Dots per inch
Encyclopedia
Dots per inch is a measure of spatial printing
Printing
Printing is a process for reproducing text and image, typically with ink on paper using a printing press. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing....

 or video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

 dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch (2.54 cm). The DPI value tends to correlate with image resolution
Image resolution
Image resolution is an umbrella term that describes the detail an image holds. The term applies to raster digital images, film images, and other types of images. Higher resolution means more image detail....

, but is related only indirectly.

DPI measurement in monitor resolution

A less misleading term, therefore, is pixels per inch
Pixels per inch
Pixels per inch or pixel density is a measurement of the resolution of devices in various contexts; typically computer displays, image scanners, and digital camera image sensors....

. Video displays are almost universally rated in dot pitch
Dot pitch
Dot pitch is a specification for a computer display, computer printer, image scanner, or other pixel-based device that describes the distance, for example, between dots of the same color on the inside of a display screen...

, which refers to the spacing between the sub-pixel red, green and blue dots which make up the pixels themselves. Monitor manufacturers use the term "dot trio pitch", the measurement of the distance between the centers of adjacent groups of three dots/rectangles/squares on the CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

 screen. Monitors commonly use pitches of 0.39, 0.33, 0.32, 0.29, 0.27, 0.25, and 0.22 mm.

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; 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
Computer printer
In computing, a printer is a peripheral which produces a text or graphics of documents stored in electronic form, usually on physical print media such as paper or transparencies. Many printers are primarily used as local peripherals, and are attached by a printer cable or, in most new printers, a...

 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
Dot matrix printer
A dot matrix printer or impact matrix printer is a type of computer printer with a print head that runs back and forth, or in an up and down motion, on the page and prints by impact, striking an ink-soaked cloth ribbon against the paper, much like the print mechanism on a typewriter...

, 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
Inkjet printer
An inkjet printer is a type of computer printer that creates a digital image by propelling droplets of ink onto paper. Inkjet printers are the most commonly used type of printer and range from small inexpensive consumer models to very large professional machines that can cost up to thousands of...

 sprays ink through tiny nozzles, and is typically capable of 300-600 DPI. A laser printer
Laser printer
A laser printer is a common type of computer printer that rapidly produces high quality text and graphics on plain paper. As with digital photocopiers and multifunction printers , laser printers employ a xerographic printing process, but differ from analog photocopiers in that the image is produced...

 applies toner
Toner
Toner is a powder used in laser printers and photocopiers to form the printed text and images on the paper. In its early form it was simply carbon powder. Then, to improve the quality of the printout, the carbon was melt-mixed with a polymer...

 through a controlled electrostatic charge, and may be in the range of 600 to 1,800 DPI.

The DPI measurement of a printer often needs to be considerably higher than the pixels per inch
Pixels per inch
Pixels per inch or pixel density is a measurement of the resolution of devices in various contexts; typically computer displays, image scanners, and digital camera image sensors....

 (PPI) measurement of a video display in order to produce similar-quality output. This is due to the limited range of colours for each dot typically available on a printer. At each dot position, the simplest type of colour printer can print no dot, or a dot consisting of a fixed volume of ink in each of four colour channels (typically CMYK with cyan
Cyan
Cyan from , transliterated: kýanos, meaning "dark blue substance") may be used as the name of any of a number of colors in the blue/green range of the spectrum. In reference to the visible spectrum cyan is used to refer to the color obtained by mixing equal amounts of green and blue light or the...

, magenta
Magenta
Magenta is a color evoked by light stronger in blue and red wavelengths than in yellowish-green wavelengths . In light experiments, magenta can be produced by removing the lime-green wavelengths from white light...

, yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...

 and black
Black
Black is the color of objects that do not emit or reflect light in any part of the visible spectrum; they absorb all such frequencies of light...

 ink) or 24 = 16 colours on laser, wax and most inkjet printers.

Higher-end inkjet printers can offer 5, 6 or 7 ink colours 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 (RGB).

While some colour printers can produce variable drop volumes at each dot position, and may use additional ink-colour channels, the number of colours is still typically less than on a monitor. Most printers must therefore produce additional colours through a halftone
Halftone
Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing...

 or dithering process. The exception to this rule is a dye-sublimation printer
Dye-sublimation printer
A dye-sublimation printer is a computer printer which employs a printing process that uses heat to transfer dye onto medium materials such as a plastic card, paper, or fabric. The sublimation name is applied because the dye transitions between the solid and gas states without going through a...

 that utilizes a printing method more akin to pixels per inch
Pixels per inch
Pixels per inch or pixel density is a measurement of the resolution of devices in various contexts; typically computer displays, image scanners, and digital camera image sensors....

.

The printing process could require a region of four to six dots (measured across each side) in order to faithfully reproduce the colour 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.

DPI or PPI in digital image files

DPI refers to the physical dot density of an image when it is reproduced as a real physical entity, for example printed onto paper, or displayed on a monitor. A digitally stored image has no inherent physical dimensions, measured in inches or centimetres. Some digital file formats record a DPI value, or more commonly a PPI (pixels per inch
Pixels per inch
Pixels per inch or pixel density is a measurement of the resolution of devices in various contexts; typically computer displays, image scanners, and digital camera image sensors....

) value, which is to be used when printing the image. This number lets the printer 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
Bitmap
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...

 image may measure 1,000 × 1,000 pixels, a resolution of 1 megapixels. 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 (prints equally well at all sizes). 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 have bitmaps pasted into them. 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
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

 operating system has set the default display "DPI" to 96 PPI, while Apple/Macintosh
Macintosh
The Macintosh , or Mac, is a series of several lines of personal computers designed, developed, and marketed by Apple Inc. The first Macintosh was introduced by Apple's then-chairman Steve Jobs on January 24, 1984; it was the first commercially successful personal computer to feature a mouse and a...

