Pixel density

The dot density is a measure of the level of detail a rasterized visual representation and thus one of the quality aspects of the technical reproduction method in image reproduction. Point densities are given for example in four-color printing or with a screen playback.

Common units of the dot density in practice:

From point density is also speaking at a rasterized image scanning, for example using a scanner. Again, it is one of the quality aspects of the sampling process.

For the human eye " perceived quality " ( visual perception) Other factors besides the point density a crucial role, among other things, the color depth and sharpness of the template itself

• 2.1 color printing
• 2.2 Ausbelichtung on photo paper
• 2.3 scanner
• 2.4 Digital Camera
• 2.5 Screen
• 2.6 Computer Mouse
• 2.7 graphic file

Conceptual distinctions

Classification

The now common difference of the point density in different units to historical reasons. The universality of digital images and their dissemination in the mass market began in the 1990s. Until then, the image processing was settled in different areas:

• Reprotechnik
• Photography
• Printing Techniques
• Television Technology
• Computer Graphics

Each area used and uses its own terminology and units of measurement. Between these areas there are often technical compatibility issues (see also "Current development of image processing " ), and it must be taken always aware of what techniques and effects of the current playback is based. There are also purely computational differences through the use of both the Anglo-American system of measurement and the metric system.

Resolution

Colloquially only the varied term, the " resolution " is used in all the above-mentioned areas often. However, it should be distinguished in the individual case whether it is, for example, pressure points (English " dots" ), pixel, a number of rows or columns, a total number of pixels or a purely mathematical quantity (for example, by interpolation ) is.

The term occasionally used " relative resolution " is misleading because the point density is an absolute physical quantity and can be measured directly.

Point

When specifying a point density must be clear from the context what is meant in an individual case under a point. Can be meant, among other

• A (any color ) pixels of a template ( for example, a graphics file )
• A (any color ) pixels of a Play ( for example, an RGB pixel of an LCD )
• A monochromatic point during playback ( for example, Dot, subpixel )
• An arbitrary point of a special color printing reproduction process, for example, dye sublimation printing
• A monochrome scanning an optical sensor
• An arbitrarily colored scanning of a sensor system

The distinction is important because, for example, at current printing methods fall on a pixel multiple pressure points and the subdivision has a direct effect on the ratio of points per length. A computer screen has subpixels horizontally with respect to three times the resolution and vertically. The distribution of the pixels in different color reproduction points is done in many imaging techniques, for example in four-color printing, but also in a normal screen. There one used for the single-color reproduction points, the terms subpixels segment or sample.

Point density

1 inch = 25.4 mm. Converted results from 0.3937 point per cm

This unit is also called dpc for dots per centimeter / centimeter.

Example 1

In the example of the fish graphic on the right following point densities are obtained for the screen display of the yellow and blue "dots":

• Horizontal: 24 pixels per length a (240 pixels)
• Vertical: 20 points per length b (200 pixels)

( 11 ppi, respectively on a screen with 110 dpi)

In conventional computer monitors and graphics today with square pixels horizontal and vertical dot densities are identical.

The point density of the graph can be determined from the density of the point representing the screen ( according to its technical data, such as 96 ppi) or the expression of a printer. The graphic itself ( as a computer graphic file ) has no resolution in dpi or ppi as it exists independently of the presentation medium.

Example 2

A resolution of 1200 dpi equivalent horizontally and 600 dpi vertically (assuming that the dots seamless and contiguous area ) of a point size of 21.2 microns x 42.3 microns. " 1200 dpi horizontally " means that distribute 1200 points in the horizontal 25.4 mm. Therefore has a point in the horizontal edge length of

Since the resolution in the vertical is only 600 dpi, here is a point twice as "long ", namely

For a single such point results in a total area of

Applications

Below are some common methods are listed in which the grid plays an important role (Table Typical dot densities ).

Four-color printing

For four-color printing each pixel according to its color in pressure points ( engl. dots ) of the four process colors (CMYK ) is dissolved. The finer the printed screen are resolved ( point density), the better these pressure points mix in the result for the human eye.

Closing date on photo paper

In a closing date on photo - chemical paper no visible pressure grid is created. The maximum achievable playback quality is determined by the optical exposure process as well as the chemical and physical properties of paper (eg, grain size, surface texture).

Scanner

Scanners capture a template usually after a screening process, line and point by point. The sampling density in dots per inch ( dpi ) is given, as it was formerly used almost exclusively in the prepress and for a later printing of the history out. Scans are now commonly based on pixels, so that the unit " ppi " use place.

See also: Fax

Digital camera

The level of detail of photos digitally recorded is usually indicated by the total number of recorded pixels of the sensor ( " image size " of the sensor in megapixels ).

It should be noted that other ( optical and electrical ) components significantly influence in the camera, the achievable image quality and limit.

Screen

For screens that are designed for the level of detail of the reproduction essentially the following values ​​of importance:

• The number of pixels in the horizontal and in the vertical direction, in pixels
• The edge length of this visible image area in cm or inch

This is calculated for the horizontal and the vertical one point density (Playback resolution ) in pixels per inch ( ppi):

• Is the number of pixels in the diagonal direction, in pixels,
• Is the number of pixels in the horizontal direction in pixels,
• Is the number of pixels in the vertical direction in pixels, and
• Is the diagonal length of the visible image area in inches. ( Screen size in inches, for example 22 ". )

In practice, the devices differ in the two ratings. In particular, the often -mentioned value of " 72 ppi " ( for " RGB pixel " ) is not applicable for all devices.

Examples:

• A 20 -inch screen (50.8 cm diagonal) 1680 × 1050 pixel has 99.06 ppi.
• A 10.1-inch netbook screen ( 25.65 cm diagonal) 1024 × 600 pixels has 118 ppi.
• A 9.7- inch screen ( 24.6 cm diagonal) 2048 × 1536 has 264 ppi.
• A 4.8- inch screen ( 12.2 cm diagonal) 1280 × 720 has 306 ppi.
• A 3.5 -inch screen ( 8.9 inches diagonal) with 960 × 640 has 329.65 ppi.

Computer Mouse

In computer mice, the number of detectable smallest individual steps per unit length is given to specify a " guiding accuracy ." The usual, albeit related to steps and not on pressure points unit is dpi.

Graphics file

The playback quality in terms of " detail " of raster graphics files (such as JPEG or PNG format) mainly depends on the reproduction method and the image size of the graphic in ( mega) pixels.

The file itself has no quality-critical point density.

In addition to graphic files with pixel grid, there are also graphic files without pixel grid: the vector graphics.

Point density transformation

To a rasterized template customize a halftone reproduction process, the output data must be transformed. The following cases are possible:

• "Implementing 1:1 ," that is, the document size ( in pixels ) corresponds exactly to the output. Example: simple matrix display
• " Reduction ", i.e., the size of the template ( in pixels ) is greater than that of the output. Example: 5 -MP JPG file, displayed on the camera's LCD
• " Magnification ", i.e., the size of the template ( in pixels ) is smaller than that of the output. Example: 1 - MP photos, displayed on a LCD screen size is 1600 × 1200 pixels ( 1.9 MP)

The implementation is almost always effected by means of interpolation to calculate not originally existing pixels. Compare also sharpness / interpolation.

In some processes (mainly in print) in addition there is a screening according to the pressure grid.