IE433 CAD/CAM Computer Aided Design and Computer Aided Manufacturing Part-2 CAD Systems Industrial Engineering Department King Saud University

CAD Systems A CAD system consists of three major parts:  Hardware: computer and input/output devices.  Operating system software.  Application software: CAD package.

Application software Graphics utility Devices drivers Input-output devices User interface Database (CAD model) Basic structure of a CAD system

 The application software is at the top level and is used to manipulate the CAD model database.  The graphics utility system performs the coordinate transformation, windowing, and display control.  Devices drivers are used to translate the data into and out of the specific format used by each device, they also control the devices.  The operating system is run in background to coordinate the entire operations.  User interface links the human and the system.

CAD hardware There are two major types of hardware used in a CAD:  Computer.  Input/output devices.

Keyboard Keypad Digitizer Trackball Joystick Tablet Mouse Printer Plotter Disk Tape Computer output microfilm CRT display with light pen Computer InputOutput I/O devices of a CAD system

CRT = Cathode ray tube The standard output device for CAD is a monitor display. Among the available technologies, the CRT is the most dominating and has produced a wide range of extremely effective graphics display.

Schematic diagram of a CRT

The electron gun generates the electrons. The focusing unit focuses the electrons into beam. The deflection system controls x and y, or the horizontal and vertical positions of the graphics information through the display controller, which typically sits between the computer and CRT. Schematic diagram of a CRT

The graphics display can be divided into two types based on the scan technology used to control the electron beam when generating graphics on the screen: 1.Random scan: the screen is not scanned in particular order. 2.Raster scan: the screen is scanned from left to right, top to bottom, all the times to generate graphics.

The three existing CRT displays that are based on these techniques are : The refresh (calligraphic) display. (based on random scan technology) Direct view storage tube (based on random scan technology) Raster display (based on raster scan technology).

Refresh Display

Direct View Storage Tube

Modern monitor displays are raster scan display monitors. Each display dot is called a pixel. A pixel is the smallest addressable area on a screen. An N * M resolution defines a screen with N rows and M columns. Each row defines a scan line.

For color displays, each pixel is represented by three closely dots with red, green and blue colors. The electron beam selects the color elements and the intensity of each color elements. The resolution of the monitor is determined by the number of pixels displayable on the screen i.e. 640 X 480 and 1024 X A rasterization process is needed in order to display either a shaded area or graphics entities. The area or entities are converted into their corresponding pixels whose intensity and color are controlled by the image display system.

Color Raster Display With Eight Planes

A rasterizer is mainly a set of scan-conversion algorithms which are used to draw an entity by generating pixels to approximate an entity. Bit map stores the scan conversion process. (each pixel value determines its brightness (grey level) or most often its colour on the screen). Every cell in the bit map is related to every pixel in the screen. The bit map memory is arranged conceptually as a series of planes, one for each bit in the pixel value. For example, an eight- plane memory provides 8 bits/pixel, this provides different gray levels or different colours The display process maps every cell into its corresponding screen pixel brightness or colour. In order to maintain a flicker-free image the screen must be refreshed at the rate 30 or 60 Hz.

The value of a pixel in the bit map memory is translated to a Gray level or a colour through a look up table ( colour table, or color map). The pixel values is used as an index for this look up table to find the corresponding table entry value which is then used by the display system (processor and beam deflection system) to control the gray level or colour.

Relationship between pixel value and look up table

If cell P in the bit map corresponds to pixel P at the location P(x,y) on the screen, then the grey level of this pixel is 50 ( ) or its corresponding colour is 50. The colour or grey-scale resolution is determined by the number of different colours or grey scales displayable at each pixel, usually measured in bits. Usually, displays are 4 bits (16 colours) 8 bits (256 colours) 16 bits (thousands of colours) 24 bits (millions of colours). Typically 4 to 8 bit/pixel are adequate for both monochrome and colour displays utilised in most engineering applications.

While raster displays are now a standard offering from nearly all CAD/CAM vendors, the quality of the displayed images is affected by flicker aliasing problems. The flicker of an image is reduced by simply reducing the time of the refresh cycle. For example if the refresh rate 60 Hz, the entire scan lines are refreshed once every 1/60 second. This high rate means more and faster accesses to the bit map. The aliasing problem is related to the resolution of the display which determines how good or bad is the raster approximation of geometric information.

The image refresh system may use an interlaced scan of two fields: In the interlaced scan (as in the home television), the refresh cycle of 1/30 second is divided into two subcycles each lasting 1/60 second. –The first subcycle displays the odd-numbered scan lines and the second displays the even-numbered scan lines. This technique produces an image with almost a refresh rate of 60 Hz instead of 30 Hz. Another scheme is to use a noninterlaced scan of one field by operating at a higher refresh rate such as 60 Hz. In this scheme, the entire scan lines are refreshed once every 1/60 second.

Example-1: An eight-plane raster display has a resolution of 1280 horizontal * 1024 vertical and refresh rate of 60 Hz noninterlaced. Find: 1. The RAM size of the bit map (refresh buffer). 2. The time required to display the scan line and a pixel. 3. The active display area of the screen if the resolution is 78 pixels per inch. 4. The optimal design if the bit map size is to be reduced by half. Solution: 1.The RAM size of the bit map = 8 * 1280 * 1024 = 1.3 Mbytes. 2.The time required to display a scan line = (1/60)/1024= 16 microseconds. The time required to display a pixel = 16/1280 = 12 nanoseconds. 3.The active display area = 1280/78 horizontal * 1024/78 vertical = 16.4 * 13.1 inch. 4. Assuming there is only one bit map available, the two solution s are to reduce the number of planes by half and keep the resolution as it is or vice versa. The two choices are: a four-plane  1280 x 1024 display or an eight-plane  640x512. The first choice is preferred, especially if 16 simultaneous colors are adequate for most applications that utilize the display.

Example-2: What is the reasonable resolution of an eight-plane display refreshed from a bit map of 256 Kbytes of RAM?. Solution: Bit map size per plane = 256 * 1000 = 256,000 bits. This could support a display with a resolution of 505*505, 640*400, or other combinations of 512,000