Pixel aspect ratio

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Pixel aspect ratio (PAR) is a mathematical ratio that describes how the width of pixels in a digital image compares to their height.

Most digital 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 Standard-definition television motion pictures, define an image as a grid of rectangular pixels in which the width of the pixel is slightly different from that of its height. 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 which comply with SMPTE standards and practices, use square pixels.


[edit] Technical Details

Pixel aspect ratio 1:1
Pixel aspect ratio 2:1

[edit] Background

A digital image is generally defined as a grid of very small but nonetheless square pixels with well-defined start and end edges and corners. Thus, digital video standards define motion pictures as a series of such images, called video frames, played back in a sequentially progressive manner.

However, standard-definition television standards and practices are incompatible with digital video. 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 standard-definition television standard that properly define image edges or explicitly demand a certain number of picture elements per line. 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.

[edit] Analog to Digital Conversion

As a result of computers becoming powerful enough to be seriously considered as video editing tools, video Digital-to-analog converters and Analog-to-digital converters were made to overcome this incompatibility. In order to convert analog video lines into a series of square pixels, the 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 (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 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 13.5MHz. Based on this information:

  • The Pixel Aspect Ratio for 480i would be 10:11 as:

    12\frac{3}{11}\div 13.5=\frac{10}{11}

  • The Pixel Aspect Ratio for 576i would be 59:54 as:

    14.75\div 13.5=\frac{59}{54}

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 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 of ITU-R BT.601, still implies that the Pixel Aspect Ratios mentioned above are correct.

[edit] Digital Video Processing

As stated above, ITU-R BT.601 specified that standard-definition television pictures are made of lines which contain exactly 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 or 576i 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{x}{480}\times \frac{10}{11}=\frac{4}{3}\Rightarrow x=\frac{480\times 11\times 4}{10\times 3}=704

  • A 4:3 576-line picture, digitized with the Rec. 601-recommended sampling rate, would be 702.915254 non-square pixels wide.

    \frac{x}{576}\times \frac{59}{54}=\frac{4}{3}\Rightarrow x=\frac{576\times 54\times 4}{59\times 3}\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, in order to maintain a safe margin of error, ITU-R BT.601 required 16 more non-square pixels per line (8 more at each edge) to be sampled to ensure that all video data near the margins were saved.

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{4}{3}\div \frac{704}{576}={\color{blue}\frac{12}{11}}

  • For PAL 16:9:

    \frac{16}{9}\div \frac{704}{576}={\color{blue}\frac{16}{11}}

[edit] 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 sources section.)

In order 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.

[edit] Modern Standards and Practices

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

[edit] 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 detail.

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 squashed in either the horizontal or vertical direction. For example, a circle generated for a computer display would look like an ellipse on a standard-definition television. This issue is more evident on 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 calculation of the distances from a certain point so that they look visually correct. An example of such effects would be radial blur or motion blur.

[edit] Use of Pixel Aspect Ratio

Pixel Aspect Ratio value is used mainly in digital video software, where the motion pictures are to be converted or reconditioned so that they are used in other video systems than the original. The video player software may use pixel aspect ratio to properly render digital video on screen. The video editing software use Pixel Aspect Ratio to properly scale and render a video into a new format.

[edit] Confusion with Picture Aspect Ratio

Screenshot of Nero Vision 8, Export, Configure dialog box: "Aspect Ratio" field that allows user to select either "4:3" or "16:9". Contrary to what the available choices suggest, this field sets the video Pixel Aspect Ratio. Given this setting and video mode ("PAL" or "NTSC"; adjusted elsewhere) Nero Vision will determine the Pixel 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 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 the Pixel Aspect Ratio of the video file by asking for information about the video system from which the video file 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 the Storage Aspect Ratio; they can simply retrieve the picture dimensions and calculate it. Non-square-pixel–aware applications often ask either Pixel Aspect Ratio or Display Aspect Ratio, from either of which they can calculate the other.

[edit] Pixel Aspect Ratios of Common Video Formats

Pixel Aspect Ratio values for common standard-definition video formats are listed below. Note that for each video format, three different types of Pixel Aspect Ratio values are listed:

  1. Rec.601, a Rec.601-compliant value, which is considered the real Pixel Aspect Ratio of standard-definition video of that type. (Read Explanation)
  2. Digital, which is roughly equivalent to Rec.601 and is more suitable to use in Digital Video Editing software. (Read Explanation)
Video System Picture Dimensions Pixel Aspect Ratio Pixel Aspect Ratio (Decimal)
Rec.601 Digital Rec.601 Digital
Standard (4:3) PAL (e.g. 576i) 720×576
1.0\overline{925} 1.\overline{09}
Widescreen (16:9) PAL 118:81 16:11 1.4567\ldots 1.\overline{45}
Standard (4:3) NTSC (e.g. 480i) 720×480
10:11 0.\overline{90}
Widescreen (16:9) NTSC 40:33 1.\overline{21}

[edit] Sources

Main Sources

Supplementary Sources

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