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Pixel aspect ratio (often abbreviated PAR) is a mathematical
ratio In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
that describes how the width of a
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the smal ...
in a digital image compares to the height of that pixel. Most
digital imaging Digital imaging or digital image acquisition is the creation of a digital representation of the visual characteristics of an object, such as a physical scene or the interior structure of an object. The term is often assumed to imply or include ...
systems display an image as a grid of tiny, square pixels. However, some imaging systems, especially those that must be compatible with
standard-definition television Standard-definition television (SDTV, SD, often shortened to standard definition) is a television system which uses a resolution that is not considered to be either high or enhanced definition. "Standard" refers to it being the prevailing sp ...
motion pictures, display an image as a grid of rectangular pixels, in which the pixel width and height are different. Pixel aspect ratio describes this difference. Use of pixel aspect ratio mostly involves pictures pertaining to standard-definition television and some other exceptional cases. Most other imaging systems, including those that comply with
SMPTE The Society of Motion Picture and Television Engineers (SMPTE) (, rarely ), founded in 1916 as the Society of Motion Picture Engineers or SMPE, is a global professional association of engineers, technologists, and executives working in the m ...
standards and practices, use square pixels. PAR is also known as ''sample aspect ratio'' and abbreviated ''SAR'', though it can be confused with storage aspect ratio.


Introduction

The ratio of the width to the height of an image is known as the aspect ratio, or more precisely the display aspect ratio (DAR) – the aspect ratio of the image ''as displayed;'' for TV, DAR was traditionally 4:3 (a.k.a. fullscreen), with 16:9 (a.k.a. widescreen) now the standard for HDTV. In digital images, there is a distinction with the storage aspect ratio (SAR), which is the ratio of
pixel dimensions The display resolution or display modes of a digital television, computer monitor or display device is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution i ...
. If an image is displayed with square pixels, then these ratios agree; if not, then non-square, "rectangular" pixels are used, and these ratios disagree. The aspect ratio of the pixels themselves is known as the ''pixel aspect ratio'' (PAR) – for square pixels this is 1:1 – and these are related by the identity: :SAR × PAR = DAR. Rearranging (solving for PAR) yields: :PAR = DAR/SAR. For example, a 640 × 480
VGA Video Graphics Array (VGA) is a video display controller and accompanying de facto graphics standard, first introduced with the IBM PS/2 line of computers in 1987, which became ubiquitous in the PC industry within three years. The term can no ...
image has a SAR of 640/480 = 4:3, and if displayed on a 4:3 display (DAR = 4:3) has square pixels, hence a PAR of 1:1. By contrast, a 720 × 576
D-1 D1, D01, D.I, D.1 or D-1 can refer to: Science and technology Biochemistry and medicine * ATC code D01 ''Antifungals for dermatological use'', a subgroup of the Anatomical Therapeutic Chemical Classification System * Dopamine receptor D1, Dopamine ...
PAL image has a SAR of 720/576 = 5:4, but is displayed on a 4:3 display (DAR = 4:3). In analog images such as film there is no notion of pixel, nor notion of SAR or PAR, but in the ''digitization'' of analog images the resulting digital image has pixels, hence SAR (and accordingly PAR, if displayed at the same aspect ratio as the original). Non-square pixels arise often in early digital TV standards, related to digitalization of analog TV signals – whose vertical and "effective" horizontal resolutions differ and are thus best described by non-square pixels – and also in some digital video cameras and computer
display mode Computer display standards are a combination of Display aspect ratio, aspect ratio, display size, display resolution, color depth, and refresh rate. They are associated with specific expansion cards, Audio and video interfaces and connectors#Vide ...
s, such as
Color Graphics Adapter The Color Graphics Adapter (CGA), originally also called the ''Color/Graphics Adapter'' or ''IBM Color/Graphics Monitor Adapter'', introduced in 1981, was IBM's first color graphics card for the IBM PC and established a de facto computer display ...
(CGA). Today they arise also in
transcoding Transcoding is the direct digital-to-digital conversion of one encoding to another, such as for video data files, audio files (e.g., MP3, WAV), or character encoding (e.g., UTF-8, ISO/IEC 8859). This is usually done in cases where a target devic ...
between resolutions with different SARs. Actual displays do not generally have non-square pixels, though digital sensors might; they are rather a mathematical abstraction used in resampling images to convert between resolutions. There are several complicating factors in understanding PAR, particularly as it pertains to digitization of analog video: * First, analog video does not have pixels, but rather a
raster scan A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image s ...
, and thus has a well-defined vertical resolution (the lines of the raster), but not a well-defined ''horizontal'' resolution, since each line is an analog signal. However, by a standardized sampling rate, the effective horizontal resolution can be determined by the
sampling theorem Sampling may refer to: *Sampling (signal processing), converting a continuous signal into a discrete signal * Sampling (graphics), converting continuous colors into discrete color components *Sampling (music), the reuse of a sound recording in ano ...
, as is done below. * Second, due to overscan, some of the lines at the top and bottom of the raster are not visible, as are some of the possible image on the left and right – see Overscan: Analog to digital resolution issues. Also, the resolution may be rounded (DV NTSC uses 480 lines, rather than the 486 that are possible). * Third, analog video signals are ''interlaced'' – each image (frame) is sent as two "fields", each with ''half'' the lines. Thus either the pixels are twice as tall as they would be without interlacing, or the image is deinterlaced.


