Pixel

Overview

A pixel (px) is the smallest addressable physical element of a raster image or the smallest controllable element of a display device. In most digital display devices, pixels are the smallest element that can be manipulated through software.

Density

Pixels Per Inch

Pixels per inch (PPI) is a measurement describing the number of pixels that can be placed in a line within the span of 1 inch. Vertical and horizontal densities are usually the same since most displays use square pixels.

The resolution of a display device refers to the number of distinct pixels in each dimension that can be displayed. Note it is independent of pixel density.

PPI of a screen are typically approximated by measuring along its diagonal. More specifically, if $w$ is the width resolution in pixels, $h$ is the height resolution in pixels, and $d$ is the diagonal size in inches, then

PPI \approx \frac{\sqrt{w^{2} + h^{2}}}{d} .

Dots Per Inch

Dots per inch (DPI) is a measurement of the number of dots that can be placed in a line within the span of 1 inch. Whereas PPI refers to display devices, DPI is used to describe digital and hard copy prints.

Confusingly, the term is often used to refer to what should be called PPI.