„Díll“: Munur á milli breytinga

Efni eytt Efni bætt við
BiT (spjall | framlög)
m Ný síða: {{dablink|This article is about the picture element. For other uses, see Pixel (disambiguation).}} [[Image:Pixel-example.png|right|frame|This example shows an image with a portion gr...
 
BiT (spjall | framlög)
mEkkert breytingarágrip
Lína 1:
[[Image:Pixel-example.png|right|frame|Þetta dæmi sýnir þar sem einn hluti myndarinnar hefur verið mikið stækkaður, og auðveldlega er hægt að sjá hvern díl (líta út eins og litlir kassar) fyrir sig.]]
{{dablink|This article is about the picture element. For other uses, see [[Pixel (disambiguation)]].}}
[[Image:Pixel-example.png|right|frame|This example shows an image with a portion greatly enlarged, in which the individual pixels are rendered as little squares and can easily be seen.]]
 
'''Díll''' er minnsta eining í [[mynd]], og hefur aðeins einn [[litur|lit]].
A '''pixel''' (short for '''pic'''ture '''el'''ement, using the common abbreviation "pix" for "picture") is a single point in a graphic image. Each such information element is not really a dot, nor a square, but an abstract [[sample (signal)|sample]]. With care, pixels in an image can be reproduced at any size without the appearance of visible dots or squares; but in many contexts, they are reproduced as dots or squares and can be visibly distinct when not fine enough. The [[Intensity (physics)|intensity]] of each pixel is variable; in color systems, each pixel has typically three or four dimensions of variability such as [[RGB color model|red, green, and blue]], or [[CMYK color model|cyan, magenta, yellow, and black]].
 
{{stubbur}}
==Technical==
 
[[Image:ReconstructionsFromPixels.png|right|frame|A pixel is not a little square. This image shows alternative ways of reconstructing an image from a set of pixel values, using dots, lines, or smooth filtering.]]
 
A pixel is generally thought of as the smallest complete sample of an image. The definition is highly context sensitive; for example, we can speak of printed pixels in a page, or pixels carried by electronic signals, or represented by digital values, or pixels on a display device, or pixels in a digital camera (photosensor elements). This list is not exhaustive, and depending on context there are several synonyms that are accurate in particular contexts, e.g. pel, sample, byte, bit, dot, spot, etc. We can also speak of pixels in the abstract, or as a unit of measure, in particular when using pixels as a measure of resolution, e.g. 2400 pixels per inch, 640 pixels per line, or spaced 10 pixels apart.
 
The measures [[dots per inch]] (dpi) and [[pixels per inch]] (ppi) are sometimes used interchangeably, but have distinct meanings especially in the printer field, where dpi is a measure of the printer's resolution of dot printing (e.g. ink droplet density). For example, a high-quality inkjet image may be printed with 200 ppi on a 720 dpi printer.
 
The more pixels used to represent an image, the closer the result can resemble the original. The number of pixels in an image is sometimes called the [[image resolution|resolution]], though resolution has a more specific definition. Pixel counts can be expressed as a single number, as in a "three-megapixel" [[digital camera]], which has a nominal three million pixels, or as a pair of numbers, as in a "640 by 480 display", which has 640 pixels from side to side and 480 from top to bottom (as in a [[VGA]] display), and therefore has a total number of 640 × 480 = 307,200 pixels or 0.3 megapixels.
 
The pixels, or color samples, that form a digitized image (such as a [[JPEG]] file used on a web page) may or may not be in one-to-one [[correspondence]] with [[screen pixel]]s, depending on how a computer displays an image.
 
In computing, an image composed of pixels is known as a ''[[bitmap|bitmapped image]]'' or a ''[[raster graphics|raster image]]''. The word ''raster'' originates from [[halftone]] printing technology, and has been widely used to describe [[raster scan|television scanning]] patterns.
 
===Display pixel size===
The size of a display pixel is determined by the screen resolution and diagonal size of the monitor displaying it.
Some Examples:
 
*Screen Res: 1024x768, Diagonal Size: 19", Pixel size: 0.377mm
*Screen Res: 800x600, Diagonal Size: 17", Pixel size: 0.4318mm
*Screen Res: 640x480, Diagonal Size: 15", Pixel size: 0.4763mm
 
=== Native vs. logical pixels in LCDs ===
 
Modern computer monitors are expected to display a range of resolutions (this was not always so, even with [[Cathode ray tube|CRTs]]). Displays capable of truly displaying only one resolution must first generate a native-resolution signal from any signal in a non-native resolution.
 