Background

Video Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) syste ...
is presented as a sequential series of images called video frames. Historically, video frames were created and recorded in analog form. As digital display technology, digital broadcast technology, and digital
video compression In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
evolved separately, it resulted in video frame differences that must be addressed using pixel aspect ratio. Digital video frames are generally defined as a grid of pixels used to present each sequential image. The horizontal component is defined by pixels (or samples), and is known as a video line. The vertical component is defined by the number of lines, as in 480 lines.
Standard-definition television Standard-definition television (SDTV, SD, often shortened to standard definition) is a television system which uses a resolution that is not considered to be either high or enhanced definition. "Standard" refers to it being the prevailing sp ...
standards and practices were developed as broadcast technologies and intended for terrestrial broadcasting, and were therefore not designed for digital video presentation. Such standards define an image as an array of well-defined horizontal " Lines", well-defined vertical "Line Duration" and a well-defined picture center. However, there is not a standard-definition television standard that properly defines image edges or explicitly demands a certain number of picture elements per line. Furthermore, analog video systems such as
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
480i 480i is the video mode used for standard-definition digital television in the Caribbean, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Paraguay, and Uruguay). The ''480 ...
and
PAL Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
576i 576i is a standard-definition television, standard-definition digital video mode, originally used for digitizing analog television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because ...
, instead of employing progressively displayed frames, employ
fields Fields may refer to: Music * Fields (band), an indie rock band formed in 2006 * Fields (progressive rock band), a progressive rock band formed in 1971 * ''Fields'' (album), an LP by Swedish-based indie rock band Junip (2010) * "Fields", a song b ...
or interlaced half-frames displayed in an interwoven manner to reduce flicker and double the image rate for smoother motion.


Analog-to-digital conversion

As a result of computers becoming powerful enough to serve as
video editing Video editing is the manipulation and arrangement of video shots. Video editing is used to structure and present all video information, including films and television shows, video advertisements and video essays. Video editing has been dramatical ...
tools, video
digital-to-analog converter In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function. There are several DAC architec ...
s and
analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...
s were made to overcome this incompatibility. To convert analog video lines into a series of square pixels, the industry adopted a default sampling rate at which
luma Luma or LUMA may refer to: Arts * La Trobe University Museum of Art, Melbourne, Australia * LUMA Projection Arts Festival, an annual event featuring building-scale projection mapping and light installations in Binghamton, NY * LUMA Foundation, ...
values were extracted into pixels. The luma sampling rate for
480i 480i is the video mode used for standard-definition digital television in the Caribbean, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Paraguay, and Uruguay). The ''480 ...
pictures was  MHz and for 576i pictures was  MHz. The term ''pixel aspect ratio'' was first coined when ITU-R BT.601 (commonly known as "
Rec. 601 ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 (or its former name CCIR 601) is a standard originally issued in 1982 by the CCIR (an organization, which has since been renamed as the Internatio ...
") specified that standard-definition television pictures are made of lines of exactly 720 non-square pixels. ITU-R BT.601 did not define the exact pixel aspect ratio but did provide enough information to calculate the exact pixel aspect ratio based on industry practices: The standard luma sampling rate of precisely  MHz. Based on this information: *The pixel aspect ratio for
480i 480i is the video mode used for standard-definition digital television in the Caribbean, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Paraguay, and Uruguay). The ''480 ...
would be 10:11 as: *: 12\tfrac \div 13\tfrac = \tfrac *The pixel aspect ratio for
576i 576i is a standard-definition television, standard-definition digital video mode, originally used for digitizing analog television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because ...
would be 59:54 as: *: 14\tfrac \div 13\tfrac = \tfrac SMPTE RP 187 further attempted to standardize the pixel aspect ratio values for
480i 480i is the video mode used for standard-definition digital television in the Caribbean, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Paraguay, and Uruguay). The ''480 ...
and
576i 576i is a standard-definition television, standard-definition digital video mode, originally used for digitizing analog television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because ...
. It designated 177:160 for
480i 480i is the video mode used for standard-definition digital television in the Caribbean, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Paraguay, and Uruguay). The ''480 ...
or 1035:1132 for
576i 576i is a standard-definition television, standard-definition digital video mode, originally used for digitizing analog television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because ...
. However, due to significant difference with practices in effect by industry and the computational load that they imposed upon the involved hardware, SMPTE RP 187 was simply ignored. SMPTE RP 187 information annex A.4 further suggested the use of 10:11 for
480i 480i is the video mode used for standard-definition digital television in the Caribbean, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Paraguay, and Uruguay). The ''480 ...
. As of this writing, ITU-R BT.601-6, which is the latest edition of ITU-R BT.601, still implies that the pixel aspect ratios mentioned above are correct.


