Computer Graphics Systems CGS
Computer graphics have had significant impact because they are an extremely effective medium for communication between people and computers- the human eye can absorb the information content of a graph, a pictorial image or a geometric shape much faster than that of a table of numbers or a text file.

There are a number of variations in the characteristics of computer graphics that may be classified into three categories as follows:
The first category defines the control the user has over the image. In passive computer graphics the user has no control; in interactive graphics the user may interact with the graphics and with the programs generating them.
The second category concerns the way the image is generated. In vector graphics the image comprises a number of lines, whereas raster graphics involve the manipulation of the color and or intensity of points, known as picture elements or pixels, in a matrix making up the image.
The third category distinguishes between image-space graphics, in which the image itself is directly manipulated to create a picture and object-space graphics, in which the image is a representation of a separate model.

CAD may be categorized as an application of interactive object-space graphics, in that the objective is to develop interactively a model of a design. It is also generally vector graphics (because many engineering applications involve line drawings), although the distinction between vector and raster graphics is tending to become blurred as raster devices are used to draw lines, and as CAD now normally incorporates both types of representation.

Computer graphics hardware (Hardware in CAD systems)

Design workstation:
The CAD workstation is the system interface with the outside word. It represents a significant factor in determining how convenient and efficient it for a designer to use the CAD system.
The workstation must accomplish five functions:
*It must interface with the CPU.
*It must generate a steady graphic image for the user.
*It must provide digital descriptions of the graphic image.
*It must translate computer commands into operating functions.
*It must facilitate communication between the user and the system.

The central processing unit
The CPU operates as the central “brain” of the CAD system. It is typically a minicomputer. It executes all mathematical computations needed to accomplish graphics and other functions, and directs the various activities within the system. These activities include:
*Managing the design workstations (operator inputs, editing, etc.)
*Directing plotters in the generation of engineering drawings
*Copying data currently on disk onto magnetic tapes for semiactive storage
*Reading magnetic tapes and hard disks containing drawing data for possible revision or other use
*Transmitting data to and from other larger computers

Secondary storage
In addition to primary computer storage, secondary storage capacity is provided in CAD system. The purpose in using secondary storage is to reduce the cost of main computer memory. The secondary storage can be used for engineering drawing files, CAD software which can be transferred to main memory as needed, and temporary files for CPU output which will be downloaded to individual graphics terminals, plotters, or other output devices.

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