Dots per inch is a measure of spatial printing or video dot density, in particular the number
of individual dots that can be placed in a line within the span of 1 inch. The metric
alternative is dots per centimeter.
DPI measurement in monitor resolution Monitors do not have dots, but do have pixels;
the closely related concept for monitors and images is pixels per inch or PPI.
Old CRT type video displays were almost universally rated in dot pitch, which refers to the spacing
between the sub-pixel red, green and blue dots which made up the pixels themselves.
Monitor manufacturers used the term "dot trio pitch", the measurement of the distance between
the centers of adjacent groups of three dotssquares on the CRT screen. Monitors commonly used
dot pitches of 0.39, 0.33, 0.32, 0.29, 0.27, 0.25, or 0.22 millimetres.
LCD monitors have a trio of subpixels, which are more easily measured.
DPI measurement in printing DPI is used to describe the resolution number
of dots per inch in a digital print and the printing resolution of a hard copy print dot
gain, which is the increase in the size of the halftone dots during printing. This is
caused by the spreading of ink on the surface of the media.
Up to a point, printers with higher DPI produce clearer and more detailed output. A printer
does not necessarily have a single DPI measurement; it is dependent on print mode, which is usually
influenced by driver settings. The range of DPI supported by a printer is most dependent
on the print head technology it uses. A dot matrix printer, for example, applies ink via
tiny rods striking an ink ribbon, and has a relatively low resolution, typically in
the range of 60 to 90 DPI. An inkjet printer sprays ink through tiny nozzles, and is typically
capable of 300–720 DPI. A laser printer applies toner through a controlled electrostatic
charge, and may be in the range of 600 to 2,400 DPI.
The DP measurement of a printer often needs to be considerably higher than the pixels
per inch measurement of a video display in order to produce similar-quality output. This
is due to the limited range of colors for each dot typically available on a printer.
At each dot position, the simplest type of color printer can either print no dot, or
print a dot consisting of a fixed volume of ink in each of four color channels or 24 = 16
colors on laser, wax and most inkjet printers, of which only 14 or 15 may be actually discernible
depending on the strength of the black component, the strategy used for overlaying and combining
it with the other colors, and if it is even used in "color" mode at all.
Higher-end inkjet printers can offer 5, 6 or 7 ink colors giving 32, 64 or 128 possible
tones per dot location. Contrast this to a standard sRGB monitor where each pixel produces
256 intensities of light in each of three channels.
While some color printers can produce variable drop volumes at each dot position, and may
use additional ink-color channels, the number of colors is still typically less than on
a monitor. Most printers must therefore produce additional colors through a halftone or dithering
process, and rely on their base resolution being high enough to "fool" the human observer's
eye into perceiving a patch of a single smooth color.
The exception to this rule are dye-sublimation printers that utilize a printing method capable
of applying a much more variable amount of dye – close to or exceeding the 256 levels
per channel available on a typical monitor – to each individual "pixel" on the page
without the need for dithering, albeit at an overall lower spatial resolution that can
make text and linear look somewhat rough, lower output speed, a wasteful dye-film roll
cartridge system, and occasional color registration errors that necessitate recalibrating the
printer to account for slippage and drift in the paper feed system. These disadvantages
mean that, despite their marked superiority in the field of producing good quality photographic
and non-linear diagrammatic output, dye-sublimation printers remain unusual niche products, and
devices using higher resolution, lower color depth and dither patterns remain the norm.
This dithered printing process could require a region of four to six dots in order to faithfully
reproduce the color contained in a single pixel. An image that is 100 pixels wide may
need to be 400 to 600 dots in width in the printed output; if a 100×100-pixel image
is to be printed inside a one-inch square, the printer must be capable of 400 to 600
dots per inch in order to accurately reproduce the image. Fittingly, 600dpi is now the typical
output resolution of entry level laser printers and some utility inkjets, with 1200/1440 or
2400/2880 being a common "high" resolution option. This contrasts with the 300/360 dpi
of early models, and the approximate 200dpi of dot-matrix printers and fax machines, that
gave faxed or computer-printed documents – especially those which made heavy use of graphics or
colored block text – a characteristic "digitized" appearance thanks to their coarse and over-obvious
dither patterns, inaccurate colors, loss of clarity in photographs, and jagged edges to
some text and lineart elements.