Digital video processing

As stated above, ITU-R BT.601 specified that standard-definition television pictures are made of lines of 720 non-square pixels, sampled with a precisely specified sampling rate. A simple mathematical calculation reveals that a 704 pixel width would be enough to contain a
480i 480i is the video mode used for standard-definition digital television in the Caribbean, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Paraguay, and Uruguay). The ''480 ...
or
576i 576i is a standard-definition television, standard-definition digital video mode, originally used for digitizing analog television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because ...
standard 4:3 picture: * A 4:3 480-line picture, digitized with the Rec. 601-recommended sampling rate, would be 704 non-square pixels wide. *: \frac\times \frac = \frac\Rightarrow x = \frac = 704 * A 4:3 576-line picture, digitized with the Rec. 601-recommended sampling rate, would be 702.915254 non-square pixels wide. *: \frac\times \frac = \frac\Rightarrow x = \frac \approx 702.915254 Unfortunately, not all standard TV pictures are exactly 4:3: As mentioned earlier, in analog video, the center of a picture is well-defined but the edges of the picture are not standardized. As a result, some analog devices (mostly PAL devices but also some NTSC devices) generated motion pictures that were horizontally (slightly) wider. This also proportionately applies to anamorphic widescreen (16:9) pictures. Therefore, to maintain a safe margin of error, ITU-R BT.601 required sampling 16 more non-square pixels per line (8 more at each edge) to ensure saving all video data near the margins. This requirement, however, had implications for PAL motion pictures. PAL pixel aspect ratios for standard (4:3) and anamorphic wide screen (16:9), respectively 59:54 and 118:81, were awkward for digital image processing, especially for mixing PAL and NTSC video clips. Therefore, video editing products chose the almost equivalent values, respectively 12:11 and 16:11, which were more elegant and could create PAL digital images at exactly 704 pixels wide, as illustrated: * For PAL 4:3: *: \frac\div \frac = \color * For PAL 16:9: *: \frac\div \frac = \color


Inconsistency in defined pixel aspect ratio values

Commonly found on the Internet and in various other published media are numerous sources that introduce different and highly incompatible values as the pixel aspect ratios of various video pictures and video systems. (See the Supplementary sources section.) To neutrally judge the accuracy and/or feasibility of these sources, please note that as the digital motion picture was invented years after the traditional motion picture, all video pictures targeted for standard definition television and compatible media, digital or otherwise, have (and must have) specifications compatible with standard definition television. Therefore, the pixel aspect ratio of digital video must be calculated from the specification of common traditional equipment rather than the specifications of digital video. Otherwise, any pixel aspect ratio that is calculated from a digital video source is only usable in certain cases for the same kind of video sources and cannot be considered/used as a general pixel aspect ratio of any standard definition television system. In addition, unlike digital video that has well-defined picture edges, traditional video systems have never standardized a well-defined edge for the picture. Therefore, the pixel aspect ratio of common standard television systems cannot be calculated based on edges of pictures. Such a calculated aspect ratio value would not be entirely wrong, but also cannot be considered as the general pixel aspect ratio of any specific video system. The use of such values would be restricted only to certain cases.


