Pixel density

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Pixels per inch (ppi) and pixels per centimetre (ppcm or pixels/cm) are measurements of the pixel density of an electronic image device, such as a computer monitor or television display, or image digitizing device such as a camera or image scanner. Horizontal and vertical density are usually the same, as most devices have square pixels, but differ on devices that have non-square pixels. Pixel density is not the same as resolution where the former describes the amount of detail on a physical surface or device, the latter describes the amount of pixel information regardless of its scale. Considered in another way, a pixel has no inherent size or unit (a pixel is actually a sample), but when it is printed, displayed, or scanned, then the pixel has both a physical size (dimension) and a pixel density (ppi). [1]

Contents

Basic principles

Since most digital hardware devices use dots or pixels, the size of the media (in inches) and the number of pixels (or dots) are directly related by the 'pixels per inch'. The following formula gives the number of pixels, horizontally or vertically, given the physical size of a format and the pixels per inch of the output:

Pixels per inch (or pixels per centimetre) describes the detail of an image file when the print size is known. For example, a 100×100 pixel image printed in a 2 inch square has a resolution of 50 pixels per inch. Used this way, the measurement is meaningful when printing an image. In many applications, such as Adobe Photoshop, the program is designed so that one creates new images by specifying the output device and PPI (pixels per inch). Thus the output target is often defined upon creating the image.

Outputting to a different device

When moving images between devices, such as printing an image that was created on a monitor, it is important to understand the pixel density of both devices. Consider a 23″ HD monitor (20″ wide), that has a known, native resolution of 1920 pixels (horizontal). Let us assume an artist created a new image at this monitor resolution of 1920 pixels, possibly intended for the web without regard to printing. Rewriting the formula above can tell us the pixel density (PPI) of the image on the monitor display:

Now, let us imagine the artist wishes to print a larger banner at 48″ horizontally. We know the number of pixels in the image, and the size of the output, from which we can use the same formula again to give the PPI of the printed poster:

This shows that the output banner will have only 40 pixels per inch. Since a printer device is capable of printing at 300 ppi, the resolution of the original image is well below what would be needed to create a decent quality banner, even if it looked good on a monitor for a website. We would say more directly that a 1920 × 1080 pixel image does not have enough pixels to be printed in a large format.

Printing on paper

Printing on paper is accomplished with different technologies. Newspapers and magazines were traditionally printed using a halftone screen, [2] which would print dots at a given frequency, the screen frequency, in lines per inch (LPI) by using a purely analog process in which a photographic print is converted into variable sized dots through interference patterns passing through a screen. Modern inkjet printers can print microscopic dots at any location, and don't require a screen grid, with the metric dots per inch (DPI). These are both different from pixel density or pixels per inch (PPI) because a pixel is a single sample of any color, whereas an inkjet print can only print a dot of a specific color either on or off. Thus a printer translates the pixels into a series of dots using a process called dithering. The dot pitch, smallest size of each dot, is also determined by the type of paper the image is printed on. An absorbent paper surface, uncoated recycled paper for instance, lets ink droplets spread — so has a larger dot pitch. [3]

Often one wishes to know the image quality in pixels per inch (PPI) that would be suitable for a given output device. If the choice is too low, then the quality will be below what the device is capable of—loss of quality—and if the choice is too high then pixels will be stored unnecessarily—wasted disk space. The ideal pixel density (PPI) depends on the output format, output device, the intended use and artistic choice. For inkjet printers measured in DPI it is generally good practice to use half or less than the DPI to determine the PPI. For example, an image intended for a printer capable of 600 dpi could be created at 300 ppi. When using other technologies such as AM or FM screen printing, there are often published screening charts that indicate the ideal PPI for a printing method. [4]

Using the DPI or LPI of a printer remains useful to determine PPI until one reaches larger formats, such as 36" or higher, as the factor of visual acuity then becomes more important to consider. If a print can be viewed close up, then one may choose the printer device limits. However, if a poster, banner or billboard will be viewed from far away then it is possible to use a much lower PPI.[ citation needed ]

The outside of the square shown above is 200 pixels by 200 pixels. To determine a monitor's ppi, set the OS DPI scaling setting at 100% and the browser's zoom at 100%, then measure the width and height, in inches, of the square as displayed on a given monitor. Dividing 200 by the measured width or height gives the monitor's horizontal or vertical ppi, respectively, at the current screen resolution. Square 200x200.svg
The outside of the square shown above is 200 pixels by 200 pixels. To determine a monitor's ppi, set the OS DPI scaling setting at 100% and the browser's zoom at 100%, then measure the width and height, in inches, of the square as displayed on a given monitor. Dividing 200 by the measured width or height gives the monitor's horizontal or vertical ppi, respectively, at the current screen resolution.