 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
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

-based CGA
Color Graphics Adapter
The Color Graphics Adapter , originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first color graphics card, and the first color computer display standard for the IBM PC....

, EGA
Enhanced Graphics Adapter
The Enhanced Graphics Adapter is the IBM PC computer display standard specification which is between CGA and VGA in terms of color and space resolution. Introduced in October 1984 by IBM shortly after its new PC/AT, EGA produces a display of 16 simultaneous colors from a palette of 64 at a...

, VGA and 8514 displays as well as the Macintosh
Macintosh
The Macintosh , or Mac, is a series of several lines of personal computers designed, developed, and marketed by Apple Inc. The first Macintosh was introduced by Apple's then-chairman Steve Jobs on January 24, 1984; it was the first commercially successful personal computer to feature a mouse and a...

 displays featured in the 128K
Macintosh 128K
The Macintosh 128K machine, released as the "Apple Macintosh", was the original Apple Macintosh personal computer. Its beige case contained a monitor and came with a keyboard and mouse. An indentation in the top of the case made it easier for the computer to be lifted and carried. It had a selling...

 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. (Points
Point (typography)
In typography, a point is the smallest unit of measure, being a subdivision of the larger pica. It is commonly abbreviated as pt. The point has long been the usual unit for measuring font size and leading and other minute items on a printed page....

 are a physical unit-of-measure in typography
Typography
Typography is the art and technique of arranging type in order to make language visible. The arrangement of type involves the selection of typefaces, point size, line length, leading , adjusting the spaces between groups of letters and adjusting the space between pairs of letters...

 dating to the days of printing presses, where 1 point by the modern definition is 1/72 of the international inch (25.4 mm), 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 (512 px ÷ 72 px/in = 7.1 in, with a 0.75 in margin down each side when assuming 8.5 in × 11 in North American paper size).

The 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
Em (typography)
An em is a unit of measurement in the field of typography, equal to the currently specified point size.The name of em is related to M. Originally the unit was derived from the width of the capital "M" in the given typeface....

 height, and 5 display pixels in x-height
X-height
In typography, the x-height or corpus size refers to the distance between the baseline and the mean line in a typeface. Typically, this is the height of the letter x in the font , as well as the u, v, w, and z...

. This is technically described as 10 pixels per em (PPEm). This made 10-point fonts crudely rendered 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 desk) 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
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 tried to solve both problems with a hack that has had long-term consequences for the understanding of what DPI/PPI means: Microsoft began writing its software to treat the screen as though it provides a PPI characteristic that is times larger than the PPI the screen actually provides; worse, because most screens at that time provided around 72 PPI, Microsoft essentially wrote its software to assume that every screen provides 96 PPI (because ). 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 does provide 72 PPI, each graphical element (such as a character of text) 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 on which to draw.

Thus, for example, 10 point font on a Macintosh (at 72 PPI) was represented with 10 pixels (i.e., 10 PPEm) whereas 10 point font on a Windows platform (at 96 PPI) using the same screen is represented with 13 pixels (i.e., Microsoft rounded 13.3333 to 13 pixels, or 13 PPEm). Likewise, 12 point font was represented with 12 pixels on a Macintosh, and 16 pixels on a Windows platform that used the same screen, 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, but this has been less of a problem with the advent of vector graphics and fonts in place of bitmap graphics and fonts. Moreover, many Windows software programs have been written since the 1980s to assume that the screen provides 96 PPI, and 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 PPI provided by the physical screen.

Proposed metrication

There are some ongoing efforts to abandon DPI in favour of metrication
Metrication
Metrication refers to the introduction and use of the SI metric system, the international standard for physical measurements. This has involved a long process of independent and systematic conversions of countries from various local systems of weights and measures. Metrication began in France in...

, giving the inter-dot spacing in dots per centimetre (dpcm) or micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

s (µm). A resolution of 72 DPI for example equals a resolution of about 28 dpcm or an inter-dot spacing of about 350 µm.
Conversion table
DPI
(dot/in)
dpcm
(dot/cm)
Pitch
(µm/dot)
72 28 350
96 38 265
150 59 169
300 118 85
2540 1000 10
4000 1575 6

See also

  • Pixels per inch
    Pixels per inch
    Pixels per inch or pixel density is a measurement of the resolution of devices in various contexts; typically computer displays, image scanners, and digital camera image sensors....

  • Samples per inch
    Samples per inch
    Samples per inch is a measurement of the resolution of an image scanner, in particular the number of individual samples that are taken in the space of one linear inch. It is sometimes misreferred to as dots per inch, though that term more accurately refers to printing resolution...

  • Lines per inch
    Lines per inch
    Lines per inch is a measurement of printing resolution in systems that use a halftone screen. Specifically, it is a measure of how close together the lines in a halftone grid are. Higher LPI indicates greater detail and sharpness....

  • Metric typographic units
    Metric typographic units
    Metric typographic units have been devised and proposed several times to overcome the various traditional point systems. After the French revolution of 1789 one popular proponent of a switch to metric was Didot, who had been able to standardise the continental European typographic measurement few...

  • Display resolution
    Display resolution
    The display resolution of a digital television or display device is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution is controlled by all different factors in cathode ray tube , flat panel or projection...

  • Mouse DPI
  • Twip
    Twip
    A twip is a typographical measurement, defined as 1/20 of a typographical point...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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