Modern computer LCDs are designed with a ''native resolution'' which refers to the perfect match between pixels and [[Triad (computers)|triads]]. CRT displays also use native red-green-blue phosphor triads, but these are not coincident with logical pixels.
 
The native resolution will produce the sharpest picture capable from the display. However, since the user can adjust the resolution, the monitor must be capable of displaying other resolutions.
Non-native resolutions have to be supported by approximate resampling in the LCD controller, using [[interpolation]] algorithms (in CRTs, the physical system interpolates between the logical pixels and the physical phosphors). This often causes the screen to look somewhat jagged or blurry (especially with resolutions that are not even multiples of the native one). For example, a display with a native resolution of 1280×1024 will look best set at 1280×1024 resolution, will display 800×600 adequately by drawing each pixel with more physical triads, but will be unable to display in 1600×1200 sharply due to the lack of physical triads.
 
Pixels can be either rectangular or square. A number called the ''[[Aspect ratio (image)|aspect ratio]]'' describes the squareness of a pixel. For example, a 1.25:1 aspect ratio means that each pixel is 1.25 times wider than it is high. Pixels on computer monitors are usually square, but pixels used in some [[digital video]] formats have non-square aspect ratios, such as those used in the PAL and NTSC variants of the [[CCIR 601]] digital video standard, and the corresponding [[anamorphic widescreen]] formats.
 
Each pixel in a monochrome image has its own value, a correlate of perceptual [[brightness]] or physical [[intensity]]. A numeric representation of zero usually represents black, and the maximum value possible represents white. For example, in an eight-bit image, the maximum unsigned value that can be stored by eight [[bit]]s is 255, so this is the value used for white.
 
In a color image, each pixel can be described using its hue, saturation, and value ([[HSV color space|HSV]]), but is usually represented instead as the red, green, and blue intensities (in its [[RGB]] [[color space]]).
 
=== Bits per pixel ===
{{main|Color depth}}
The number of distinct colors that can be represented by a pixel depends on the number of bits per pixel (bpp). The maximum number of colors a pixel can take can be found by taking two to the power of the color depth. For example, common values are
* 8 bpp, 2<sup>8</sup> = 256 colors
* 16 bpp, 2<sup>16</sup> = 65536 colors; known as [[Highcolor]] or Thousands
* 24 bpp, 2<sup>24</sup> = 16,777,216 colors; known as [[Truecolor]] or Millions
* 48 bpp; for all practical purposes a continuous colorspace; used in many flatbed scanners and for professional work
 
Images composed of 256 colors or fewer are usually stored in the computer's [[video memory]] in [[packed pixel]] (chunky) format, or sometimes in [[Planar (computer graphics)|planar]] format, where a pixel in memory is an index into a list of colors called a [[Palette (computing)|palette]]. These modes are therefore sometimes called ''[[indexed color|indexed]]'' modes. While only 256 colors are displayed at once, those 256 colors are picked from a much larger palette, typically of 16 million colors. Changing the values in the palette permits a kind of animation effect. The animated startup logos of [[Windows 95]] and [[Windows 98]] are probably the best-known example of this kind of animation. On older systems, 4 bpp (16 colors) was common.
 
For depths larger than 8 bits, the number is the sum of the bits devoted to each of the three RGB (red, green and blue) components. A 16-bit depth is usually divided into five bits for each of red and blue, and six bits for green, as most human [[eye]]s are more sensitive to green than the other two primary colors. For applications involving transparency, the 16 bits may be divided into five bits each of red, green, and blue, with one bit left for transparency. A 24-bit depth allows 8 bits per component. On some systems, 32-bit depth is available: this means that each 24-bit pixel has an extra 8 bits to describe its [[opacity]] (for purposes of combining with another image).
 
When an [[image file]] is displayed on a [[screen]], the number of bits per pixel is expressed separately for the [[image file format|raster file]] and for the [[display]]. Some raster file formats have a greater bit-depth capability than others. The [[GIF]] format, for example, has a maximum depth of 8 bits, while [[TIFF]] files can handle 48-bit pixels. There are no consumer display adapters that can output 48 bits of color, so this depth is typically used for specialized professional applications with [[film scanner]]s, [[computer printer|printer]]s and very expensive [[Workstation|workstation computers]]. Such files are only [[rendering (computer graphics)|rendered]] on screen with 24-bit depth on most computers.
 
