CS101 - Introduction to Computing - Lecture Handout 33

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Graphics & Animation

During the last lecture …(Internet Services):

We looked at several services provided by the Internet
FTP
Telnet
Web
eMail
Instant messaging
VoIP
We also found out about the addressing scheme used on the Internet

IP Address:

A unique identifier for a computer on a TCP/IP network
Format: four 8-bit numbers separated by periods. Each 8-bit number can be 0 to 255

Domain Names:

IP addresses are fine for computers, but difficult to recognize and remember for humans
A domain name is a meaningful, easy-to-remember ‘label’ for an IP address

DNS: Domain Name System:

DNS is the way that Internet domain names are located & translated into IP addresses

FTP:

Used to transfer files between computers on a TCP/IP network (e.g Internet)

Telnet Protocol:

Using Telnet, a user can remotely log on to a computer (connected to the user’s through a TCP/IP network, e.g. Internet) & have control over it like a local user, including control over running various programs

The Web :

The greatest, shared resource of information created by humankind
A user may access any item on the Web through a URL, e.g.
http://www.vu.edu.pk/cs/index.html

The Web

eMail:

Computer-to-computer messaging
Inexpensive, and quite quick, but not instant!

But first, the components:

eMail client
SMTP server
POP3 server

The Trouble with eMail:

Slow response times
No way of knowing if the person we are sending eMail to is there to read it

The process of having a conversation through eMail by exchanging several short messages is too cumbersome
Instant messaging (IM) solves these problems

Instant Messaging:

The IM services available on the Internet (e.g. ICQ, AIM, MSN Messenger, Yahoo!
Messenger) allow us to maintain a list of people (contacts) that we interact with regularly
We can send an instant messages to any of the contacts in our list as long as that contact is online

Key Point:

Once the IM server provides the communication info to the user and his/her contact’s IM client, the two are able to communicate with each other without the IM server’s assistance
This server-less connection is termed as a P2P connection

VoIP: Voice over IP:

Voice delivered from one device to another using the Internet Protocol
Inexpensive, but of poor quality

Today’s Goal:

Graphics & Animation

We will become familiar with the role that graphics and animations play in computing
We will look at how graphics & animation are displayed
We will look at a few popular formats used for storing graphics and animation

Computer Graphics:

Images created with the help of computers
2-D and 3-D (displayed on a 2-D screen but in such a way that they give an illusion of depth)
Used for scientific research, artistic expression, or for industrial applications
Graphics have made the computer interfaces more intuitive by removing the need to memorize commands

Displaying Images:

Most all computer displays consist of a grid of tiny pixels arranged in a regular grid of rows and columns
Images are displayed by assigning different colors to the pixels located in the desired portion of the computer display
Let’s discuss the pixel a bit more …

Pixel:

The smallest image forming element on a computer display
The computer display is made up of a regular grid of these pixels
The computer has the capability of assigning any color to any of the individual pixels on the display
Let’s now see how the computer displays a square

Pixel Colors :

The color of each pixel is generally represented in the form a triplet
In a popular scheme – the RGB scheme – each part of the triplet represents the intensity of one of out of three primary colors: red, green, blue
Often, the intensity of each color is represented with a byte, resulting in 256x256x256 (16+ million) unique color combinations
If this scheme is used to display an image that is equal to the size of an XGA (1024x768 pixels) display, the image will require 2.25MB of storage, which is just too much A number of clever schemes have been invented to reduce the number of bytes that are required for storing graphics. 2 popular ones:
Color mapping
Dithering

Color Mapping :

Instead of letting each pixel assume one out of 16 million possible colors, only a limited number of colors – called the platelet – are allowed

For example, the platelet may be restricted to 256 colors (requiring 1 byte/pixel instead of 3)
Each value, from 0 to 255, is mapped to a selected RGB color through a table, reducing the size of a 2.25MB graphic to 0.75MB
The quality of the displayed image will not suffer at all if the image only uses colors that are a part of the platelet

Color Mapping

Color Platelet Example:

Color Platelet Example

Dithering:

In this scheme, pixels of alternating colors are used to simulate a color that is not present in the platelet
For example, red and green pixels can be alternated to give the impression of bright yellow
The quality of the displayed image is poorer

Aliasing:

The computer screen consists of square-ish pixels arranged in a fixed grid
At times, when a diagonal line is drawn on this grid, it looks more like a staircase, instead of a straight line
This effect – called aliasing – can be managed by reducing the size of pixels

Anti-Aliasing:

Anti-aliasing is another technique used for managing the ‘staircase’ effect
Let’s say that we need to draw a white straight-line such that it overlaps 60% with one pixel, and 40% with another initially, and near the end, 58%, 41%, and 1%, respectively, with three pixels

