It cannot mean anything else, printing
is about spacing pixels on paper. The
two terms are fully interchangeable, use
either according to your whim. If we
have a 300 dpi image, both terms mean it
will print at 300 pixels per inch (pixel
spacing on paper), so 300 pixels will
cover one inch.
If the usage context pertains to inkjet
printer ink dot ratings, dpi means "ink
dots per inch" (but since the ink dots
are actually larger than their spacing,
the rating is more specifically about
carriage and paper motor stepping
intervals). If the printer rating is
1440 dpi, it means its motors can space
1440 ink dots per inch while trying to
simulate the color of the pixels in that
300 dpi image
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