Pixel aspect ratio

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

Most digital imaging 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 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 standards and practices, use square pixels.

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. 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:

Aspect ratio

The aspect ratio of a geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side - the ratio of width to height, when the rectangle is oriented as a "landscape".

The aspect ratio is expressed as two numbers separated by a colon (x:y). The values x and y do not represent actual width and height but, rather, the "relation" between width and height. As an example, 8:5, 16:10 and 1.6:1 are the same aspect ratio.

In objects of more than two dimensions, such as Hyperrectangles, the aspect ratio can still be defined as the ratio of the longest side to the shortest side.

Applications and uses

The term is most commonly used with reference to:

Newspaper format

Newspaper formats vary substantially, with different formats more common in different countries. The size of a newspaper format refers to the size of the paper page; the printed area within that can vary substantially depending on the newspaper.

In some countries, particular formats have associations with particular types of newspaper; for example, in the United Kingdom, there is a distinction between "tabloid" and "broadsheet" as references to newspaper content quality, which originates with the more popular newspapers using the tabloid format; hence "tabloid journalism".

Trends

Manfred Werfel Research Director and Vice President of IFRA predicts a trend towards the Berliner format.

In a recent trend, many newspapers have been undergoing what is known as "web cut down", in which the publication is redesigned to print using a narrower (and less expensive) roll of paper. In extreme examples, some broadsheet papers are nearly as narrow as traditional tabloids. An average roll of 26.4 lb (12.0 kg), 45 in (110 cm) diameter newsprint rolled out is 9.7 mi (15.6 km) long.

Aspect ratio (aeronautics)

In aeronautics, the aspect ratio of a wing is the ratio of its span to its aerodynamic breadth or chord. A long, narrow wing has a high aspect ratio, whereas a short, stubby wing has a low aspect ratio.

For a constant-chord wing of chord c and span b, the aspect ratio is given by:

If the wing is swept, c is measured parallel to the direction of forward flight.

For most wings the length of the chord is not a constant but varies along the wing, so the aspect ratio AR is defined as the square of the wingspan b divided by the wing area S. In symbols,

For such a wing with varying chord, the standard mean chord SMC is defined as

Aspect ratio of aircraft wings

Aspect ratio and other features of the planform can be used to predict the aerodynamic performance of a wing.

For a given wing area, the aspect ratio, which is proportional to the square of the wingspan, is of particular significance in determining the performance. Roughly, an airplane in flight can be imagined to affect a circular cylinder of air with a diameter equal to the wingspan. A large wingspan is working on a large cylinder of air, and a small wingspan is working on a small cylinder of air. For two aircraft of the same weight but different wingspans the small cylinder of air must be pushed downward by a greater amount than the large cylinder in order to produce an equal upward force. The aft-leaning component of this change in velocity is proportional to the induced drag. Therefore the larger downward velocity produces a larger aft-leaning component and this leads to larger induced drag on the aircraft with the smaller wingspan and lower aspect ratio.