A CONCEPT OF GRAPHICS

Written BY k.dasAnd S.mishra

Dept of computer science

What is computer Graphics?

Computer graphics is a field of computer science that concerned with digitally Synthesizing and manipulating visual contents.

Computer graphics(cg) includes almost everything on computer that is not text or sound.

The term cg refer to several different thing 1.The representation and manipulation of

pictorial data by a computer.2.Various technologies used to create and

manipulate image.

Bitmap and Vector Image

There are two kinds of computer graphics :Raster(composed of pixel) and Vector(composed of path).

Raster image are called bitmap images. Bitmap image are mapped to a grid or an

array of pixels. In bitmap images, each pixel has assigned

color. Bitmap image are edited at the pixel level

that allows the color of any one pixel to change.

Bitmap image

Bitmap images are not easily scalable.

Bitmap images are edited at the pixel level that allows the color of any one pixel to change.

Bitmap images are usually created and edited in photo or paint program such as adobe Photoshop.

Vector image

Vector image are not based on picture patterns but instead use mathematical formulas to draw lines and curves that can be combined to create from geometric object such as circle and polygons.

Vector images are editing by manipulating the lines and curves that make up an image using a program such as coral draw.

Application of Graphics

Paint program:

It is a graphics program that enables you to draw pictures on the display screen which is represented as bit maps (bit-mapped graphics).

Sophisticated paint applications are often called image editing programs.

Application of Graphics

Animation software:

Enables you to chain and sequence a series of images to simulate movement. Each image is like a frame in a movie.

. A cartoon on television is one example of animation. Animation on computers is one of the chief ingredients of multimedia presentations.

Graphics Application

Card and Desktop publishing

A CAD system is a combination of hardware and software that enables engineers and architects to design everything from furniture to airplanes.

Provides a full set of word-processing features as well as fine control over placement of text and graphics, so that you can create newsletters, advertisements, books, and other types of documents. It means by using a personal computer or workstation high-quality printed documents can be produced.

CRT

CRT

CRT

Cathode ray tube

Cathode ray tube is a vacuum tube containing one or more electric gun and a fluorescence screen used to view images.

It has a mean to accelerated and deflect the electron beam onto the screen to create the images.

Working Principle of cathode ray tube

Heat is supplied to the cathode by passing current through a heater element.

The cathode is a cylindrical metallic structure that is rich in electrons.

On heating ,electron released from cathode surface.

Control grid is the next element that follow the cathode.

Working principle of cathode ray tube

Intensity of electron beam is controlled by setting voltage level on the control grid.

A high negative voltage applied to the control grid shuts off the beam by repelling electrons and stopping them from passing through the small hole at the end of the control grid structure.

Smaller negative voltage on the control grid simply decrease the number of electrons passing through the cathode. Thus we can control the brightness of display by varying the voltage on control grid.

Working principle of CRT

Positive charged anodes accelerate the electrons towards the phosphor screen.

Focusing and deflection coil are together needed to force the electron beam to converge into small spot , as it strike the screen, otherwise the electron would repel each other and beam is spread out as it approaches the screen.

Finally when the accelerating electron beam collide on phosphor coating ,a part of kinetic energy is converted into light and heat.

Raster scan Display

A raster scan display is most common method of drawing images on a CRT screen.

In this method horizontal and vertical deflection signals are generated to move a beam all over the screen in the pattern for displaying any image.

The electron beam is swept across the screen one row at a time from top to bottom.

The electron beam swept back and forth from left to right across the screen.

Raster scan display resolution is poor because it in contrast produces Zig-Zag lines that are plotted as discrete point set.

Horizontal and Vertical Retrace

Raster scan display

HORIZONTAL RETRACE VERTICAL RETRACE

The beam is on ,while it moves from left to right. The beam is off ,when it moves back from right to left. This phenomenon is called the horizontal retrace.

As soon as the beam reaches the bottom of the screen ,it is turned off and is rapidly retraced back to top to start again. This is called vertical retrace.

Random Scan Display

A raster scan display has an electron beam directed only to the parts of the screen where a picture is to be drawn .

Random scan display draw a picture one line at a time for this reason it is also known as vector display picture.

A picture definition is stored as a set of line drawing command in the area of refresh buffer.

Random scan display resolution is good than random scan display as picture definition is stored as a set of line-drawing instructions instead of a set of intensity values for all screen point.

Working principle of graphics display

A graphics display consists of three components: frame buffer, display controller and a monitor.

Frame buffer: The frame buffer stores an image as a matrix of intensity values . In personal computer, the frame buffer is located on the graphics card that manage the video subsystem of the computer. It have some memory stored intensity values are then retrieved from the refresh buffer and displayed on the screen one row at a time. Each intensity value is represented by bit zero(0) or one(1) in the frame buffer.

Display controller and display device

Working principle of display device(graphics)

The video controller has direct access to memory locations in the frame buffer and passing it to the display device. It reads each successive byte of data from frame buffer and converts this 0’s and 1’s in one line into a corresponding video signals and this line is called scan line. If intensity is one(1) then the controller sends a signal to display a dot in corresponding position on the screen. if the intensity is zero then no dot is displayed.

Frame Buffer

1 1 1 1 1 1 0 0

0 1 1 0 0 1 1 0

0 1 1 0 0 1 1 0

0 1 1 1 1 1 0 0

0 1 1 0 0 1 1 0

0 1 1 0 0 1 1 0

1 1 1 1 1 1 0 0

0 0 0 0 0 0 0 0