Modern standards and practices

In modern digital imaging systems and
high-definition television High-definition television (HD or HDTV) describes a television system which provides a substantially higher image resolution than the previous generation of technologies. The term has been used since 1936; in more recent times, it refers to the g ...
s, especially those that comply with SMPTE standards and practices, only square pixels are used for broadcast and display. However, some formats (ex.,
HDV HDV is a format for recording of high-definition video on DV cassette tape. The format was originally developed by JVC and supported by Sony, Canon, and Sharp. The four companies formed the HDV Consortium in September 2003. Conceived as an af ...
,
DVCPRO HD DV refers to a family of codecs and videotape, tape formats used for storing digital video, launched in 1995 by a consortium of camcorder, video camera manufacturers led by Sony and Panasonic. In the late 1990s and early 2000s, DV was strongly ...
) use non-square pixels internally for image storage, as a way to reduce the amount of data that must be processed, thus limiting the necessary transfer rates and maintaining compatibility with existing interfaces.


Issues of non-square pixels

Directly mapping an image with a certain pixel aspect ratio on a device whose pixel aspect ratio is different makes the image look unnaturally stretched or squashed in either the horizontal or vertical direction. For example, a circle generated for a computer display with square pixels looks like a vertical ellipse on a standard-definition NTSC television that uses vertically rectangular pixels. This issue is more evident on wide-screen TVs. Pixel aspect ratio must be taken into consideration by video editing software products that edit video files with non-square pixels, especially when mixing video clips with different pixel aspect ratios. This would be the case when creating a video montage from various cameras employing different video standards (a relatively rare situation). Special effects software products must also take the pixel aspect ratio into consideration, since some special effects require calculation of the distances from a certain point so that they look visually correct. An example of such effects would be radial blur, motion blur, or even a simple image rotation.


Use of pixel aspect ratio

Pixel aspect ratio value is used mainly in digital video software, where motion pictures must be converted or reconditioned to use video systems other than the original. The
video player Media player software is a type of application software for playing multimedia computer files like audio and video files. Media players commonly display standard media control icons known from physical devices such as tape recorders and CD play ...
software may use pixel aspect ratio to properly render digital video on screen. Video editing software uses pixel aspect ratio to properly scale and render a video into a new format. The pixel aspect ratio support is also required to display, without distortion, legacy digital images from computer standards and video-games what existed in the 80s. In that generation, square pixels were too expensive to produce, so machines and video cards like the
SNES The Super Nintendo Entertainment System (SNES), commonly shortened to Super NES or Super Nintendo, is a 16-bit home video game console developed by Nintendo that was released in 1990 in Japan and South Korea, 1991 in North America, 1992 in Euro ...
, CGA, EGA,
Hercules Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures. The Romans adapted the Gr ...
, C64, MSX,
PC-88 The , commonly shortened to PC-88, are a brand of Zilog Z80-based 8-bit home computers released by Nippon Electric Company (NEC) in 1981 and primarily sold in Japan. The PC-8800 series sold extremely well and became one of the three major Japane ...
,
X68000 The is a home computer created by Sharp Corporation. It was first released in 1987 and sold only in Japan. The initial model has a 10 Megahertz, MHz Motorola 68000 Central processing unit, CPU, 1 Megabytes, MB of Random Access Memory, RAM ...
etc had non-square pixels.


