This is the original and proper name for the unit of resolution in visual display units (VDUs). They are now referred to as 'pixels', however this is merely a contraction of the phrase 'picture element'. A computer monitor with a resolution of 1280x1024 has 1280 picture elements (or pixels) horixontally, and 1024 vertically. A higher resolution means more detailed images can be displayed on the screen due to the fact there are more pixels, therefore monitors supporting higher resolutions are usually more expensive.

Pixel colours

A pixel is further defined by the display device colour-depth. This is a definition of the ability for a pixel to display a range of colours, or grey shades or simply to be on or off (white or black). Usually expressed in digital bits, displays operate in a specific 'Bits-Per-Pixel' (BPP) mode indicating the amount of computer bits used in all pixels on the display. In early graphical VDU displays, there was simply one bit for every pixel. The display was said to be monochrome, or black and white. Over time, with the advent of more capable display electronics, we now have 16, 24 or even 32 bits per pixel. The more information (BPP) we use for each pixel, the better a reproduction of colour in individual pixels we have. A display device is a device for visual or tactile presentation of images (including text) acquired, stored, or transmitted in various forms. ... This article is about the unit of information. ... Color depth is a computer graphics term describing the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. ... Something which is monochromatic has a single color. ...

The easiest example of this can be described with a 24 bit pixel. The primary colours of light - red, green and blue, can easily be divided into 24 bits, giving eight bits for red, eight for green and eight for blue. Eight digital bits, each being a 0 or 1 gives a total of 256 (2 to the power of 8) possible variations of each primary colour. Three primary colours times 256 (256 x 256 x 256) gives you a total 'colour palette' of 16,777,216 individual colours that any pixel can be at any one time on the display.

Digital storage of picture elements

Display devices represent the colour of a pixel using exactly that - values from 0 to 255 (256 in all) for red, green and blue 'components' of the pixel (if the display is operating in 24bpp mode). A complete 1280 x 1024 pixel screen, being made up of three bytes for every pixel, would mean we need to store 3,932,160 bytes, or just under 4 Megabytes, just for one screen.

For older display hardware that was not as capable, we did not have the luxury of storing 4Mb for the screen, so instead we used a palettised display. Palletising a screen involves not simply granting each pixel an ablility to display its own colour, but instead to represent each pixel with index within a separate table of colours, called a palette.

A common palettised arrangement was an 8-bit palettised display. Each pixel is not an individual colour, but instead is an 8-bit number (256). This number is used as a 'lookup' to the palette table. The table will also be 256 'rows', each row would contain a full representation of a particular colour. So if row one contains light purple, row two contains dark brown, row three contains bright orange, etc, a particular pixel would be able to point to one of these colours, and hence BE that colour.

This was usually fully handled by the display hardware in the computer system and so would operate quickly. The upside - less memory storage for each display, since each pixel would only be one byte, a 1280 x 1024 screen would take up 1,310,720, or 1.3Mb plus a very small palette table (24 bpp = 3 bytes times 256 rows = 768 bytes for the palette table). The downside - The whole display would only be able to show colours out of 256 different pre-defined colours at one time. This usually resulted in less than perfect representations of photographs and other high quality graphics, but was nonetheless effective and worthwhile. In practise though, older display hardware wouldn't be able to show 1280 x 1024 pixels, but more likely at 640 x 480 or 800 x 600 (480,768, or 480KB). A far more practical proposition given the high costs of display memory in the 80s and early 90s.

Palettised picture files

.GIF (Graphics Interchange Format) files are stored in a similar palettised fashion. If a GIF file were displayed occupying the whole screen of a modern full colour computer display, you would see similar poorer colour reproduction than that offered in other full colour image formats, such as .JPG. With pictures like this you can see the restriction of 256 colours. ... A photo of a flower compressed with successively higher compression ratios from left to right. ...