Dots per inch
A printer does not necessarily have a single DPI measurement; it is dependent on print mode, which is usually influenced by driver settings.
An inkjet printer sprays ink through tiny nozzles, and is typically capable of 300–720 DPI.
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 (typically CMYK with cyan, magenta, yellow and black ink) or 24 = 16 colours on laser, wax and most inkjet printers, of which only 14 or 15 (or as few as 8 or 9) may be actually discernible depending on the strength of the black component, the strategy used for overlaying and combining it with the other colours, and whether it is in "color" mode.
Higher-end inkjet printers can offer 5, 6 or 7 ink colours giving 32, 64 or 128 possible tones per dot location (and again, it can be that not all combinations will produce a unique result).
Contrast this to a standard sRGB monitor where each pixel produces 256 intensities of light in each of three channels (RGB).
While some color printers can produce variable drop volumes at each dot position, and may use additional ink-color channels, the number of colours is still typically less than on a monitor.
The exception to this rule is dye-sublimation printers, which can apply a much more variable amount of dye—close to or exceeding the number of the 256 levels per channel available on a typical monitor—to each "pixel" on the page without dithering, but with other limitations: These disadvantages mean that, despite their marked superiority in producing good photographic and non-linear diagrammatic output, dye-sublimation printers remain niche products, and thus other 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 (measured across each side) to accurately reproduce the color in a single pixel.
This contrasts with the 300–360 (or 240) dpi of early models, and the approximate 200 dpi of dot-matrix printers and fax machines, which gave faxed and computer-printed documents—especially those that made heavy use of graphics or coloured block text—a characteristic "digitized" appearance, because of their coarse, obvious dither patterns, inaccurate colours, loss of clarity in photographs, and jagged ("aliased") edges on some text and line art.
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.
Changing the PPI to 100 in an image editing program would tell the printer to print it at a size of 10 × 10 inches.
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 0.7 in margin down each side when assuming 8+1⁄2 in × 11 in North American paper size; in the rest of the world, it is 210 mm × 297 mm – called A4.
[citation needed] Microsoft tried to solve both problems with a hack that has had long-term consequences for the understanding of what DPI and PPI mean.
[citation needed] Displays with high pixel densities were not common up to the Windows XP era.
Windows Vista introduced support for programs to declare themselves to the OS that they are high-DPI aware via a manifest file or using an API.
Developers using the old GDI API and Windows Forms on .NET Framework runtime need to update their apps to be DPI aware and flag their applications as DPI-aware.
Windows 7 adds the ability to change the DPI by doing only a log off, not a full reboot and makes it a per-user setting.