DPI or PPI in digital image files In printing, DPI refers to the output resolution
of a printer or imagesetter, and PPI refers to the input resolution of a photograph or
image. DPI refers to the physical dot density of an image when it is reproduced as a real
physical entity, for example printed onto paper. A digitally stored image has no inherent
physical dimensions, measured in inches or centimeters. Some digital file formats record
a DPI value, or more commonly a PPI value, which is to be used when printing the image.
This number lets the printer or software know the intended size of the image, or in the
case of scanned images, the size of the original scanned object. For example, a bitmap image
may measure 1,000 × 1,000 pixels, a resolution of 1 megapixel. If it is labeled as 250 PPI,
that is an instruction to the printer to print it at a size of 4 × 4 inches. Changing the
PPI to 100 in an image editing program would tell the printer to print it at a size of
10×10 inches. However, changing the PPI value would not change the size of the image
in pixels which would still be 1,000 × 1,000. An image may also be resampled to change the
number of pixels and therefore the size or resolution of the image, but this is quite
different from simply setting a new PPI for the file.
For vector images, there is no equivalent of resampling an image when it is resized,
and there is no PPI in the file because it is resolution independent. However there is
still a target printing size. Some image formats, such as Photoshop format, can contain both
bitmap and vector data in the same file. Adjusting the PPI in a Photoshop file will change the
intended printing size of the bitmap portion of the data and also change the intended printing
size of the vector data to match. This way the vector and bitmap data maintain a consistent
size relationship when the target printing size is changed. Text stored as outline fonts
in bitmap image formats is handled in the same way. Other formats, such as PDF, are
primarily vector formats which can contain images, potentially at a mixture of resolutions.
In these formats the target PPI of the bitmaps is adjusted to match when the target print
size of the file is changed. This is the converse of how it works in a primarily bitmap format
like Photoshop, but has exactly the same result of maintaining the relationship between the
vector and bitmap portions of the data. Computer monitor DPI standards
Since the 1980s, the Microsoft Windows operating system has set the default display "DPI" to
96 PPI, while Apple/Macintosh computers have used a default of 72 PPI. These default specifications
arose out of the problems rendering standard fonts in the early display systems of the
1980s, including the IBM-based CGA, EGA, VGA and 8514 displays as well as the Macintosh
displays featured in the 128K computer and its successors. The choice of 72 PPI by Macintosh
for their displays arose from the convenient fact that the official 72 points-per-inch
mirrored the 72 pixels-per-inch that actually appeared on their display screens., which
therefore makes 1 point approximately 0.0139 in or 352.8 µm). Thus, a 72 pixels-per-inch
seen on the display was exactly the same physical dimensions as the 72 points-per-inch later
seen on a printout, with 1 pt in printed text equal to 1 px on the display screen.
As it is, the Macintosh 128K featured a screen measuring 512 pixels in width by 342 pixels
in height, and this corresponded to the width of standard office paper.
A consequence of Apple's decision was that the widely used 10 point fonts from the typewriter
era had to be allotted 10 display pixels in em height, and 5 display pixels in x-height.
This is technically described as 10 pixels per em. This made 10-point fonts render crudely
and difficult to read on the display screen, particularly for lowercase characters. Furthermore,
there was the consideration that computer screens are typically viewed at a distance
1/3 or 33% greater than printed materials, causing a mismatch between the perceived sizes
seen on the computer screen versus those on the printouts.
