Bit-mapped display

From Computer History Wiki
Jump to: navigation, search

Bit-mapped displays are the current standard for output in user interfaces to computers, particularly when using a graphical user interface. They provide a screen which can display images or text, and is the main output to the user.

The video display has an array of pixels, each one of which can be independently set - in most contemporary units, to a color value defined by three 8-bit values, one for each primary colour (red, green and blue).

Originally, before semiconductor RAM prices fell, bit-mapped dislays provided fewer bits per pixel; originally only one bit per pixel. (Some low-end or low-power displays still provide only this.) At an intermediate stage, an 8-bit value for each pixel was used: either for a grey-scale; or for a limited number of colours. (I.e. only 256 distinct colours could be in use at any time, although each colour could be selected arbitrarily, and mapped into three 8-bit values, as above.)

The resolution depends in part on the physical size of the screen; and in the early stages, also on RAM cost. Some early displays were as little as 320×200; later on, 1024x768 pixels was widespread, and then 1280x1024. As the screen aspect ratio of high-definition television has become common, resolutions such as 1280×720 and 1920×1080 have become more common.

To provide the ability to draw things at high speed, thereby fully utilizing the capabilities of the display, they are usually directly connected to the computer's main bus, with the display memory directly visible to the CPU as main memory. Due to the intimate relationship with the rest of the system, they are usually found on personal computers.

The original bit-mapped displays were an evolution of video terminals; once RAM prices fell low enough, the ability to display graphics, etc, instead of just characters, made the step forward inevitable.