Unit-2: Useful technologies related to computer graphics


Hardcopy is a printed copy of information from a computer. Sometimes it refer to as a printout, so it called hardcopy because it exists as a physical object. Hardcopy is tangible output that usually printed. The principal examples are printouts, whether text or graphics, form printers and also films including microfilms and microfirche is also considered as hardcopy output. 

Output Devices

It is an electromechanical device, which accepts data from a computer and translates them into form understand by users.

Following are Output Devices:

  1. Printers
  2. Plotters


Printer is the most important output device, which is used to print data on paper.

1. Impact Printers: The printers that print the characters by striking against the ribbon and onto the papers are known as Impact Printers.

These Printers are of two types:

  1. Character Printers
  2. Line Printers

2. Non-Impact Printers: The printers that print the characters without striking against the ribbon and onto the papers are called Non-Impact Printers. These printers print a complete page at a time, therefore, also known as Page Printers.

Page Printers are of two types:

  1. Laser Printers
  2. Inkjet Printers

LASER PRINTER : laser printer is a type of printer which makes use of a laser beam to produce an image on the drum or you can say that it is a printer that makes use of a focused beam of light to transfer text and images onto paper. Laser printer makes use of laser technology for the printing of copies onto the paper.

INKJET PRINTER : Inkjet printers are a category of printer in which printing is done with the help of inkjet technology. The technology works by spraying ionized ink directed by magnetic plates onto the paper, which is fed through the printer.


Visual Displays – Basic Technologies

 • Cathode Ray Tubes

 • Flat Panel Displays

• Electroluminescence Displays

• LCD Displays

• Active Matrix TFT

• Light Valves

 • Micro mirror devices.

Raster-Scan Displays

Raster Scan Displays are most common type of graphics monitor which employs CRT. It is based on television technology. In raster scan system electron beam sweeps across the screen, from top to bottom covering one row at a time.A pattern of illuminated pattern of spots is created by turning beam intensity on and off as it moves across each row. A memory area called refresh buffer or frame buffer stores picture definition. This memory area holds intensity values for all screen points. Stored intensity values are restored from frame buffer and painted on screen taking one row at a time.Each screen point is referred to as pixels.

In raster scan systems refreshing is done at done at a rate of 60-80 frames per second. Refresh rates are also sometimes described in units of cycles per second / Hertz (Hz). At the end of each scan line, electron beam begins to display next scan line after returning to left side of screen. The return to the left of screen after refresh of each scan line is known as horizontal retrace of electron beam. At the end of each frame electron beam returns to top left corner and begins the next frame.

Raster-Scan Display Processor:
An important function of display process is to digitize a picture definition given in an application program into a set of pixel-intensity values for storage in refresh buffer. This process is referred to as scan conversion. The purpose of display processors is to relieve the CPU from graphics jobs.

Display processors can perform various other tasks like: creating different line styles, displaying color areas, etc. Typically display processors are utilized to interface input devices, such as mouse, joysticks.

Random-Scan Display

In Random-Scan Display electron beam is directed only to the ares of screen where a picture has to be drawn. It is also called vector displays, as it draws picture one line at time. It can draw and refresh component lines of a picture in any specified sequence. Pen plotter is an example of random-scan displays.

The number of lines regulates refresh rate on random-scan displays. An area of memory called refresh display files stores picture definition as a set of line drawing commands. The system returns back to first line command in the list, after all the drawing commands have been processed. High-quality vector systems can handle around 100, 00 short lines at this refresh rate. Faster refreshing can burn the phosphor. To avoid this every refresh cycle is delayed to prevent refresh rate greater than 60 frames per second.

Random-Scan Display Processors:

Input in the form of an application program is stored in the system memory along with graphics package. Graphics package translates the graphic commands in application program into a display file stored in system memory. This display file is then accessed by the display processor to refresh the screen. The display processor cycles through each command in the display file program. Sometimes the display processor in a random-scan is referred as Display Processing Unit / Graphics Controller.