Computer displays

The PPI/PPCM of a computer display is related to the size of the display in inches/centimetres and the total number of pixels in the horizontal and vertical directions. This measurement is often referred to as dots per inch, though that measurement more accurately refers to the resolution of a computer printer.

For example, a 15-inch (38 cm) display whose dimensions work out to 12 inches (30.48 cm) wide by 9 inches (22.86 cm) high, capable of a maximum 1024×768 (or XGA) pixel resolution, can display around 85 PPI, or 33.46 PPCM, in both the horizontal and vertical directions. This figure is determined by dividing the width (or height) of the display area in pixels by the width (or height) of the display area in inches. It is possible for a display to have different horizontal and vertical PPI measurements (e.g., a typical 4:3 ratio CRT monitor showing a 1280×1024 mode computer display at maximum size, which is a 5:4 ratio, not quite the same as 4:3). The apparent PPI of a monitor depends upon the screen resolution (that is, the number of pixels) and the size of the screen in use; a monitor in 800×600 mode has a lower PPI than does the same monitor in a 1024×768 or 1280×960 mode.

The dot pitch of a computer display determines the absolute limit of possible pixel density. Typical circa-2000 cathode ray tube or LCD computer displays range from 67 to 130 PPI, though desktop monitors have exceeded 200 PPI, and certain smartphone manufacturers' flagship mobile device models have been exceeding 500 PPI since 2014.

In January 2008, Kopin Corporation announced a 0.44 inch (1.12 cm) SVGA LCD with a pixel density of 2272 PPI (each pixel only 11.25 μm). [5] [6] In 2011 they followed this up with a 3760-DPI 0.21-inch diagonal VGA colour display. [7] The manufacturer says they designed the LCD to be optically magnified, as in high-resolution eyewear devices.

Holography applications demand even greater pixel density, as higher pixel density produces a larger image size and wider viewing angle. Spatial light modulators can reduce pixel pitch to 2.5  μm, giving a pixel density of 10,160 PPI. [8]

Some observations indicate that the unaided human generally can't differentiate detail beyond 300 PPI. [9] However, this figure depends both on the distance between viewer and image, and the viewer’s visual acuity. The human eye also responds in a different way to a bright, evenly lit interactive display from how it does to prints on paper.

High pixel density display technologies would make supersampled antialiasing obsolete, enable true WYSIWYG graphics and, potentially enable a practical “paperless office” era. [10] For perspective, such a device at 15 inch (38 cm) screen size would have to display more than four Full HD screens (or WQUXGA resolution).

The PPI pixel density specification of a display is also useful for calibrating a monitor with a printer. Software can use the PPI measurement to display a document at "actual size" on the screen.

Calculation of monitor PPI

Display resolution and pixel density.png

PPI can be calculated from knowing the diagonal size of the screen in inches and the resolution in pixels (width and height). This can be done in two steps:

  1. Calculate diagonal resolution in pixels using the Pythagorean theorem, then the actual PPI:

where

  1. is width resolution in pixels
  2. is height resolution in pixels
  3. is diagonal size in inches (this is the number advertised as the size of the display).

