Characteristics of Multimedia Systems

Characteristics of Multimedia Systems

·  Multimedia systems as information systems that include combinations of the following media; including:

o  Text, hypertext and numbers

Text is a fundamental source of information for human civilizations. As such it is an important component of multimedia and is found in almost all multimedia applications.

The simplest file type for text is the "txt" file type. This is a direct ASCII representation of textual information and contains no formatting information, like typeface, font information or page layout. The file format "txt" can be opened by using simple "text editors" like "Edit" (DOS) "Notepad" (Windows) or "pico" (unix). Word processors like Microsoft Word will also open "txt" files, though they must be specifically instructed to save a file in this format.

Word processors allow the user to format their text with respect to fonts, typeface and page layout. The instructions for this formatting are usually proprietary and are different for each different word processor. For example, Microsoft's default file format is "doc", which includes, not only the textual information, but also information relating to how the text should appear on the screen and printed page. For this reason it is usually not possible to open a document created in Microsoft Word with Word Perfect - unless some form of conversion takes place. The use of differing operating systems like Windows, Macintosh or unix further complicates the matter. To avoid this problem RTF (Rich Text Format) may be used. This is a "universal" format that can be read by many different word processors and operating systems.

Early multimedia applications were computerised versions of encyclopaedias - mostly text and pictures with a few words and/or animations. These were basically copies of the original paper based encyclopaedias, with little advantage to the user, except for the computer's search capabilities (replacing the index of the book version).

An important addition to these early multimedia applications is the use of hypertext. You are familiar with hypertext links used in applications like the world wide web (a small part of which you are using now). Hyperlinks are text which is highlighted like this. Such text allows the user to be "transported" to another location by clicking (with the mouse) on the text.

Effective use of hypertext allows the information to be presented in a "non-linear" form. Most books re read in a "linear" form from beginning to end. Hypertext, being "non-linear", allows the user to jump from place to place and access the information in a way that may be more meaningful for the individual.

Today the most common form of hypertext is found in the format called HTML - Hypertext Markup Language. This is the language of the World Wide Web, and is the language this document is written in. HTML requires a special piece of software called a browser to be read. This browser converts the HTML into a form, which is pleasing to the eye. For example the following heading:

This is a second level heading in italics

looks like this in HTML:

<H2<i>This is a second level heading in italics</i</H2>

HTML can be written in any text editor - for example notepad and saved with the file extension htm or html. As the HTML can be confusing for the uninitiated many programs have been written which allow the user to write in a fashion similar to a word processor. These programs will convert the WYSIWYG text into correct HTML

(WYSIWYG = What You See Is What You Get)

A common example is Microsoft Frontpage -though many other programs will save in the HTML file format - eg MS Word and Corel Draw.

o  Audio

Sound is important in a multimedia presentation for:

·  Reinforcement : pictures of an object can be enhanced with sound - for example a picture of a lion looks better accompanied by the sound of a lion's roar.

·  Enhancement : music playing can make a presentation more enjoyable.

·  Warnings or associations : a strong beep or ding is often used to indicate errors, while the sound played when windows shutdown is familiar and reassuring.

Sounds may be stored on computers in two ways - as waves (or similar file formats) or as MIDI (or similar file formats). These two formats can be compared to bitmapped and vector file formats used in graphics. Waves are similar to Bitmaps, while MIDI files are similar to vector graphics.

·  Wave Files: digitised versions of analogue sound waves. These may be large files, depending upon the quality of the sound and the amount of compression used in storage. You can learn more about Wave format sound.

·  MIDI Files: Musical Instrument Digital Interface - mathematical data used to drive the sound system directly. These are small files as the actual "sound" isn't stored, rather "instruction" for the sound card or musical instrument to "play" the sound. You can learn more about the MIDI formate

o  Images and/or animations

Graphics in Multimedia

It was the invention of the CD-ROM, which made graphics and animations feasible in multimedia - indeed there was little true multimedia, available at a price the general public could afford, before the CD-ROM became a popular peripheral for home computer systems. This is mainly due to the large file size of graphics and animations. For example, an entire page of text may take up as little as 2k of memory, while a single picture 800x600 with 256 colours will take over 500k. An animation increases this file size enormously as each frame in the animation may take up 800 k each - to be displayed at up to 24 frames per second.

There are two broad categories of image formats: Bitmapped and Vector. We will look at each type of image in turn.

Bitmapped Graphics

A bitmapped graphic is an image made up of dots, where the precise colour and location of each dot is stored in memory. The term bit-map reflects this idea of a map of pixels on the screen, each pixel being represented by one (or more) bits of data.

As the amount of detail of a picture is increased, the size of the bit-map increases also. For this reason, very high quality full colour images can be very large indeed.

However, despite the large file size, graphics add an important dimension to a multimedia presentation, and the old adage "A picture is worth a thousand words" is particularly applicable to multimedia applications. Therefore, many ways have been invented to reduce the file size of graphics and make them more easily "transportable".

To make a graphic "transportable" a standard graphic type was needed. In the past each different type of computers handled graphics in different ways. For this reason many different (and incompatible) file types were invented. The Macintosh computer used TIFF or MCP (Mac paint) file types, while the IBM computer used BMP or PCX. It was once difficult to view files from one computer on another and file conversion programs were needed.