A video controller, often referred to as a video or graphics card, is a key hardware component that allows computers to generate graphic information to any video display devices, such as a monitor or projector. They are also known as graphics or video adapters. Some modern computers do not include video cards, but rather have graphics processing units directly integrated into the computer’s motherboard.

Following are some of the important input devices which are used in a computer −

  • Keyboard
  • Mouse
  • Joy Stick
  • Light pen
  • Track Ball
  • Scanner


Keyboard is the most common and very popular input device which helps to input data to the computer. The layout of the keyboard is like that of traditional typewriter, although there are some additional keys provided for performing additional functions.

Keyboards are of two sizes 84 keys or 101/102 keys, but now keyboards with 104 keys or 108 keys are also available for Windows and Internet.

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Mouse is the most popular pointing device. It is a very famous cursor-control device having a small palm size box with a round ball at its base, which senses the movement of the mouse and sends corresponding signals to the CPU when the mouse buttons are pressed.


Generally, it has two buttons called the left and the right button and a wheel is present between the buttons. A mouse can be used to control the position of the cursor on the screen, but it cannot be used to enter text into the computer.


Joystick is also a pointing device, which is used to move the cursor position on a monitor screen. It is a stick having a spherical ball at its both lower and upper ends. The lower spherical ball moves in a socket. The joystick can be moved in all four directions.


The function of the joystick is similar to that of a mouse. It is mainly used in Computer Aided Designing (CAD) and playing computer games.

Light Pen

Light pen is a pointing device similar to a pen. It is used to select a displayed menu item or draw pictures on the monitor screen. It consists of a photocell and an optical system placed in a small tube.


When the tip of a light pen is moved over the monitor screen and the pen button is pressed, its photocell sensing element detects the screen location and sends the corresponding signal to the CPU.

Track Ball

Track ball is an input device that is mostly used in notebook or laptop computer, instead of a mouse. This is a ball which is half inserted and by moving fingers on the ball, the pointer can be moved.


Since the whole device is not moved, a track ball requires less space than a mouse. A track ball comes in various shapes like a ball, a button, or a square.


Scanner is an input device, which works more like a photocopy machine. It is used when some information is available on paper and it is to be transferred to the hard disk of the computer for further manipulation.


Scanner captures images from the source which are then converted into a digital form that can be stored on the disk. These images can be edited before they are printed.


‘Working exposure’ means ‘working for free’ in exchange for exposure or experience.

 Dreamweaver is Adobe’s flagship tool, which primarily focuses on creating and editing HTML and CSS, while giving a preview of the output. Web designing and application deveopment can be done simultaneously, with real-time checking and editing.

You, possibly, must have seen this phrase in a Multimedia course curriculum where the university/college/teaching body will give you a hands-on experience with tools such as the Dreamweaver.


Clipping, in the context of computer graphics, is a method to selectively enable or disable rendering operations within a defined region of interest. Mathematically, clipping can be described using the terminology of constructive geometry. A rendering algorithm only draws pixels in the intersection between the clip region and the scene model. Lines and surfaces outside the view volume (aka. frustum) are removed.

Clip regions are commonly specified to improve render performance. A well-chosen clip allows the renderer to save time and energy by skipping calculations related to pixels that the user cannot see. Pixels that will be drawn are said to be within the clip region. Pixels that will not be drawn are outside the clip region. More informally, pixels that will not be drawn are said to be “clipped.”

Clipping in 3D graphics:

In three-dimensional graphics, the terminology of clipping can be used to describe many related features. Typically, “clipping” refers to operations in the plane that work with rectangular shapes, and “culling” refers to more general methods to selectively process scene model elements. This terminology is not rigid, and exact usage varies among many sources.

Scene model elements include geometric primitives: points or vertices; line segments or edges; polygons or faces; and more abstract model objects such as curves, splines, surfaces, and even text. In complicated scene models, individual elements may be selectively disabled (clipped) for reasons including visibility within the viewport (frustum culling); orientation (backface culling), obscuration by other scene or model elements (occlusion culling, depth- or “z” clipping). Sophisticated algorithms exist to efficiently detect and perform such clipping. Many optimized clipping methods rely on specific hardware acceleration logic provided by a graphics processing unit  (GPU).