For example:

  1. For 15.6 inch screen with a 5120×2880 resolution you get = 376.57 PPI.
  2. For 50 inch screen with a 8192×4608 resolution you get = 188 PPI.
  3. For 27 inch screen with a 3840×2160 resolution you get = 163 PPI.
  4. For 32 inch screen with a 3840×2160 resolution you get = 138 PPI.
  5. For an old-school 10.1 inch netbook screen with a 1024×600 resolution you get = 117.5 PPI.
  6. For 27 inch screen with a 2560×1440 resolution you get = 108.8 PPI.
  7. For a 21.5 inches (546.1 mm) screen with a 1920×1080 resolution you get = 102.46 PPI;

These calculations may not be very precise. Frequently, screens advertised as “X inch screen” can have their real physical dimensions of viewable area differ, for example:

Calculating PPI of camera view screens

Camera manufacturers often quote view screens in 'number of dots'. This is not the same as the number of pixels, because there are 3 'dots' per pixel – red, green and blue. For example, the Canon 50D is quoted as having 920,000 dots. [15] This translates as 307,200 pixels (×3 = 921,600 dots). Thus the screen is 640×480 pixels. [16]

This must be taken into account when working out the PPI. 'Dots' and 'pixels' are often confused in reviews and specs when viewing information about digital cameras specifically.

Scanners and cameras

"PPI" or "pixel density" may also describe image scanner resolution. In this context, PPI is synonymous with samples per inch. In digital photography, pixel density is the number of pixels divided by the area of the sensor. A typical DSLR, circa 2013, has 1–6.2 MP/cm2; a typical compact has 20–70 MP/cm2.

For example, Sony Alpha SLT-A58 has 20.1 megapixels on an APS-C sensor having 6.2 MP/cm2 since a compact camera like Sony Cyber-shot DSC-HX50V has 20.4 megapixels on an 1/2.3" sensor having 70 MP/cm2. The professional camera has a lower PPI than a compact camera, because it has larger photodiodes due to having far larger sensors.

Smartphones

Smartphones use small displays, but modern smartphone displays have a larger PPI rating, such as the Samsung Galaxy S7 with a quad HD display at 577 PPI, Fujitsu F-02G with a quad HD display at 564 PPI, [17] the LG G6 with quad HD display at 564 PPI or – XHDPI or Oppo Find 7 with 534 PPI on 5.5" display – XXHDPI (see section below). [18] Sony's Xperia XZ Premium has a 4K display with a pixel density of 807 PPI, the highest of any smartphone as of 2017. [19]

Logical DPI values on Android

Android supports the following logical DPI values for controlling how large content is displayed: [20]

NameScale FactorDPI
ldpi0.75x~120
mdpi1x~160
tvdpi1.33x~213
hdpi1.5x~240
xhdpi2x~320
xxhdpi3x~480
xxxhdpi4x~640

Metrication

The digital publishing industry primarily uses pixels per inch but sometimes pixels per centimeter is used, or a conversion factor is given. [21] [22] [23]

The PNG image file format only allows the meter as the unit for pixel density. [24]

Image file format support

The following table show how pixel density is supported by popular image file formats. The cell colors used do not indicate how feature-rich a certain image file format is, but what density support can be expected of a certain image file format.

Even though image manipulation software can optionally set density for some image file formats, not many other software uses density information when displaying images. Web browsers, for example, ignore any density information. As the table shows, support for density information in image file formats varies enormously and should be used with great care in a controlled context.

Format Units of measurement [lower-alpha 1] Raster/vector Multi-pagePer-page sizeSize in lengths for image or pageDensity
Exif LengthRasterPPI or PPCM, 8 bytes (64bit rational unsigned) each for horizontal and vertical directions [25]
AI Length or pixelBothNoExplicit for length. No for pixelImplicit for included raster images
EPS LengthBothYesYesExplicitExplicit DPI (PPI) for rasterized images, fonts or effects
GIF PixelRasterYesNoNoNo
ICO PixelRasterYesYesNoNo
JPEG PixelRasterNoImplicit when density is setOptional PPI or PPCM, 2 bytes each for horizontal and vertical directions [26]
PDF LengthBothYesYesExplicitExplicit DPI (PPI) for rasterized images, fonts or effects
PNG PixelRasterNoImplicit when density is setOptional PPM, 4 bytes each horizontal and vertical directions [27]
PPM PixelRasterYesNoNoNo
PSD and PSB Length or pixelBothNoExplicit for length. No for pixelOptional
SVG Length or pixelBothYesNoExplicit for length. [lower-alpha 2] No for pixelImplicit for included raster images
TIFF PixelBothYesYesImplicit when density is setOptional PPI or PPCM, two 32-bit unsigned integers each for horizontal and vertical directions [30]
WebP PixelRasterYesUn­knownUn­knownWEBP has no tags/attributes of its own which specify density on the output medium. But WEBP uses Resource Interchange File Format as its container format which supports Metadata to be included as XMP and EXIF chunks. And EXIF supports resolution, see entry in this table. Practical example: GraphicConverter v11.7.1 correctly shows resolution information for WEBP files with EXIF metadata.
XCF PixelBothNoNoNo
Format Units of measurement Raster/vector Multi-pagePer-page sizeSize in lengths for image or pageDensity
  1. Length refers to horizontal and vertical size in inches, centimeters, etc., whereas pixel refers only to the number of pixels found along the horizontal and vertical dimension.
  2. Support in SVG differs. The standard supports the floats pixelUnitToMillimeterX, pixelUnitToMillimeterY, screenPixelToMillimeterX and screenPixelToMillimeterY for use in CSS2. [28] Inkscape SVG supports density for PNG export only inkscape:export-xdpi and inkscape:export-ydpi. [29] Adobe stores it even differently.