Confusion with display aspect ratio

Pixel aspect ratio is often confused with different types of image aspect ratios; the ratio of the image width and height. Due to non-squareness of pixels in Standard-definition TV, there are two types of such aspect ratios: ''storage aspect ratio'' (''SAR'') and ''
display aspect ratio The aspect ratio of a display device is the proportional relationship between the width and the height of the display. It is expressed as two numbers separated by a colon (''x'':''y''), where ''x'' corresponds to the width and ''y'' to the heigh ...
'' (abbreviated ''DAR'', also known as '' image aspect ratio'' and '' picture aspect ratio''). Also, ''pixel aspect ratio'' (''PAR'') is also known as ''sample aspect ratio'' (abbreviated ''SAR'') in some industrial standards (such as
H.264 Advanced Video Coding (AVC), also referred to as H.264 or MPEG-4 Part 10, is a video compression standard based on block-oriented, motion-compensated coding. It is by far the most commonly used format for the recording, compression, and distr ...
) and output of programs (such as ''
ffmpeg FFmpeg is a free and open-source software project consisting of a suite of libraries and programs for handling video, audio, and other multimedia files and streams. At its core is the command-line ffmpeg tool itself, designed for processing of vid ...
''). Note the reuse of the abbreviations ''PAR'' and ''SAR''. This article uses only the terms pixel aspect ratio, display aspect ratio and storage aspect ratio to avoid ambiguity. Storage aspect ratio is the ratio of the image width to height in pixels, and can be easily calculated from the video file. Display aspect ratio is the ratio of image width to height (in a unit of length such as centimeters or inches) when displayed on screen, and is calculated from the combination of pixel aspect ratio and storage aspect ratio. However, users who know the definition of these concepts may get confused as well. Poorly crafted user-interfaces or poorly written documentations can easily cause such confusion: Some video-editing software applications often ask users to specify an "aspect ratio" for their video file, presenting him or her with the choices of "4:3" and "16:9". Sometimes, these choices may be "PAL 4:3", "NTSC 4:3", "PAL 16:9" and "NTSC 16:9". In such situations, the video editing program is implicitly asking for the pixel aspect ratio of the video file by asking for information about the video system from which the video file originated. The program then uses a table (similar to the one below) to determine the correct pixel aspect ratio value. Generally speaking, to avoid confusion, it can be assumed that video editing products never ask for the storage aspect ratio as they can directly retrieve or calculate it. Non-square-pixel–aware applications also need only to ask for either pixel aspect ratio or display aspect ratio, from either of which they can calculate the other.


Pixel aspect ratios of common video formats

Pixel aspect ratio values for common standard-definition video formats are listed below. Note that for PAL video formats, two different types of pixel aspect ratio values are listed: #'' Rec.601'', a Rec.601-compliant value, which is considered the real pixel aspect ratio of standard-definition video of that type. #'' Digital'', which is roughly equivalent to Rec.601 and is more suitable to use in Digital Video Editing software. Note that sources differ on PARs for common formats – for example, 576 lines (PAL) displayed at 4:3 (DAR) corresponds to either PAR of 12:11 (if 704×576, SAR = 11:9), or a PAR of 16:15 (if 720×576, SAR = 5:4). See
references Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''name'' ...
for sources giving both, and SDTV: Resolution for a table of storage, display and pixel aspect ratios. Also note that CRT televisions do not have pixels, but scanlines.


References


Main sources

*
As of the retrieval date, a free membership of ITU Online Bookstore would allow free download of up to three ITU-R Recommendations. * **This standard, which is the basis for
HDMI High-Definition Multimedia Interface (HDMI) is a proprietary audio/video interface for transmitting uncompressed video data and compressed or uncompressed digital audio data from an HDMI-compliant source device, such as a display controller, ...
, specifies 16:15 (1.0666) as the pixel aspect ratio of 4:3 576i/p and 8:9 (0.888) as the pixel aspect ratio of 4:3 480i/p. * * * *


Supplementary sources

* **A PDF version o
Adobe Premiere Pro CS4 Documentations
is also available from
Adobe Adobe ( ; ) is a building material made from earth and organic materials. is Spanish for ''mudbrick''. In some English-speaking regions of Spanish heritage, such as the Southwestern United States, the term is used to refer to any kind of e ...
web site. **This source specifies 12:11 (1.09) as the pixel aspect ratio of 576i. * **A PDF version o
Adobe After Effects CS4 Documentations
is also available from
Adobe Adobe ( ; ) is a building material made from earth and organic materials. is Spanish for ''mudbrick''. In some English-speaking regions of Spanish heritage, such as the Southwestern United States, the term is used to refer to any kind of e ...
web site. **This source specifies 12:11 (1.09) as the pixel aspect ratio of 576i. * **This source calculates different pixel aspect ratio values for 480i and 576i pictures. * **An in depth analysis on the discrepancies of the pixel aspect ratios provided in various specifications. **Creative Commons Attribution, Noncommercial-Share Alike 3.0 Germany (CC by-sa) **English translation


Notes


External links


The Pixel Aspect Ratio Acid Test

Pixel Calculator
{{DEFAULTSORT:Pixel aspect ratio Engineering ratios Digital television Film and video technology Computer graphics data structures Image processing Digital geometry Digital imaging