Square sampling

"In modern digital imaging, including computing, digital photography, graphics arts, and HD, samples (pixels) are ordinarily equally spaced vertically and horizontally – they have equal horizontal and vertical sample density. These systems have square sampling (‘square pixels’); they have sample aspect ratio (SAR) of unity. With square sampling, the count of image columns is simply the aspect ratio times the count of image rows.

The term square refers to the sample arrangement: Square does not mean that image information is uniformly distributed throughout a square area associated with each pixel! Some 1080i HD compression systems resample to 4/3 or ¾ pixel aspect ratio.

Legacy imaging and video systems including digital SD systems had unequal horizontal and vertical sample pitch (nonsquare sampling). That situation was sometimes misleadingly referred to as ‘rectangular sampling,’ but a square is also a rectangle! A heated debate in the early 1990s led to the adoption of square sampling for HD. In 1995 the New York Times wrote,

If you use the term pitch, it isn’t clear whether you refer to the dimension of an element or to the number of elements per unit distance. I prefer the term density.

HDTV signals can be sent in a variety of formats that rely not only on progressive or interlaced signals, but on features like square versus round pixels …

A technical person finds humour in that statement; surprisingly, though – and unintentionally – it contains a technical grain of truth: In sampled continuous-tone imagery, the image information associated with each sample is spread out over a neighbourhood which, ideally, has circular symmetry." - from Digital Video and HD, 2nd Edition, by Charles Poynton.