See also

Related Research Articles

<span class="mw-page-title-main">Computer monitor</span> Computer output device

A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a visual display, support electronics, power supply, housing, electrical connectors, and external user controls.

<span class="mw-page-title-main">Pixel</span> Physical point in a raster image

In digital imaging, a pixel, pel, or picture element is the smallest addressable element in a raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, pixels are the smallest element that can be manipulated through software.

<span class="mw-page-title-main">Raster graphics</span> Matrix-based data structure

In computer graphics and digital photography, a raster graphic represents a two-dimensional picture as a rectangular matrix or grid of pixels, viewable via a computer display, paper, or other display medium. A raster is technically characterized by the width and height of the image in pixels and by the number of bits per pixel. Raster images are stored in image files with varying dissemination, production, generation, and acquisition formats.

In computing, WYSIWYG, an acronym for What You See Is What You Get, refers to software which allows content to be edited in a form that resembles its appearance when printed or displayed as a finished product, such as a printed document, web page, or slide presentation. WYSIWYG implies a user interface that allows the user to view something very similar to the result while the document is being created. In general, WYSIWYG implies the ability to directly manipulate the layout of a document without having to type or remember names of layout commands.

<span class="mw-page-title-main">Dot matrix</span>

A dot matrix is a 2-dimensional patterned array, used to represent characters, symbols and images. Most types of modern technology use dot matrices for display of information, including mobile phones, televisions, and printers. The system is also used in textiles with sewing, knitting and weaving.

<span class="mw-page-title-main">Dots per inch</span> Measure of dot density

Dots per inch is a measure of spatial printing, video or image scanner 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). Similarly, dots per centimetre refers to the number of individual dots that can be placed within a line of 1 centimetre (0.394 in).

<span class="mw-page-title-main">Dot pitch</span> Distance between RGB dots (sub-pixels) on a display

Dot pitch is a specification for a computer display, computer printer, image scanner, or other pixel-based devices that describe the distance, for example, between dots (sub-pixels) on a display screen. In the case of an RGB color display, the derived unit of pixel pitch is a measure of the size of a triad plus the distance between triads.

<span class="mw-page-title-main">Display resolution</span> Number of distinct pixels in each dimension that can be displayed

The display resolution or display modes of a digital television, computer monitor 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 different factors in cathode ray tube (CRT) displays, flat-panel displays and projection displays using fixed picture-element (pixel) arrays.

Lines per inch (LPI) is a measurement of printing resolution. A line consists of halftones that is built up by physical ink dots made by the printer device to create different tones. Specifically LPI is a measure of how close together the lines in a halftone grid are. The quality of printer device or screen determines how high the LPI will be. High LPI indicates greater detail and sharpness.

<span class="mw-page-title-main">Metric typographic units</span>

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 a few decades earlier. The conversion did not happen, though. The Didot point was metrically redefined as 12660 m (≈ 0.376 mm) in 1879 by Berthold.

Image resolution is the level of detail of an image. The term applies to digital images, film images, and other types of images. "Higher resolution" means more image detail. Image resolution can be measured in various ways. Resolution quantifies how close lines can be to each other and still be visibly resolved. Resolution units can be tied to physical sizes, to the overall size of a picture, or to angular subtense. Instead of single lines, line pairs are often used, composed of a dark line and an adjacent light line; for example, a resolution of 10 lines per millimeter means 5 dark lines alternating with 5 light lines, or 5 line pairs per millimeter. Photographic lens are most often quoted in line pairs per millimeter.

