Pixel Aspect Ratio
Pixel-to-aspect ratio (often abbreviated PAR) is a mathematical ratio that describes the width of a pixel in a digital image that compares to the height of that pixel. Most digital imaging systems display an image as a grid of small square pixels.
However, some display systems, especially those that must be compatible with standard definition television motion pictures, display an image as a grid of rectangular pixels where the pixel width and height differ.
The pixel aspect ratio explains this difference. The use of the pixel aspect ratio mostly includes images related to standard definition television and some other exceptions. Many other display systems, including those that comply with SMPTE standards and practices, use square pixels.
The ratio of an image's width to its height is known as the aspect ratio or more precisely the image aspect ratio (DAR) – aspect ratio of the image as it is displayed; DAR for TV was traditionally 4:3 (also known as fullscreen), with 16:9 (aka widescreen) now the standard for HDTV. In digital images, there is a distinction between the storage aspect ratio (SAR), which is the ratio of pixel dimensions.
If an image is displayed with square pixels, these proportions are congruent; otherwise, non-square "rectangular" pixels are used and these ratios do not match.
The aspect ratio of pixels is known as pixel aspect ratio (PAR) - for square pixels this is 1:1 - and they are related to identity: SAR × PAR = DAR. Rearrangement (decoding for PAR) efficiency: PAR = DAR / SAR.
For example, a 640 × 480 VGA image has a SAR value of 640/480 = 4:3, and if it is displayed on a 4:3 screen (DAR = 4:3) it has square pixels, so it has a 1:1 PAR value.
On the contrary, the SAR value of 720 × 576 D-1 PAL image is 720/576 = 5:4, but 4:3 is displayed on the screen (DAR = 4:3).