1. Field of the Invention
This invention is related generally to display terminals and more specifically to apparatus, for use with a bit-mapped raster scanned graphics terminal, to store and retrieve graphics display data in either a word or a pixel format.
2. Description of the Prior Art
The picture displayed on a video monitor screen can be considered to consist of a large number of generally horizontal, parallel "raster lines" or lines of displayed information. As the monitor's electron beam scans along each line, locations on the face of the cathode ray tube are stimulated. Each such location is commonly termed a pixel and the resolution of the screen is specified as the number of lines displayed multiplied by the number of pixels on each line. In a color monitor, the colors are generated by differentially activating red, blue and green electron guns which are aimed at three very close, but not coincident, points on the screen face.
Raster scan graphics terminals are typically designed as either character or bit-mapped terminals. In character graphics, the displayed picture is made up of a combination of predefined alphanumeric characters and simple shapes, with each character or shape being a matrix of pixels (e.g. 8.times.10) which has a single address in terminal memory. In bit-mapped graphics, every pixel is associated with one or more locations in memory. In a monochrome terminal, a single bit per pixel would be adequate to describe the pixel state. In a color terminal, however, multiple bits per pixel are required. For example, if it is desired to be able to display up to 16 different colors on the screen simultaneously, each pixel must have four bits associated with it to define its color. The bits of color information associated with each pixel may, in some implementations, not define a color, but rather define a location in a table which contains the color information currently assigned to those bits.
Prior art terminals commonly have only a single mode of accessing and modifying pixel information in the memory of a bit-mapped terminal. Usually pixels are accessed individually. This method allows each pixel to be independently modified but is time consuming and not desirable for all possible terminal applications.