Pixel
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This example shows an image with a portion greatly enlarged. The different shades of colour blend together to create the perception of a smooth image. |
A
pixel (short for
picture
element, using the common abbreviation "pix" for "picture") is one of the many tiny
dots that make up the
representation of a
picture in a
computer's memory. Each such information element is not really a dot, nor a square, but an abstract
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 of each pixel is variable; in color systems, each pixel has typically three or four dimensions of variability such as
red, green and blue, or
cyan, magenta, yellow and black.
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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
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 pixels, depending on how a computer displays an image.
In computing, an image composed of pixels is known as a
bitmapped image or a
raster image. The word
raster originates from
halftone printing technology, and has been widely used to describe
television scanning patterns.
Native vs. logical pixels in LCD displays
Since the resolution of most
computer displays can be adjusted from the computer's
operating system, a display's pixel resolution may not be an absolute measurement.
Modern LCD computer displays are designed with a
native resolution which refers to the perfect match between pixels and
triads. (CRT displays also use red-green-blue phosphor triads, but these are not coincident with image pixels, and cannot therefore be said to be equivalent to 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. This often causes the screen to look somewhat jagged or blurry. 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, and may 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 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
digital video 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 represenation 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
bits is 255, so this is the value used for white.
In a colour image, each pixel can be described using its hue, saturation, and value, but is usually represented instead as red, green and blue intensities (see
RGB).
Bits per pixel
The number of distinct colours 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
8=256; (256 colours)],
* 16 bpp [2
16=65536; (65,536 colours, known as
Highcolour or Thousands)],
* 24 bpp [2
24=16777216; (16,777,216 colours, known as
Truecolor or Millions)].
* 48 bpp [2
48; (for a human's practical purpose, a continuous colorspace; used in many flatbed scanners and for professional work)
Images composed of 256 colours or fewer are usually stored in the computer's
video memory in
chunky or
planar format, where a pixel in memory is an index into a list of colours called a
palette. These modes are therefore sometimes called
indexed modes. While only 256 colours are displayed at once, those 256 colours are picked from a much larger palette, typically of 16 million colours. 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 (most human
eyes are more sensitive to green than the other two primary colors). 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
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 displays that can display 48 bits of colour, so this depth is typically used for specialized professional applications with
film scanners and
printers. Such files are
rendered on a screen with 24-bit depth.
Subpixels
Many display and image-acquisition systems are, for various reasons, not capable of displaying or sensing the different
colour channels at the same site. This approach is generally resolved by using multiple
subpixels, each of which handles a single colour channel. For example,
LCD displays 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 sensors also use subpixels, by using coloured filters. (
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 colour displays. The technique, referred to as
subpixel rendering, uses knowledge of
pixel geometry to manipulate the three coloured 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).
A megapixel is 1 million pixels, and is used not only for the number of pixels in an image, but also often to express the number of sensor elements of
digital cameras or the number of display elements of
digital displays. 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 image sensors, 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[
1]. 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.
*0.3 Megapixels = 640x480 or VGA
*0.5 Megapixels = 800x600 or SVGA
*0.8 Megapixels = 1024x768 or XVGA
*1.3 Megapixels = 1280x1024 or SXGA
*1.9 Megapixels = 1600x1200 or UXGA
*3.1 Megapixels = 2048x1536 or QXGA
*5.2 Megapixels = 2560x2048 or QSXGA
Several other types of objects derived from the idea of the pixel, such as the
voxel (volume element),
texel (texture element) and
surfel (surface element), have been created for other
computer graphics and
image processing uses.
The word
pixel was first published in 1965 by
Frederic C. Billingsley of
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) doesn't know where he got it, but says it was "in use at the time" (circa 1963).
The word is a
portmanteau of
picture and
element, via
pix.
Pix was first coined in 1932 in a headline in Variety magazine, 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. The earlist publication of the term
picture element itself was in
Wirelss World magazine in 1927.
A brief but detailed history of
pixel and
picture element, with references, is linked
below.
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Computer display standard*
Image resolution*
Raster scan*
Voxel*
Vector graphics*
Rasterisation*
Electronic maps*
Pixel art*
Gigapixel image*
Pixel advertising*
Antipixel*
Intrapixel and Interpixel processing*
Frederic C. Billingsley, first to publish the word
pixel*
A Quick Guide to Digital Video Resolution and Pixel Aspect Ratios*
Megapixel calculator: Calculate image dimensions, DPI and file sizes, storage (in Spanish).
*
Megapixel calculator: Calculate image dimensions, resolution and file sizes
*
Digital Camera Pixels: Basics of the pixel, dithering, PPI and print size
*
A Pixel Is Not A Little Square: Microsoft Memo by computer graphics pioneer Alvy Ray Smith
*
Megapixels Chart: Graph displaying megapixels versus maximum photo print size.
*
A Brief History of 'Pixel' More than you need to know about the history of pixel, pel, and picture element
*
Pixels and Me video of a history talk at the
Computer History Museum