=== Subpixels ===
[[Image:CRT phosphor dots.png|thumb|200px|[[Phosphor]] dots in a color CRT display bear no relation to pixels or subpixels]]
Many display and image-acquisition systems are, for various reasons, not capable of displaying or sensing the different [[color]] channels at the same site. This approach is generally resolved by using multiple '''subpixels''', each of which handles a single color channel. For example, [[LCD]]s typically divide each pixel horizontally into three subpixels. Most [[LED]] displays divide each pixel into four subpixels; one red, one green, and two blue. Most [[digital camera]] [[sensor]]s also use subpixels, by using colored filters. ([[Cathode Ray Tube|CRT]] displays also use red-green-blue phosphor dots, but these are not aligned with image pixels, and cannot therefore be said to be subpixels.)
 
For systems with subpixels, two different approaches can be taken:
:*The subpixels can be ignored, with pixels being treated as the smallest addressable imaging element; or
:*The subpixels can be included in rendering calculations, which requires more analysis and processing time, but can produce apparently superior images in some cases.
 
The latter approach has been used to increase the apparent resolution of color displays. The technique, referred to as [[subpixel rendering]], uses knowledge of [[pixel geometry]] to manipulate the three colored sub-pixels separately, and is most effective with flat-panel displays set to their native resolutions (because the pixel geometry of such displays is usually fixed and predictable). This works best with black-on-white images and thus is often used to make text sharper and easier to read. An added bonus of this effect is that while it does not work on CRTs, it still produces an [[anti-aliasing]] effect, and thus still improves image quality to some extent.
 
== Megapixel ==
 
A megapixel is 1 million pixels, and is a term used not only for the number of pixels in an image, but also to express the number of sensor elements of [[digital camera]]s or the number of display elements of [[computer display|digital display]]s. For example, a camera with an array of 2048×1536 sensor elements is commonly said to have "3.1 megapixels" (2048 × 1536 = 3,145,728).
 
Digital cameras use photosensitive electronics, either [[charge-coupled device]] (CCD) or [[CMOS|complementary metal–oxide–semiconductor]] (CMOS) [[image sensor]]s, consisting of a large number of single sensor elements, each of which records a measured intensity level. In most digital cameras, the sensor array is covered with a patterned color filter mosaic having red, green, and blue regions in the [[Bayer filter]] arrangement, so that each sensor element can record the intensity of a single primary color of light. The camera interpolates the color information of neighboring sensor elements, through a process called [[demosaicing]], to create the final image. These sensor elements are often called "pixels", even though they only record 1 channel (only red, or green, or blue) of the final color image. Thus, a so-called ''N-megapixel'' camera that produces an N-megapixel image provides only one-third of the information that an image of the same size could get from a scanner. Thus, certain color contrasts may look fuzzier than others, depending on the allocation of the primary colors (green has twice as many elements as red or blue in the Bayer arrangement).
 
In contrast to conventional image sensors, the [[Foveon X3 sensor]] uses three layers of sensor elements, so that it detects red, green, and blue intensity at each array location. This structure eliminates the need for de-mosaicing and eliminates the associated image artifacts, such as color blurring around sharp edges. Citing the precedent established by mosaic sensors, Foveon counts each single-color sensor element as a pixel, even though the native output file size has only one pixel per three camera pixels.[http://www.x3f.info/technotes/x3pixel/pixelpage.html] With this method of counting, an N-megapixel Foveon X3 sensor therefore captures the same amount of information as an N-megapixel Bayer-mosaic sensor, though it packs the information into fewer image pixels, without any interpolation.
 
== Standard display resolutions ==
[[Image:Vector Video Standards.png|thumb|Display standards comparison]]
<!--Replace with [[Image:Video Standards.svg|thumb|Display standards comparison]] when rendering is fixed-->
Standard [[display resolution]]s include:
* [[VGA]] 0.3 Megapixels = 640×480
* [[SVGA]] 0.5 Megapixels = 800×600
* [[XGA]] 0.8 Megapixels = 1024×768 (sometimes called [[XVGA]])
* [[SXGA]] 1.3 Megapixels = 1280×1024
* [[SXGA+]] 1.4 Megapixels = 1400×1050
* [[UXGA]] 1.9 Megapixels = 1600×1200
* [[WUXGA]] 2.3 Megapixels = 1920×1200
* [[QXGA]] 3.1 Megapixels = 2048×1536
* [[WQXGA]] 4.1 Megapixels = 2560×1600
* [[QSXGA]] 5.2 Megapixels = 2560×2048
* [[WQSXGA]] 6.6 Megapixels = 3200×2048
* [[QUXGA]] 7.7 Megapixels = 3200×2400
* [[WQUXGA]] 9.2 Megapixels = 3840×2400
 