Anti-Aliasing

The staircase effect is caused because the proper drawing of the line requires a pixel that does not exist
There are three options in this case:
Assign the white color to the pixel corresponding to the largest overlap
Assign the white color to both pixels
Either of these will cause the staircase effect
The 3rd option is to color the pixel with 60% overlap to a 40% gray color & the other one to 60% gray
Result: A smoother - pleasing to the eye - look

Graphics File Formats:

The choice of the format generally depends upon the nature of the image. For example:
An image of natural scenery contains many irregular, non-gemetric shapes, therefore is stored in bit-map format
A CAD drawing consists of many geometric shapes like straight lines, arcs, etc. and therefore is stored in a vector format
A third situation arises when dealing with graphics that contain both regular and irregular shapes

Vector or Object-Oriented Graphics:

Treats everything that is drawn as an object
Objects retain their identity after they are drawn
These objects can later be easily moved, stretched, duplicated, deleted, etc
Are resolution independent
Relatively small file size
Examples: swf, svg, wmf, ps

Bit-Mapped or Raster Graphics:

Treats everything that is drawn as a bit-map
If an object is drawn on top of another, it is difficult to move just one of them while leaving the other untouched
Changing the resolution often requires considerable touch-up work
Relatively large file size
Examples: gif, jpg, bmp

File Formats Popular on the Web (1):

Bit-map images compressed using the LZW algo.
The number of colors is first reduced to 256 and then consecutive pixels having the same color are encoded in a [color, numberOfPixels] format
Works well with computer-generated graphics (e.g. CAD, block diagrams, cartoons) but not with natural, realistic images
Loss-less for images having 256 colors or less jpg (JPEG – Joint Photographic Experts Group)
Compressed, full-color and gray-scale bit-map images of natural, real-world scenes, where most every pixel differs in color from its neighbor
It does not work as well as gif with non-realistic images, such as cartoons or line drawings
Does not handle compression of B&W images
Lossy
swf (Shockwave Flash)
Designed for 2-D animations, but can also be used for storing static vector images as well
A special program (called a plug-in) is required to view swf files in a Web browser svg (Structured Vector Graphics)
New format; may become more popular than swf

Image Processing:

A branch of computer science concerned with manipulating and enhancing computer graphics

Examples:
Converting 2-D satellite imagery into a 3-D model of a terrain
Restoring old, faded photographs into something closer to the original
Determining the amount of silting in Tarbela lake from a satellite image

D Graphics:

Flat images enhanced to impart the illusion of depth
We perceive the world and the objects in it in 3-D - breadth, width, depth - although the images formed on the retinas of our eyes are 2-D
The secret of 3-D perception: stereo vision
The two eyes are spaced a few cm apart
Result: The images formed on the two retinas are slightly different
The brain combines these two into a single 3-D image, enabling us to perceive depth

3-D Graphics: Applications:

Games
Medical images
3-D CAD

3-D Rendering:

The process of converting information about 3-D objects into a bit-map that can be displayed on a 2-D computer display
Computationally, very expensive!
Steps:
Draw the wire-frame (skeleton, made with thin lines)
Fill with colors, textures, patterns
Add lighting effects (reflections, shadows)

Animation:

Graphics in motion, e.g. cartoons
Illusion of motion is created by showing the viewer a sequence of still images, rapidly
Drawing those images - each slightly different from the previous one - used to be quite tedious work
Computers have helped in cutting down some of the tediousness
See next slides

Computer Animation: Examples

Games
Cartoons, movies
Visualization of processes, e.g the IM process
Displaying the results of scientific experiments, e.g. nuclear fusion

Tweening:

Creating a reasonable illusion of motion requires the drawing of 14-30 images per second of animation – very tedious!
In practice, only 4-5 images (called key images) instead of 14-30 are drawn, and then the computer is asked to create the remaining in-between images
This process of creating these in-between images from key images is called inbetweening (or tweening for short)
The simplest algorithm for tweening calculates the position of a particular segment of an image by calculating the average of the positions of that same image segment belonging to adjacent key images

The Future of Graphics & Animation:

New graphic-file storage formats will appear with better compression efficiencies
3-D animation will become more popular as computers become faster and algorithms become smarter
More realistic games; better realism in movies – may, one day, make the human actors extinct

Today’s Goal:Graphics & Animation

We became familiar with the role that graphics and animations play in computing

We discussed how graphics & animation are displayed
We also looked at several formats used for storing graphics and animation

Next Lecture:(Intelligent Systems)

To become familiar with the distinguishing features of intelligent systems with respect to other software systems
To become able to appreciate the role of intelligent systems in scientific, business and consumer applications
To look at several techniques for designing intelligent systems