Pixel

With pixel, pixel image cell or pixel (rare Pel ) the individual color values ​​of a digital raster graphics are referred to as well as the data necessary for collection or representation of a color value surface elements with an image sensor or screen with grid control. "Pixel" ( nominative singular: the pixel; genitive: the pixel; plural: the pixel ) is a portmanteau of the abbreviations of English words pictures ( colloquially shortened to " pix " ) and element and is often abbreviated as px.

Pixels of raster graphic

The pixels in a raster graphics are like a grid points arranged, which is assigned a color.

Pixels as discrete samples

Often pixels are considered as rectangular or square. However, this is not a universal model concept. For the purposes of digital signal processing, a pixel is a discrete sample; Make others points than the pixel No conclusions. This is evident in the increase of raster graphics: The appearance varies depending on the chosen scaling method, and the pixels of the output image will appear in the magnification is not necessarily the squares ( see picture right). A similar error would be interpreted as the samples of a digital audio signal over a certain time interval equal residual value because the signal is displayed by the audio editor in the step-like enlargement.

The conceptual model of a square pixel is inappropriate, because it neglects the different options in the conversion of image content to raster graphics. In computer graphics, are created in the artificial images, the desired image content as a vector graphic, 3D scene description or another type of image description can be present. This image description defines a continuous signal must be converted to a raster graphics ( scanned or rendered ) by the image contents are scanned. The reconstruction filter used determines how the colors of the original image description are weighted in the vicinity of a pixel and feeds into the pixel color. In an image-capturing system that digitizes a natural image signal on an optical image plane or row, the corresponding theoretical " reconstruction filter " statements (the point spread ) through the optical and electronic components of the system.

While it is possible to consider pixels as squares, but this is at best useful when a box filter was chosen as the reconstruction filter, because here the color value of a pixel would correspond to the average of all color contributions within the square. However, the resulting pixel must generally be considered as a discrete sample for subsequent image processing. If by " a pixel width" is mentioned, then it refers to the actual distance between two adjacent pixels; the "center " of a pixel refers to in reality the pixel itself with phrases such as " overlap of half a pixel " the overlapping of the image content by the reconstruction filter used is meant.

Pixel values

The encoding used in the pixel color is defined, inter alia, about the color space and the color depth. The simplest case is a binary image in which a pixel stores a monochrome value.

In addition to color information screen graphics can also contain a so-called alpha channel that contains transparency information. The storage of any further information is conceivable; the exact format depends on the graphics format being used. Only at the output on the screen a specific format must be observed, which is specified by the image memory of the graphics card.

Problems

Both the resolution (and hence the local sampling rate ) and the size of the information stored in the pixels ( for example, the color depth) is restricted in practice, therefore, a pixel may represent an approximation of the reality.

The limitations of the local sampling rate means that image information is lost. According to the Nyquist -Shannon sampling theorem, it may in certain image content and a low sampling rate or resolution to aliasing or jaggies ( " pixelated " representation ) can occur. These effects can be counteracted by anti- aliasing; the anti-aliasing in computer graphics avails itself of different methods. Antialiasing optical signals may be performed by a low pass combined with aperture correction.

The reduce, enlarge or rotate a raster graphics can be blurred or cause faulty acting images.

Pixels in image sensors and screens

The pixels of an image sensor or the screen surfaces usually consist of a respective color ( red, green and blue). Wherein the liquid crystal - display (LCD ) of each visible pixel is driven with a color value. The authorities responsible for the basic colors of the pixel areas, called subpixels are often located adjacent to each other. The finer compared to Pixel subpixel can be used in order to increase the horizontal resolution with the screening ( subpixel rendering ). Furthermore, triangular, irregularly great, alternately arranged, or additional white subpixels are possible, for example in the PenTile pixel geometries from Samsung. For some, especially older, flat panel production reasons so-called pixel errors may occur.

CRT screens project the image signal by means of electron beams on a fluorescent matrix with a fixed resolution. Although mounted in front of the phosphor layer slot, strip or shadow mask ensures that only belonging to the respective electron primary colors are encouraged. Because of the relatively wide and approximately normally distributed intensity profile of the electron and distortion and stray light, however, the pixels of the phosphor matrix do not exactly match the expected pixels, even if the output resolution of the physical resolution of the screen corresponds tube.

The physical size of a pixel depends on the device. The resolution of a monitor or scanner is measured in pixels per inch (ppi ) or dots per inch (dpi). Commercially available computer screens achieve a pixel density of about 100 ppi, corresponding to 0.3 mm per pixel. For TVs, the pixel density is usually low and with newer smartphones higher order multiples, while the sensors of scanners and digital cameras can reach several thousand ppi. The number of pixels in image sensors maximum usable is often specified in megapixels, but usually only the color points of a Bayer sensor are meant and not pixels. The aspect ratio of a pixel on the screen (English pixelAspectRatio ) need not necessarily be one; most SDTV video standards describe irregular pixel aspect ratios. The pixel size and the pixel spacing in relation to the image resolution have a decisive influence on the legibility and visibility of text and graphics on computer monitors and televisions.

Conceptual history and related terms

The term " pixel " in the sense of a small display unit of a device was first 1884 patent for his electric telescope used in Paul Nipkow, but the term was previously in optics usual.

The term " picture element" has been used since 1911 in various U.S. patents. When scanning, processing, and display of images by means of computers became possible in the 1950s and 1960s, the literature most commonly used other terms such as "resolution element ", " spot", "sample ", " grid point" or "matrix element ".

The oldest known document in which the term "pixel " occurs are Fred C. Billingsley 1965 published articles Digital Video Processing at JPL and Processing Ranger and Mariner Photography in the Proceedings of the SPIE Vol 0003 or 0010. The less common name Pel was Vol 55 published by William F. Schreiber as part of his article, Picture Coding in the IEEE Proceedings in March 1967.

The term "pixel" is also used in names of certain applications of raster graphics, such as pixel fonts, pixel art and pixel banner. Derived from " pixel" is the concept of voxels, which indicates, inter alia, the three-dimensional equivalent of a pixel, and the term texels for pixel of a texture image used in the synthesis.