Understanding
the PPI, DPI and Resolution Dilemma
By:
Shiv Verma
This article is designed to address PPI and
DPI. LPI and SPI will not be covered in detail.
Definitions
PPI (pixels per inch) is the number of pixels
displayed in an image. A digital image is
information that your screen displays in pixels.
The PPI is the display resolution not the image
resolution.
DPI (dots per inch) is a measure of resolution
counting dots (pixels) running across one inch
of the image. It is a measure of the resolution
of a printer. It refers to the dots of ink or
toner used by an ink jet printer, laser printer,
or other printing device to print your text and
graphics. In general, the more dots, the better
and sharper the image. DPI is printer
resolution.
LPI (lines per inch) also called screen
frequency or ruling, is how many rows of
halftone spots make up a halftone screen that is
used to convert a continuous tone image
(photograph) into a series of dots. How fine or
coarse the screen is.
SPI (samples per inch) is scanner and digital
image resolution. To scan an image the scanner
takes a sampling of portions of the image. The
more samples it takes per inch, the closer the
scan is to the original image. The higher the
resolution, the higher the SPI.
-
Image Size -
There are three measures that are used to
define the size of an image:
-
Pixel count -
e.g 3000x2000 pixels
-
Physical size
- e.g. 8" x 10"
-
Resolution -
e.g. 240 pixels per inch (PPI)
Lets start with a the digital image file – each
image is tagged with a lot of information beyond
the pixel data captured for the image. This is
the EXIF data - the width and height in pixels,
the resolution (PPI), camera data, lens data,
exposure (speed and aperture) data, focus data,
flash data. EXIF stands for Exchangeable Image
File Format, and is a standard for storing
interchange information in image files.
To resolve some of the confusion and to
understand the relationship of these terms lets
start with the Display or Monitor or LCD/DLP
Projector. Monitors are measured in pixels
(e.g. a 1024x768 display), and visual screen
width typically in inches – 15inch monitor 17
inch monitor, etc. (confusion - this is the
diagonal measure not the true width). If image
display and monitor display had a one to one
correlation it would not be possible to view an
image in its entirety if the image has a higher
pixel count than the display in the visual
portion of the screen. Although monitors do have
a measure for PPI (pixels per inch), the PPI
information embedded in an image is discarded
when it is displayed on a monitor.
When you display a digitally captured image on a
monitor, the only determination of the size of
the image is its width and height in pixels. All
the other data embedded in the image file
including resolution (PPI data) is ignored. If
your image is a 480Kbyte file which is 1024
pixels wide by 768 pixels wide, it will display
as a full screen image if you are using an 1024
x 768 display or higher. It doesn't matter if
your PPI is set to 75 or 300 or 1000 This is as
far as any monitor display goes – software
intervenes and interpolates the image to resize
it to fit on the screen. In most cases the
embedded PPI information is ignored.
DETAILS:
PPI – Pixels per Inch
When using an image editor if you
change the PPI information you will see that the
width and height in the image size boxes will
change. "Pixels per inch" is almost exclusively
used for printing, not for display using a
monitor or projector. If you take an image that
is 1024 pixels wide and 768 pixels high, and you
print it with a PPI setting of 200 pixels per
you will get a print that has dimensions of 5.12
inches by 3.84 inches. If you resample the image
down to 512 pixels wide by 384 pixels wide and
you keep the PPI setting at 200, the print will
now be 2.778 inches wide by 1.92 inches high.
To still get an image of 5.12 inches by 3.84
inches you can decrease the PPI to 100 – you
will however, deteriorate the quality.
DPI - Dots per Inch
DPI is associated with printers
and printing. DPI is a property of a printer and
printer driver software, not the digital image.
DPI is a means of defining the spacing of
droplets of ink that are pot on the paper to
produce a print. Printer have settings of
360DPI, 720DPI, 1440DPI and 2880DPI but the
difference between these setting is subtle nad
not really perceptible o the naked eye. The key
point here is that changing the DPI setting has
no effect on the size of the print. PPI controls
the size and DPI controls print quality. In
reality, printers often use a halftone screen
and dither many small dots of ink in order to
reproduce the tone of each pixel from the
original image, so the PPI that is actually
reproduced is lower than the DPI of the printer.
In conclusion DPI describes the resolution of
printers. PPI describes the density of pixels in
an image file. In simplistic terms – to produce
a print the system (computer, printer, software
and settings) map the PPI of the image file to
the DPI of the print
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