Pixel
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A pixel (a portmanteau of picture element) is one of the many tiny dots that make up the representation of a picture in a computer's memory. Usually the dots are so small and so numerous that, when printed on paper or displayed on a computer monitor, they appear to merge into a smooth image. The colour and intensity of each dot is chosen individually by the computer to represent a small area of the picture. Pixel is sometimes abbreviated px or pel (for "picture element"), although pel sometimes refers to sub-pixels.
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Technical
Pixels are generally thought of as the smallest complete element of an image. The definition is highly context sensitive. For example, we can speak of pixels in a visible image (e.g. a printed page) or pixels carried by one or more electronic signal(s), or represented by one or more digital value(s), or pixels on a display device. This list is not exhaustive and depending on context there are several synonyms which are accurate in particular contexts, e.g. pel, sample, bytes, bits, dots, spots, superset, triad, stripe set, window, etc. We can also speak of pixels in the abstract, in particular when using pixels as a measure of resolution, e.g. 2400 pixels per inch or 640 pixels per line. Dots is often used to mean pixels, especially by computer sales and marketing people, and gives rise to the abbreviation DPI or dots per inch.
Note that a pixel may be comprised of smaller parts known as sub-pixels. For example a pixel on a color display may be composed of red, green and blue sub-parts (sub-pixels, sub-pels, etc.) the three of which may be referred to as a triad. The terms "pixel" and "image element" are often used interchangeably, although this is technically incorrect.
Image elements is a broader term than pixels and is also highly context sensitive. Image elements includes both complete pixels as well as those various sub-parts of pixels and other elements of images which are not pixel related such as DCT coefficients. For example, it is correct to say that the red part of an RGB pixel is an image element but it is not normally considered correct to refer to the red part as a pixel itself (although persons who are not skilled in the television industry often do).
The more pixels used to represent an image, the closer the result will resemble the original. The number of pixels in an image is called the resolution. This 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.
The coloured dots that form a digitized image (such as a JPG file used on a web page) are also called pixels. Depending on how a computer displays an image, these may not be in one-to-one correspondence with screen pixels. In areas where the distinction is important, the dots in the image file may be called texels.
In computer programming, an image composed of pixels is known as a bitmapped image or a raster image. The word raster originates from analogue television technology. Bitmapped images are used to encode digital video and to produce computer-generated art.
Native vs. logical pixels
Since the resolution of most computer displays can be adjusted from the computer's operating system, a pixel is a purely relative 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 the resolution. Non-native resolutions have to be approximated by the software in the LCD screen, using interpolation algorithms. This often causes the screen to look jagged and 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 will be unable to display in 1600×1200 at all 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 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 brightness. 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 has its own hue, saturation and value, usually represented as three numbers representing red, green and blue intensities respectively (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). Common values are
- 8 bpp (256 colours),
- 16 bpp (65,536 colours, known as Highcolour),
- 24 bpp (16,777,216 colours, known as Truecolour).
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 logo of Windows 95 and Windows 98 is probably the best-known example of this kind of animation.
For depths larger than 8 bits, the number is the total 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 (the eye being more sensitive to green). 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. On older systems, 4 bpp (16 colours) is also common.
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.
Sub-pixels
Many display and image-acquisition systems are, for various reasons, not capable of displaying the different colour channels at the same site. This approach is generally resolved by using multiple sub-pixels, each of which handles a single colour channel. For example, LCD displays typically divide each pixel horizontally into three sub-pixels. Most LED displays divide each pixel into four sub-pixels; one red, one green, and two blue. Most digital camera sensors also use sub-pixels, 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 sub-pixels.)
For systems with subpixels, two different approaches can be taken:
- ignore the fact that the sub-pixels exist, and treat the system as if it used uniform pixels
- take the sub-pixels into account, complicating the analysis, but potentially producing better images
An example of a technology that uses the latter approach is the use of colour fringing on computer fonts to encode sub-pixel resolution luminance information.
A recent technique for increasing the apparent resolution of a colour display, named subpixel rendering, uses knowledge of pixel geometry to manipulate the three coloured sub-pixels separately, which seems to be most effective with LCD displays set at native resolution. This is a form of anti-aliasing, and is mostly used to improve the appearance of text. Microsoft's ClearType, which is available in Windows XP, is an example of this.
Megapixel
A megapixel is 1 million pixels, and is usually used to express the resolution capabilities of digital cameras. For example, a camera that can take pictures with a resolution of 2048×1536 pixels is commonly said to have "3.1 megapixels" (2048 × 1536 = 3,145,728).
Some digital cameras (digicams) use CCDs, which record brightness levels. Older digital cameras that do not use Foveon X3 CCDs have red, green, and blue colour filters so that each pixel can record the brightness of a single primary colour. Thus, the pixels of digital cameras that don't use Foveon X3 CCDs are similar to sub-pixels. The camera interpolates the colour information to create the final image. Thus, an 'x'-megapixel image from a digital camera can have as little as 1/4th the colour resolution of the same image as taken by a scanner. The detail resolution is unimpaired. Thus, a picture of a blue or red object will tend to look fuzzy compared to the same object in shades of grey. Green objects appear less fuzzy, since green is allocated more pixels (due to the eye's increased sensitivity for green). See [1] (http://megamyth.homestead.com/imageres.html) for a more detailed discussion.
Similar concepts
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.
See also
- Computer display standard
- Image resolution
- voxel
- Vector graphics
- hunt-the-pixel
- putpixel
- Electronic maps
- Pixel art
- Gigapixel image
External links
- A Quick Guide to Digital Video Resolution and Pixel Aspect Ratios (http://www.uwasa.fi/~f76998/video/conversion/)ar:بكسل
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