Eventually a file "standard" developed. The common file types in use today are gif and jpg. GIF stands for "Graphics Interchange Format", a proprietary system developed by "Compuserve" (an online computer system popular before the internet was released to the general public). JPG was developed by the Joint Picture Group and does not have the copyright restrictions of the gif format. Common animation and video formats include avi, quicktime (Apple) and mpg (motion picture group), though vector animations like flash are becoming popular.

Once a standard was developed, the need to reduce file size was addressed. There are a number of ways to reduce file size including compression and the use of vector graphics.

Compression of graphics involves the reduction of redundant data in a picture. Consider the following picture :

This picture has large tracts of the colours green and blue. A bitmap refers to the storage of information for a collection of pixels as a series of numbers. That information refers to the colour of the pixel and its position on the screen. The picture above has a large amount of redundant data, because many of the pixels are exactly the same as the pixel next to them.

Compression involves identifying large areas where the colour is the same and "tokenising" the information for these pixels. Rather than simply repeating the information for each pixel, over and over, it is better to store the information for one pixel (a token) and then store the fact that there is (say) 50 similar pixels (tokens) in a line next to this first pixel. This compression can greatly reduce the size of a bit mapped image. The image above, stored as a BMP (a raw bit mapped picture format) is 91 k, when it is stored as a gif (a compressed file format) its size reduces to only 6 k.

The second way to reduce a file's size is through a technique, which reduces the amount of detail in a picture. This type of compression is sometimes called "lossy" - referring to a loss of detail. JPG and mpg formats are "lossy" compression formats.

This kind of compression is most suitable for pictures like that below - ie one in which there are not large areas of similar colours.

The picture above stored as a BMP (raw) is 267 k. Compressed as a gif it reduces to 82k. Stored in jpg format the file size reduces to only 37k, though there is some (barely perceptible) loss of detail. Most high quality graphic editors will allow you to specify the "quality" of the jpg compression - from highest compression with most loss of detail, to low compression with little loss of detail.

JPG compression is not always the best form of compression in all circumstances. Under certain conditions, gif will be more efficient than jpg. Pictures with large areas of similar colour will compress better as a gif than as a jpg. The first picture (of the house) is 6 k as a gif, but 21k as a jpg.

A new image format is PNG.

PNG (pronounced PEENG) is a file format for image compression that, in time, is expected to replace the Graphics Interchange Format that is widely used on today's Internet. Owned by Unisys, the GIF format and its usage in image-handling software involve licensing or other legal considerations. (Web users can make, view, and send GIF files freely but they can't develop software that builds them without an arrangement with Unisys.) The PNG format, on the other hand, was developed by an Internet committee expressly to be patent-free. It provides a number of improvements over the GIF format.

Like a GIF, a PNG file is compressed in lossless fashion (meaning all image information is restored when the file is decompressed during viewing). A PNG file is not intended to replace the JPEG format, which is "lossy" but lets the creator make a trade-off between file size and image quality when the image is compressed. Typically, an image in a PNG file can be 10 to 30% more compressed than in a GIF format.

The PNG format includes these features:

Ø  You can not only make one colour transparent, but you can control the degree of transparency (this is also called "opacity"). interlaced GIF of the image is supported and is faster in developing than in the GIF format. Gamma correction allows you to "tune" the image in terms of colour brightness required by specific display manufacturers. Images can be saved using true colour as well as in the palette and grey-scale formats provided by the GIF.

Ø  Unlike the GIF89a, the PNG format doesn't support animation since it can't contain multiple images. The PNG is described as "extensible," however. Software houses will be able to develop variations of PNG that can contain multiple, scriptable images.

Ø 

Compression, either lossy or lossless, is not the only way to save space when using images. Another way to make a picture small is to represent the information in a completely different way.

Vector Graphics

The next way to reduce the size of a picture is to use a vector representation of the image. This form of "compression" involves taking the image and reducing it to a series of mathematical equations.

For example, the equation for a "circle shape" on a computer could be:

(Though it’s likely to be much more complicated)

Now, given this equation we can make a circle of any size we want simply by changing the value of "r" (radius). The amount of memory this takes up will be the same, whether the circle is a tiny dot, or takes up the entire screen. Further, if we zoom into the image, the formula can be used to "re-draw" the image. Therefore, the quality of the image is the same, no matter how far we zoom in - unlike bit-mapped pictures, which break up into blocks (pixellate) when we zoom in too far.

Macromedia "flash" format is a vector animation format. Vectors certainly reduce the files size, however, it is difficult to represent "living" images (like people) in vector format. Further, if the multimedia is presented as a series of web documents, "plug-ins" are required to view most vector graphic formats.

Animation in Multimedia

A graphic may be considered a static (still) image, while an animation is a moving image. Animations differ from video according to the way it was produced. Video is live or "real" footage of an event taken with a video camera. Animation is computer generated moving pictures produced in either 2 or 3 dimensions by various software. For this reason a movie like Toystory or The Lion King is considered animation, while Star Wars is considered a video (despite the fact that Star Wars contains a great deal of computer animation sequences and is not really "real").