Pixel aspect ratio

Pixel aspect ratio

Pixel aspect ratio (PAR), in computer graphics terminology, is a mathematical ratio that describes how the horizontal length of a pixel in an imaging system compares to its vertical height.

Most modern imaging systems describe an image as a grid of very small but nonetheless square pixels. However, some imaging systems, especially those which must maintain compatibility with analog standard-definition, define image as a grid of rectangular pixels in which the width of the pixel is different from that of its height. To describe this proportion, we state it in form of Pixel Aspect Ratio.

Use of Pixel Aspect Ratio mostly involves pictures pertaining to standard-definition television. Most other imaging systems, including those complying with SMTPE standards and practices, use square pixels.

Technical Details


Digital image is defined as a grid of very small but nonetheless square pixel with well-defined start and end edges and corners. Thus, digital video is defined as a series of such images, called video frames, played back at a fixed frame rate in a sequentially progressive manner.

However, standard-definition television picture is essentially incompatible with digital image as it has well-defined lines but it does not have standardized or well-defined edges or well-defined picture elements similar to pixels. Furthermore, analog video systems such as NTSC 480i and PAL 576i, instead of employing progressively-displayed frames, employs fields or interlaced half-frames displayed in an interwoven manner to create motion.

Analog to Digital Conversion

As a result of computers becoming powerful enough to be seriously considered as video editing tools, the Video Digital-to-analog converters were made, whose duty was to overcome this incompatibility. In order to convert analog video lines into a series of square pixels, industry adopted a default sampling rate at which luma values were extracted into pixels. The luma sampling rate for 480i pictures was 12+3/11 MHz and for 576i pictures was 14.75 Mhz.

However, the term Pixel Aspect Ratio was first coined when ITU-R BT.601-4 (commonly known as "Rec. 601") specified that standard-definition television pictures are made of lines which contain exactly 720 non-square pixels. ITU-R BT.601-4 did not defined 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 13.5Mhz. Based on this information:
*The Pixel Aspect Ratio for 480i would be 10:11 as:::12frac{3}{11}div 13.5=frac{10}{11}

*The Pixel Aspect Ratio for 576i would be 59:54 as:::14.75div 13.5=frac{59}{54}::Digital video-editing products use the aspect ratio of 12:11 for 576-line PAL digital video files. (See below.)

SMPTE RP 187 further attempted to standardize the Pixel Aspect Ratio values for 480i and 576i. It designated 177:160 for 480i or 1035:1132 for 576i. However, due to significant difference with practices in effect by in 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 11:10 for 480i.

As of this writing, ITU-R BT.601-6, which is the latest edition or ITU-R BT.601, haven't explicitly specified the aspect ratio for any of standard-definition television pictures.

Digital Video Processing

As we stated above, ITU-R BT.601 specified that standard-definition television pictures are made of lines which contain exactly 720 non-square pixel, sampled with a precisely-specified sampling rate. A simple mathematical calculation reveals that 704 pixels would be enough to contain a 480i or 576i standard 4:3 picture.

Unfortunately, not all the standard TV pictures are exactly 4:3: As stated above, in analog video, the center of 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 for anamorphic widescreen (16:9) pictures. Therefore, in order to maintain a safe margin of error, ITU-R BT.601 required 16 more non-square pixels per each line (8 more at each edge), which may not be visible in all TVs, to be sampled to ensure that all video data near the margins are saved.

This requirement, however, had two implications:
**First: Pixel Aspect Ratios for standard (4:3) and anamorphic wide screen (16:9) motion pictures, 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.
**Second: The Pixels Aspect Ratios approved by ITU-R BT.601-6 were no longer applicable for non-linear editing digital video editing products which edited digital video clips with full 720 pixel which were to be mixed with pictures and video sources of various types and Pixel Aspect Ratios including computer renderings. These applications had to employ the computationally-correct Pixel Aspect Ratios for 720×480 and 720×576 motion pictures that were to be rendered on 4:3 or/and 16:9 screens. Unfortunately, due to mass of 576i pictures being more 4:3-defiant than the mass of 480i motion pictures, the NLE Pixel Aspect ratio of 576i found its way into video editing product's documentation vis-à-vis the Rec.601–compliant Pixel Aspect ratio of 480i. For example, Adobe Premiere Pro CS3 specifies 16:15 as the Pixel Aspect Ratio for 576i/p while specifying 10:11 as the Pixel Aspect Ratio for 480i/p.

Modern Image and Video Systems

On modern digital imaging systems and high-definition television pictures, especially those which comply with SMPTE standards and practices, only square pixels are used. For compatibility reasons of course, a number of high-definition television picture formats with non-square pixels also exist.

Issues of non-square pixels

In terms of picture quality, the optimum pixel aspect ratio for any given picture dimension is 1:1. Any other pixel aspect ratio will cause the number of pixels in one direction (either horizontally or vertically) to be reduced and thus image will lose resolution and will contain less perceived details.

Displaying an image with a certain pixel aspect ratio on a device whose pixel aspect ratio is different will cause the image to look unnaturally stretched or squished in on of the either horizontal or vertical directions. Picture of a circle, generated for a computer display would look like an ellipse on a standard-definition television. This issue is more evident on a wide-screen TVs.

Pixel Aspect Ratio must be taken into consideration by video editing software products which 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. Special effects software products must also take the Pixel Aspect Ratio into consideration, since some special effects require to calculate the distances from a certain point so that they look visually correct. An example of such effects would be radial blur or motion blur.

Confusion with Picture Aspect Ratio

Pixel Aspect Ratio is often confused with Picture Aspect Ratio. Picture Aspect Ratio is the ratio of the image width and height.Due to non-squareness of pixels in Standard-definition TV, there are two types of Picture Aspect Ratio: Storage Aspect Ratio (SAR) and Display Aspect Ratio (DAR). Storage Aspect Ratio is the ratio of the image width to height in pixels and can be easily calculated from 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 also 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 4:3" and "PAL 16:9"!) In such situations, the video editing program is implicitly asking the Pixel Aspect Ratio of the video file by asking for information about the Video System from which the video file in question is originated. The program will then use a table (similar to the one in "Pixel Aspect Ratio of Common Video Formats" section below) to determine the correct Pixel Aspect Ratio value.

The rule of thumb is: Video editing products never ask for Storage Aspect Ratio; they can simply retrieve picture dimensions and calculate it. Non-square-pixel–aware applications would often ask either Pixel Aspect Ratio or Display Aspect Ratio, from either of which they can calculate the other.

Pixel Aspect Ratios of Common Video Formats


Main Sources:
* Pirazzi, Chris, " [http://lurkertech.com/lg/video-systems/ Programmers' Guide to Video Systems] ", retrieved October 1st, 2008
* Pirazzi, Chris, " [http://lurkertech.com/lg/pixelaspect/ Square and Non-Square Pixels] ", retrieved October 1st, 2008
* Poynton, Charles. "Digital Video and HDTV: Algorithms and Interfaces". San Francisco: Morgan Kaufmann Publishers, 2002, ISBN: 1-55860-792-7

Supplementary Sources:
* Adobe Systems Incorporated, "Common Pixel Aspect Ratios," in "Adobe Premiere Pro CS3 Documentations", retrieved October 9th, 2008. An [http://help.adobe.com/en_US/PremierePro/3.0/premierepro_cs3_help.pdf on-line version] is available.

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