The concept of clipping can be extended to higher dimensionality using methods of abstract algebraic geometry

Cohen–Sutherland algorithm

The Cohen–Sutherland algorithm is a computer-graphics algorithm used for line clipping. The algorithm divides a two-dimensional space into 9 regions and then efficiently determines the lines and portions of lines that are visible in the central region of interest (the viewport).

The algorithm was developed in 1967 during flight-simulator work by Danny Cohen and Ivan Sutherland.

The algorithm includes, excludes or partially includes the line based on whether:

  • Both endpoints are in the viewport region (bitwise OR of endpoints = 00): trivial accept.
  • Both endpoints share at least one non-visible region, which implies that the line does not cross the visible region. (bitwise AND of endpoints ≠ 0): trivial reject.
  • Both endpoints are in different regions: in case of this nontrivial situation the algorithm finds one of the two points that is outside the viewport region (there will be at least one point outside). The intersection of the outpoint and extended viewport border is then calculated (i.e. with the parametric equation for the line), and this new point replaces the outpoint. The algorithm repeats until a trivial accept or reject occurs.

 figure below are called outcodes. An outcode is computed for each of the two points in the line. The outcode will have 4 bits for two-dimensional clipping, or 6 bits in the three-dimensional case. The first bit is set to 1 if the point is above the viewport. The bits in the 2D outcode represent: top, bottom, right, left. For example, the outcode 1010 represents a point that is top-right of the viewport.


Cyrus–Beck algorithm

The Cyrus–Beck algorithm is a generalized line clipping algorithm. It was designed to be more efficient than the Cohen–Sutherland algorithm, which uses repetitive clipping.[1] Cyrus–Beck is a general algorithm and can be used with a convex polygon clipping window, unlike Sutherland–Cohen, which can be used only on a rectangular clipping area.

Here the parametric equation of a line in the view plane is

where {\displaystyle 0\leq t\leq 1}0\leq t\leq 1.

Now to find the intersection point with the clipping window, we calculate the value of the dot product. Let pE be a point on the clipping plane E.

Calculate {\displaystyle \mathbf {n} \cdot (\mathbf {p} (t)-\mathbf {p} _{E})}{\displaystyle \mathbf {n} \cdot (\mathbf {p} (t)-\mathbf {p} _{E})}:

if < 0, vector pointed towards interior;

if = 0, vector pointed parallel to plane containing p;

if > 0, vector pointed away from interior.

Here n stands for normal of the current clipping plane (pointed away from interior).

By this we select the point of intersection of line and clipping window where (dot product is 0) and hence clip the line.

Midpoint Subdivision Algorithm:-

Midpoint subdivision algorithm is an extension of the Cyrus Beck algorithm. This algorithm is mainly used to compute visible areas of lines that are present in the view port are of the sector or the image. It follows the principle of the bisection method and works similarly to the Cyrus Beck algorithm by bisecting the line in to equal halves. But unlike the Cyrus Beck algorithm, which only bisects the line once, Midpoint Subdivision Algorithm bisects the line numerous times.

Also the Sutherland Cohen subdivision line clipping algorithm requires the calculation of the intersection of the line with the window edge. These calculations can be avoided by repetitively subdividing the line at its midpoint.

Step1: Calculate the position of both endpoints of the line

Step2: Perform OR operation on both of these endpoints

Step3: If the OR operation gives 0000
                    Line is guaranteed to be visible
                  Perform AND operation on both endpoints.
                  If AND ≠ 0000
            then the line is invisible
            then the line is clipped case.

Step4: For the line to be clipped. Find midpoint
        Xmis midpoint of X coordinate.
                  Ymis midpoint of Y coordinate.

Step5: Check each midpoint, whether it nearest to the boundary of a window or not.

Step6: If the line is totally visible or totally rejected not found then repeat step 1 to 5.

Step7: Stop algorithm.