<span class="mw-page-title-main">IBM T220/T221 LCD monitors</span> High resolution LCD monitors

The IBM T220 and T221 are LCD monitors that were sold between 2001 and 2005, with a native resolution of 3840×2400 pixels (WQUXGA) on a screen with a diagonal of 22.2 inches (564 mm). This works out to 9,216,000 pixels, with a pixel density of 204 pixels per inch, much higher than contemporary computer monitors and approaching the resolution of print media. The display family was nicknamed "Big Bertha" in some trade journals. Costing around $8,400 in 2003, the displays saw few buyers. Such high-resolution displays would remain niche products for nearly a decade until modern high-dpi displays such as Apple's Retina display line saw more-widespread adoption.

Resolution independence is where elements on a computer screen are rendered at sizes independent from the pixel grid, resulting in a graphical user interface that is displayed at a consistent physical size, regardless of the resolution of the screen.

<span class="mw-page-title-main">Hanlin eReader</span>

The Hanlin is an e-Reader, an electronic book (e-book) reading device. The Hanlin v3 features a 6" (15 cm), 4-level grayscale electrophoretic display with a resolution of 600×800 pixels, while the v3+ features a 16-level grayscale display. The Hanlin v5 Mini, features a 5" (15 cm), 8-level grayscale electrophoretic display with a resolution of 600×800 pixels. The device runs a Linux-based OS.

"21:9" is a consumer electronics (CE) marketing term to describe the ultrawide aspect ratio of 64:27, designed to show films recorded in CinemaScope and equivalent modern anamorphic formats. The main benefit of this screen aspect ratio is a constant display height when displaying other content with a lesser aspect ratio.

<span class="mw-page-title-main">4K resolution</span> Video or display resolutions with a width of around 4,000 pixels

4K resolution refers to a horizontal display resolution of approximately 4,000 pixels. Digital television and digital cinematography commonly use several different 4K resolutions. In television and consumer media, 3840 × 2160 is the dominant 4K standard, whereas the movie projection industry uses 4096 × 2160.

<span class="mw-page-title-main">Retina display</span> High-resolution display brand by Apple


The Retina display is a branded series of IPS LCD and OLED displays by Apple Inc. and have a higher pixel density than its traditional displays. Apple has registered the term "Retina" as a trademark with regard to computers and mobile devices with the United States Patent and Trademark Office and Canadian Intellectual Property Office. The applications were approved in 2012 and 2014 respectively. The Canadian application cited a 2010 application in Jamaica.

<span class="mw-page-title-main">Graphics display resolution</span> Width and height of an electronic visual display device, such as a computer monitor, in pixels

The graphics display resolution is the width and height dimension of an electronic visual display device, measured in pixels. This information is used for electronic devices such as a computer monitor. Certain combinations of width and height are standardized and typically given a name and an initialism which is descriptive of its dimensions. A graphics display resolution can be used in tandem with the size of the graphics display to calculate pixel density. An increase in the pixel density often correlates with a decrease in the size of individual pixels on a display.

The aspect ratio of an image is the ratio of its width to its height, and is expressed with two numbers separated by a colon, such as 16:9, sixteen-to-nine. For the x:y aspect ratio, the image is x units wide and y units high. Common aspect ratios are 1.85:1 and 2.40:1 in cinematography, 4:3 and 16:9 in television photography, and 3:2 in still photography. The film was filmed in 2.40:1 widescreen.

<span class="mw-page-title-main">5K resolution</span> Video or display resolutions with a width of around 5,000 pixels

5K resolution refers to display formats with a horizontal resolution of around 5,000 pixels. The most common 5K resolution is 5120 × 2880, which has an aspect ratio of 16∶9 with around 14.7 million pixels, with exactly twice the linear resolution of 1440p and four times that of 720p. This resolution is typically used in computer monitors to achieve a higher pixel density, and is not a standard format in digital television and digital cinematography, which feature 4K resolutions and 8K resolutions.

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