THE PIXEL: A FUNDAMENTAL UNIT OF DIGITAL IMAGES

Every digital image consists of a fundamental small-scale descriptor: THE PIXEL, invented by combining the words "PICture ELement." Just as how pointillist artwork uses a series of paint blotches, millions of pixels can also combine to create a detailed and seemingly continuous image.

Short for Picture Element, a pixel is a single point in a graphic image. Graphics monitors display pictures by dividing the display screen into thousands (or millions) of pixels, arranged in rows and columns. The pixels are so close together that they appear connected.

The number of bits used to represent each pixel determines how many colors or shades of gray can be displayed. For example, in 8-bit color mode, the color monitor uses 8 bits for each pixel, making it possible to display 2 to the 8th power (256) different colors or shades of gray.

On color monitors, each pixel is actually composed of three dots -- a red, a blue, and a green one. Ideally, the three dots should all converge at the same point, but all monitors have some convergence error that can make color pixels appear fuzzy.

Comparison of optical zoom and digital zoom

Bits per pixel

Main article: Color depth

The number of distinct colors that can be represented by a pixel depends on the number of bits per pixel (bpp). A 1 bpp image uses 1-bit for each pixel, so each pixel can be either on or off. Each additional bit doubles the number of colors available, so a 2 bpp image can have 4 colors, and a 3 bpp image can have 8 colors:

• 1 bpp, 2¹ = 2 colors (monochrome)
• 2 bpp, 2² = 4 colors
• 3 bpp, 2³ = 8 colors

...

• 8 bpp, 2⁸ = 256 colors
• 16 bpp, 2¹⁶ = 65,536 colors ("Highcolor" )
• 24 bpp, 2²⁴ ≈ 16.8 million colors ("Truecolor")

Subpixels

Geometry of color elements of various CRT and LCD displays;phosphor dots in a color CRT display (top row) bear no relation to pixels or subpixels.

Many display and image-acquisition systems are, for various reasons, not capable of displaying or sensing the different color channels at the same site. Therefore, the pixel grid is divided into single-color regions that contribute to the displayed or sensed color when viewed at a distance. In some displays, such as LCD, LED, and plasma displays, these single-color regions are separately addressable elements, which have come to be known as subpixels. For example,LCDs typically divide each pixel horizontally into three subpixels. When the square pixel is divided into three subpixels, each subpixel is necessarily rectangular. In the display industry terminology, subpixels are often referred to as pixels, as they are the basic addressable elements in a viewpoint of hardware, and they call pixel circuits rather than subpixel circuits.