Computer graphics

Computer graphics (CG) is an art of creating pictures and films on the computer screen using programming. In other words it is the creation, storage and manipulation of images and models. It can be a series of images combined or it can be a single image also. It is made up of a number of pixels which is the smallest graphical unit represented on the computer screen.

Basically there are two types of computer graphics namely 

1) Interactive Computer graphics

2) Non interactive Computer graphics

In Interactive Computer graphics the user has some control over the image by using an input device through which he can send his request to the computer. The computer on receiving the request can change the image accordingly.eg: a flight simulator

In Non interactive Computer graphics the user has no control over the image and the image will work according to the instruction given in the pre-defined program.eg: screen savers

Application of Computer graphics

Nowadays the area of computer graphics is widely used in a variety of applications for specific purposes like

• Computer aided design (CAD)
• Presentation graphics
• Image processing
• Virtual reality
• Entertainment like videogames, animation etc.
• Graphical user interface(GUI)
• Education & training
• Computer art

Computer aided design(CAD)  is used in the design of aircrafts,buildings,automobiles ,spacecraft's and many other products.

Presentation Graphics is used to produce various illustrations in reports and presentations. Examples are bar charts, line graphs etc.

Image Processing includes techniques to modify or improve existing pictures which can be used in the medical field for tomography, ultrasonic medical scanners etc.

Virtual Reality allows representations that allows a real life look and feel that enables the user to immerse in the virtual world .e.g.: development of online classrooms

In Entertainment, CG is mostly used in the movie industry for creating music, videos and cartoon animated movies .It is also widely used in the game industry.

For GUI, the interface between computer and the user has been radically altered by the use of computer graphics. CG is effectively used to make menus, icons (graphical symbols).

In Education and training field computer generated models are created for better understanding of the topic.CG is also being used to help people with special needs. Various Simulators are used for practice sessions and training of ship captains and pilots.

In Computer art, CG is used for creating logos, cartoon drawings, morphing etc. It includes artist's paintbrush programs, paint packages and animation packages.

Types of computer graphics

There are two types of computer graphics

• Raster (composed of pixels)
• Vector(composed of paths)

Raster images are commonly called bitmap images. A bitmap image uses a grid of individual pixels where each pixel can be different in shade or color. Digital photographs are the most common type of raster graphics. An important property of raster graphics is its resolution. Resolution indicates the amount of detail, so a higher resolution means more detailed image. But a bitmap image appears to be jagged when u zoom in the image.

Vector graphics on the other hand uses points, lines, curves and shapes to create illustrations. Vector graphics are composed of paths. It uses the mathematical relationships between the connecting points and lines to describe an image. One of the key characteristics of vector image is that the line work is sharp even when you zoom in closely. Unlike a bitmap image Vector based graphics appears to be smooth at any size or resolution.

Advanced concepts in computer graphics

Antialiasing

When a text or a graphic image is displayed on a monitor or screen the smoothness of the edges of the images is limited to the resolution of the screen which means the edges tend to be a little jagged. This jagged appearance is called aliasing.

Antialiasing is a software technique to minimize the jagged effect (stair step - like lines) in the images. Jagged appearance occurs because the output device does not have enough resolution to represent the smooth line. Antialiasing surrounds the stair steps with intermediate shades of grey or color thus reducing the jagged effect. Antialiasing is also called oversampling.

There are three main classes of antialiasing algorithms:

1) As aliasing problem is due to low resolution one easy method is to increase the resolution. This increases the cost of image production.

2) The image can be calculated by considering the intensities of a particular region. This is called prefiltering.

3) The image is created at high resolution and then digitally filtered. This method is called super sampling or post filtering as filtering is carried out after sampling.

4) Unweighted Area Sampling is finding pixel intensity by calculating the areas of overlap of each pixel within the object to be displayed.

Texture mapping and bump mapping

Bump mapping like texture mapping is a technique to add more realism to synthetic images without adding a lot of geometry. It works by altering the brightness of the pixels in specific patterns. By using bump mapping a sphere can be made to look like an orange.

Ray tracing

Ray tracing is a technique for rendering three dimensional graphics with very complex light interactions. This technique is used in the design of optical systems such as cameras, lenses, microscopes etc. It is best suited for applications where the image can be rendered slightly ahead of time such as in still images and film and television visual effects but poorly suited for real time applications like videogames where speed is very critical.

Six things every student should know to solve Computer graphics problems

• Screen coordinate- This 2D coordinate system refers to the physical coordinates of the pixels on the computer screen based on the current screen resolution. This is usually measured in pixels. The origin (0,0)is at the top left of the screen.
• 2D transformation- One of the most important and common task in computer graphics is to transform the coordinates of either objects within the graphical scene .It is important to transform from coordinate system to other. When transformations occur in 2D plane it is a 2D transformation. All these transformations can be efficiently handled using matrix representations.
• 3D Transformation-Similar to 2D transformation which uses 3*3 (x,y,z)matrices but 3D transformation uses 4*4 Matrices(x,y,z,w)
• Clipping Algorithm-Clipping is the process of removing unwanted lines or any other portions in the image outside the line of interest. The region against which an object is to be clipped is called the clip window. There are two commonly used line clipping algorithms -Cohen Sutherland and Liang- Barsky.The purpose of the clipping algorithms are to determine which points, lines or portions of line lie within the clipping window. In polygon clipping it takes as input the vertices of the polygon and return one or more polygons.
• Animation-This means giving life to any object in CG.The basic idea behind animation is to playback the recorded image at a rate that is faster enough to fool the human eye.
• Rendering -It is the process of adding shade, color and lamination to a 2-D or 3-D frame to create life like images on the screen. There are two types of rendering- Real time rendering and Pre rendering. Real time rendering is used most prominently in gaming and interactive graphics whereas Pre rendering or offline rendering is used where time is less of an issue.

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