Microsoft tried to solve both problems with a hack that has had long-term consequences
for the understanding of what DPI and PPI mean. Microsoft began writing its software
to treat the screen as though it provided a PPI characteristic that is of what the screen
actually displayed. Because most screens at the time provided around 72 PPI, Microsoft
essentially wrote its software to assume that every screen provides 96 PPI. The short-term
gain of this trickery was twofold: It would seem to the software that more pixels
were available for rendering an image, thereby allowing for bitmap fonts to be created with
greater detail. On every screen that actually provided 72
PPI, each graphical element would be rendered at a size larger than it "should" be, thereby
allowing a person to sit a comfortable distance from the screen. However, larger graphical
elements meant less screen space was available for programs to draw; indeed, although the
default 720-pixel wide mode of a Hercules mono graphics adaptor – or a "tweaked" VGA
adaptor – provided an apparent 7.5-inch page width at this resolution, the more common
and color-capable display adaptors of the time all provided a 640-pixel wide image in
their high resolution modes, enough for a bare 6.67 inches at 100% zoom. Consequently,
the default margins in Microsoft Word were set, and still remain at 1 full inch on all
sides of the page, keeping the "text width" for standard size printer paper within visible
limits; despite most computer monitors now being both larger and finer-pitched, and printer
paper transports having become more sophisticated, the Mac-standard half-inch borders remain
listed in Word 2010's page layout presets as the "narrow" option.
Without using supplemental, software-provided zoom levels, the 1:1 relationship between
display and print size was lost; the availability of different-sized, user-adjustable monitors
and display adaptors with varying output resolutions exacerbated this, as it was not possible to
rely on a properly-adjusted "standard" monitor and adaptor having a known PPI. For example,
a 12" Hercules monitor and adaptor with a thick bezel and a little underscan may offer
90 "physical" PPI, with the displayed image appearing nearly identical to hardcopy but
a thin-bezel 14" VGA monitor adjusted to give a borderless display may be closer to 60,
with the same bitmap image thus appearing 50% larger; yet, someone with an 8514 adaptor
and the same monitor could achieve 100 DPI using its 1024-pixel wide mode and adjusting
the image to be underscanned. A user who wanted to directly compare on-screen elements against
those on an existing printed page by holding it up against the monitor would therefore
first need to determine the correct zoom level to use, largely by trial and error, and often
not be able to obtain an exact match in programs that only allowed integer percent settings,
or even fixed pre-programmed zoom levels. For the examples above, they may need to use
respectively 94% – or 95/90, 63% – or 60/66.7; and 104% – or 105, with the more
commonly accessible 110% actually being a less precise match.
Thus, for example, a 10-point font on a Macintosh was represented with 10 pixels, whereas a
10-point font on a Windows platform at the same zoom level is represented with 13 pixels
– and, on a typical consumer grade monitor, would have physically appeared around 15/72
to 16/72 of an inch high instead of 10. Likewise, a 12-point font was represented with 12 pixels
on a Macintosh, and 16 pixels on a Windows platform at the same zoom, and so on. The
negative consequence of this standard is that with 96 PPI displays, there is no longer a
1-to-1 relationship between the font size in pixels and the printout size in points.
This difference is accentuated on more recent displays that feature higher pixel densities.
This has been less of a problem with the advent of vector graphics and fonts being used in
place of bitmap graphics and fonts. Moreover, many Windows software programs have been written
since the 1980s which assume that the screen provides 96 PPI. Accordingly, these programs
do not display properly at common alternative resolutions such as 72 PPI or 120 PPI. The
solution has been to introduce two concepts: logical PPI: The PPI that software claims
a screen provides. This can be thought of as the PPI provided by a virtual screen created
by the operating system. physical PPI: The PPI that a physical screen
actually provides. Software programs render images to the virtual
screen and then the operating system renders the virtual screen onto the physical screen.
With a logical PPI of 96 PPI, older programs can still run properly regardless of the actual
physical PPI of the display screen, although they may exhibit some visual distortion thanks
to the effective 133.3% pixel zoom level. Proposed metrication
There are some ongoing efforts to abandon the DPI unit in favor of metrication, giving
the inter-dot spacing in dots per centimeter or micrometers between dots. A resolution
of 72 DPI for example equals a resolution of about 28 dpcm or an inter-dot spacing
of about 350 µm. In BMP images 2835 pixels per meter correspond to 72 DPI.
See also Pixels per inch
Samples per inch – a related concept for image scanners
Lines per inch Metric typographic units
Display resolution Mouse DPI
Twip References
External links All About Digital Photos – The Myth of DPI
Monitor DPI detector