== Similar concepts ==
 
Several other types of objects derived from the idea of the pixel, such as the [[voxel]] (volume element), [[texel (graphics)|texel]] (texture element) and [[surfel]] (surface element), have been created for other [[computer graphics]] and [[image processing]] uses.
 
== Etymology ==
 
The word '''''pixel''''' was first published in 1965 by [[Frederic C. Billingsley]] of [[Jet Propulsion Laboratory|JPL]], to describe the picture elements of video images from space probes to the moon and Mars; but he did not coin the term himself, and the person he got it from (Keith E. McFarland at the Link Division of General Precision in Palo Alto) does not know where he got it, but says it was "in use at the time" (circa 1963).
 
The word is a combination of ''picture'' and ''element'', via ''pix''. ''Pix'' was first coined in 1932 in a [[Variety Magazine]] headline, as an abbreviation for the word ''pictures'', in reference to movies; by 1938 ''pix'' was being used in reference to still pictures by photojournalists.
 
The concept of a '''picture element''' dates to the earliest days of television, for example as '''''Bildpunkt''''' (the German word for ''pixel,'' literally ''picture point'') in the 1888 German patent of [[Paul Nipkow]]. According to various etymologies, the earliest publication of the term ''picture element'' itself was in ''Wireless World'' magazine in 1927,[http://select.nytimes.com/search/restricted/article?res=F60612FA35550C718CDDAD0894DD494D81] though it had been used earlier in various U.S. patents filed as early as 1911.[http://www.google.com/patents?vid=USPAT1175313&id=Df9EAAAAEBAJ&dq=%22picture+element%22&as_drrb_ap=b]
 
A brief but detailed history of ''pixel'' and ''picture element'', with references, is linked [[#External links|below]].
 
== See also ==
<div style="-moz-column-count:3; column-count:3;">
*[[Subpixel rendering]]
*[[Computer display standard]]
*[[Image resolution]]
*[[Raster scan]]
*[[Voxel]]
*[[Vector graphics]]
*[[Rasterisation]]
*[[Map#Electronic maps|Electronic maps]]
*[[Pixel art]]
*[[Gigapixel image]]
*[[Pixel advertising]]
*[[Intrapixel and Interpixel processing]]
*[[Frederic C. Billingsley]], first to publish the word ''pixel''
*[[QVGA]]
</div>
 
== External links ==
 
* [http://www.uwasa.fi/~f76998/video/conversion/ A Quick Guide to Digital Video Resolution and Pixel Aspect Ratios]
* [http://web.forret.com/tools/megapixel.asp Megapixel calculator]: Calculate image dimensions, resolution and file sizes.
* [http://www.cambridgeincolour.com/tutorials/digital-camera-pixel.htm Digital Camera Pixels]: Basics of the pixel, dithering, PPI and print size.
* [http://fred.dsimprove.be/__offSite/pixel/ Pixel & Resolution Tables]: Practical tables, and theory for beginner's education about resolution issues in digital imaging & printing.
* [http://alvyray.com/Memos/MemosMicrosoft.htm#PixelIsNotSquare A Pixel Is Not A Little Square]: Microsoft Memo by computer graphics pioneer Alvy Ray Smith.
* [http://design215.com/toolbox/megapixels.php Megapixels Chart]: Graph displaying megapixels versus maximum photo print size.
* [http://www.foveon.com/article.php?a=74 A Brief History of 'Pixel']: More than you need to know about the history of pixel, pel, and picture element.
 
* [http://video.google.com/videoplay?docid=-5655850487750051532 Pixels and Me]: Video of a history talk at the [[Computer History Museum]].
 
[[Category:Computer graphics]]
[[Category:Image processing]]
[[Category:Digital geometry]]
[[Category:Display technology]]
 
[[ar:بكسل]]
Lína 146 ⟶ 11:
[[da:Pixel]]
[[de:Pixel]]
[[en:DíllPixel]]
[[es:Píxel]]
[[eo:Rastrumero]]