While I wrote this article a few years ago for
a class I was teaching, I have found people still refer to
it. Thus, I am including it and updating it.
The term Resolution in the field of computer
graphics has various meanings depending upon the context in
which it is used. When an image is scanned into a computer,
this image is called the input image. The technical
term for the number of pixels per linear inch of this input
image is ppi (pixels per inch) or spi (samples per inch).
However, the output, especially when referring to an inkjet,
laser, or dye sub type of printer, is in terms of dpi (dots
per inch). Unfortunately, the terms are often used synonymously
even though they have technical differences. Most scanner
manuals refer to dpi and do not even discuss ppi. Some books
also talk about monitor resolution in terms of ppi.
When referring to bitmapped or
raster images, the higher the ppi or spi, the larger
the file size. For example, if one created two TIFF files
of a 6" diameter circle, one at 72 dpi and one at 300
dpi, the 72 dpi size would be 596KB while the 300 dpi size
would be 10MB or almost 20 times larger in terms of bytes.
At this point, one might say Awhy
is the author talking about ppi, spi, and then using dpi for
her example?@ This
probably seems to be confusing; but it need not be.
When printer resolution is discussed, it
is in terms of dpi (dots per inch). However, a printer, that
has a maximum resolution of 720 x 720 is commonly stated
as having a dpi of 720 (however, usually it is not as high
as stated although it is not done to mislead the user). That
dpi has a different meaning than the dpi of an image. For
example, let's take
the 6" in diameter circle that has an output resolution
of 300 dpi. I can print that on an inkjet printer, for example,
at 180 x 180, 360 x 360, 720 x 720, or 720 x 1440. What the
above numbers are theoretically saying is that the output
of the printer will create an image where the dots are spaced
vertically at 180 dots per inch and horizontally at 180 dots
per inch. However, these are physical Ablobs@
of ink on the paper. Moreover, for better quality they are
randomly spaced; thus, the term 180 x 180 or 720 x 1440 do
not refer to actual "blobs"
of ink. But people will speak of printing at a dpi of 720
or 360. Thus printer dpi is different from image output dpi.
The best way to understand this is to try experiments on your
own. Create an image at 72 dpi, 200 dpi, and 300 dpi. Print
these same images at various settings on your printer. Then,
compare the results.
One last term to dissect is lines per
inch or lpi. One will sometimes see this term used when
discussing scanners in terms of output resolution. This
term refer to screen frequencies and only screen frequencies.
These screen frequencies refer to screens used to produce
halftone images. When one has had the unfortunate experience
of scanning in a beautiful scene from a magazine only to have
horrid intersecting lines (called moiré patterns) appear
on it, one is capturing the imprints of the halftone dots
used to produce that image on an image setter using halftone
screens. A Halftone image is produced by various size
dots. A halftone screen is used in this process. A process
called rasterization produces tiny dots, spots or pixels (imagesetter
pixels are different from the regular definition of pixels).
These are contained in cells per linear inch.
The home user does not have to concern himself
with lines per inch other than to recognize that the common
relationship of ppi to lpi is 2:1. With the input size of
the image equaling the output size. If the ppi (input resolution)
is 300, the lpi will be 150. Some scanning software designates
output using the screen frequency terms of lpi. This can appear
to be very confusing. However, if the input linear size of
the image, say the 5x7 photo is to equal the output linear
size, (5x7) and the final image resolution in Photoshop, Painter,
PhotoPaint, Fireworks, etc. is to be 300, the image can be
scanned in using an image input of 300 ppi (although often
stated incorrectly as dpi) or an output of 300 dpi or 150
lpi. If at this point, one says "how
will I ever understand this?"
I recommend that you go to your computer, scanner, printer,
etc. and see how these numbers work.
When scanning in an image, I always scan
it in at a ppi or dpi (used incorrectly by many scanner manufacturers
to be the same as ppi) of 300. I do this so that if I ever
want to have the work printed on an imagesetter, it will contain
enough information. However, according to various printer
manufacturers, the dpi of an image can be as low as 180 or
200. I usually do not go below 240.
Many digital cameras advertise themselves
as having a certain number of Megapixels; This
does not really give the purchaser any knowledge about how
large (linearly) a picture can be printed and still maintain
high quality. If one digs hard enough, the actual pixel dimensions
can usually be found.
The following chart will show the linear
size of digital pictures taken at certain resolutions as well
as their linear and pixel file sizes working in RGB, assuming
a dpi of 240, and a video card set at 24 bit depth or millions
LINEAR FILE SIZE INCHES
640 x 480
2.667 x 2.00
1024 x 768
4.267 x 3.2
1152 x 864
4.8 x 3.6
1280 x 960
5.33 x 4
1280 x 1024
5.33 x 4.267