Dots per inch

A close-up of the dots produced by an inkjet printer at draft quality. Actual size is approximately 0.25 inches square (0.635 centimetres square). Individual coloured droplets of ink are visible; this sample is about 150 DPI.

Dots per inch (DPI) 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 (2.54 cm). The DPI value tends to correlate with image resolution, but is related only indirectly.

DPI measurement in monitor resolution

A less misleading term, therefore, is pixels per inch. Video displays are almost universally rated in dot pitch, 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 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 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 300-600 DPI.^[1] A laser printer applies toner 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 (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, magenta, yellow and black 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 or dithering process. The exception to this rule is a dye-sublimation printer that utilizes a printing method more akin to pixels per inch.

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.

A 10 × 10-pixel image on a computer display usually requires many more than 10 × 10 printer dots to accurately reproduce, due to limitations of available ink colours in the printer. The whole blue pixels making up the sphere are reproduced by the printer using cyan, magenta, and black.

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) 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 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 operating system has set the default display "DPI" to 96 PPI, while Apple/Macintosh computers have used a default of 72 PPI.^[2] 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. (Points are a physical unit-of-measure in typography 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 height, and 5 display pixels in x-height. 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 tried to solve both problems with a hack that has had long-term consequences for the understanding of what DPI/PPI means^[3]: Microsoft began writing its software to treat the screen as though it provides a PPI characteristic that is $\tfrac{1}{3}$ 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 $72 * (1+\tfrac{1}{3}) = 96$). The short-term gain of this trickery was twofold:

• It would seem to the software that $\tfrac{1}{3}$ 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 $\tfrac{1}{3}$ 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.^[4] 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^[3]:

• 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

See also: dots per centimetre

There are some ongoing efforts to abandon DPI in favour of metrication, giving the inter-dot spacing in dots per centimetre (dpcm) or micrometres (µm).^[5][6] 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
• Samples per inch
• Lines per inch
• Metric typographic units
• Display resolution
• Mouse DPI
• Twip

References

1. ^ Ask OKI—"Inkjet Printers"
2. ^ Hitchcock, Greg (2005-10-08). "Where does 96 DPI come from in Windows?". Microsoft Developer Network Blog. Microsoft. http://blogs.msdn.com/fontblog/archive/2005/11/08/490490.aspx. Retrieved 2009-11-07. 
3. ^ ^{a b} "Where does 96 DPI come from in Windows?". blogs.msdn.com. 2005-09-08. http://blogs.msdn.com/fontblog/archive/2005/11/08/490490.aspx. Retrieved 2010-05-09. 
4. ^ Connare, Vincent (1998-04-06). "Microsoft Typography - Making TrueType bitmap fonts". Microsoft. http://www.microsoft.com/typography/tt/sbit.aspx. Retrieved 2009-11-07. 
5. ^ "Class ResolutionSyntax". Sun Microsystems. http://java.sun.com/j2se/1.4.2/docs/api/javax/print/attribute/ResolutionSyntax.html. Retrieved 2007-10-12. 
6. ^ "Media Queries". http://www.w3.org/TR/css3-mediaqueries/. 

External links

• All About Digital Photos - The Myth of DPI
• Monitor DPI detector