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| Katarzyna J. Macura, M.D., Ph.D. |
| The Russell H. Morgan Department of Radiology and Radiological Science • Johns Hopkins Medical Institutions |
Part 5 — Multimedia on the Internet
by Katarzyna J. Macura, M.D., Ph.D.
Multimedia refers to any computer-based presentation or application that integrates two or more of the following elements: text, graphics, animation, audio, video and virtual reality. A PowerPoint presentation combining text, graphics and video clips is an example of a multimedia application. It is a noninteractive multimedia presentation in which the slides are displayed in a predefined linear fashion. Interactive multimedia applications accept input from the user by means of a keyboard, voice or mouse and perform an action in response. An interactive multimedia program allows users to select the material, define the order in which it is presented and obtain feedback on their actions. The ability for users to interact with a multimedia application is one of its more unique and important features that enhances learning by engaging and challenging users.
Text is a fundamental element used to convey information. It can be enhanced by a variety of textual effects to emphasize and clarify information, e.g., font size, color, style or special effects, such as shadow, blinking, bouncing or fading.
A graphic is a digital representation of non-text information, such as a drawing, charts or photographs. Graphics were the first media used to enhance the text-based Internet. The introduction of graphical Web browsers allowed Web page developers to incorporate illustrations, logos and pictorial navigation into Web pages. Graphics files on the Web must be saved in a certain format. The two most common file formats for graphical files are JPEG (Joint Photographic Experts Group) and GIF (Graphics Interchange Format). Files are saved in both JPEG and GIF format using compression techniques to reduce the file size for faster downloading from the Web.
JPEG is designed for compressing full-color, gray scale images or continuous-tone artwork. Any smooth variation in color, such as occurring in highlighted or shaded areas, will be represented more faithfully and in less space by JPEG than by GIF. GIF does significantly better on images with only a few distinct colors, such as line drawings and simple cartoons. Plain black-and-white images should never be converted to JPEG. There has to be at least 16 gray levels before JPEG is useful for gray-scale images.
JPEG is lossy, meaning that the decompressed image isn't quite the same as the original. A lossless compression algorithm is one that guarantees its decompressed output to be bit-for-bit identical to the original input. This scheme does not discard any data during the encoding process, while the lossy scheme throws useless data away during encoding. That is, in fact, how lossy schemes manage to obtain superior compression ratios over most lossless schemes. JPEG was designed specifically to discard information that the human eye cannot easily see. Because the human eye is much more sensitive to brightness variations in gray-scale than to color variations, JPEG can compress color data more heavily than brightness data. Gray-scale images do not compress well by large factors. It should be noted that GIF is lossless for gray-scale images of up to 256 levels, while JPEG is not. However, the more complex and subtly rendered the image, the more likely that JPEG will do well.
There are two good reasons for using JPEG: to make image files smaller and to store 24 bits/pixel color. Using smaller image files makes it easier to transmit files across networks and saves storage space when creating archiving libraries. JPEG stores full color information 24 bits/pixel (16 million colors) unlike GIF, which can store only 8 bits/pixel (256 or fewer colors). Hence JPEG is considerably more appropriate than GIF for use as a WWW standard photo format. However, it takes longer to decode and view a JPEG image than to view an image of a simpler format such as GIF. A useful property of JPEG is that adjusting compression parameters can vary the degree of lossiness. This means that the image maker can trade file size against output image quality.
JPEG is a useful format for compact storage and transmission of images, but it should not be used as an intermediate format for sequences of image manipulation steps. The disadvantage of lossy compression with JPEG is that if the user repeatedly compresses and decompresses an image, a little more quality is lost each time. A lossless 24-bit format such as PNG (Portable Network Graphics), TIFF (Tag[ged]) Image File Format) or PPM (Portable PixMap) should be used while working on the original image and then JPEG format can be used when the file is ready to store or send out on the net. If the user expects to edit the image again in the future, the original lossless master copy should be used. Both GIF and PNG support transparent backgrounds, while JPEG does not have this ability. This issue is currently making the GIF image very popular with Web page creators and designers.
Because graphics files can be time consuming to download, some Web sites use thumbnails on their pages. A thumbnail is a small version of a larger image that the user can click on to display the full-sized image. The graphical elements for computer applications can be obtained from a clip art/image gallery, which is a collection of previously created clip art and photographs grouped by themes. Graphics can also be created using paint/image editing software, for example, Windows Paint, PaintShop Pro or Adobe Photoshop. Photographs for multimedia applications can be obtained using a scanner to digitize photos, taking the photographs with a digital camera or buying them in a photograph collection on a CD-ROM. Graphics can also be downloaded from the Web.
Displaying a series of still graphics in rapid sequence creates an animation, which is a graphic that has the illusion of motion. Animated graphics make Web pages more visually attractive and draw attention to important information or links. There are graphics animation and authoring software packages that allow creation of animations. One popular type of animation, called an animated GIF, is created using special software (e.g., Ulead GIF Animator freeware) to combine several images into a single GIF file. With GIF Animator the user can create pictures, banners, buttons and even movies using the built-in image editor or any stand-alone graphics editor, to create image files that will be assembled into GIF animation. Also, the user can obtain previously created animations from a CD-ROM or download them from the Web.
Audio is music, speech or any other sound. Using audio in a multimedia application to supplement text and graphics enhances understanding. Audio for multimedia can be obtained in several ways. One method is to capture the sound using a microphone, CD-ROM, radio, musical device or any other audio input device that is plugged into a port on a sound card. As with graphics and animation, audio clips can be purchased on a CD-ROM, DVD-ROM or downloaded from the Web. Audio files can be quite large, and therefore they are frequently compressed to reduce the size and increase the speed of downloading. MP3 is a popular technology to compress audio. Files compressed using this format have an extension of .mp3 and have to be downloaded completely. Most currently used browsers contain a program called a player that can play the audio in MP3 files. Web applications also use streaming audio that allows the user to listen to the sound as it downloads to the computer. Streaming is the process of transferring data in a continuous and even flow, which allows users to access and use a file before it has been transmitted completely. Streaming is important because most users do not have fast enough Internet connections to download a large multimedia file quickly. Web-based audio can be also used for Internet telephone service, sometimes called Internet telephony. This technology allows users to talk to other people over the Internet. Internet telephony uses the Internet, instead of a telephone network, so the cost of the communication is reduced to the cost of the Internet connection. Internet telephone software and the computer's sound card digitize and compress the conversation and then transmit the digitized audio over the Internet to the called parties.
Video consists of photographic images that are played back at speeds of 15-30 frames per second and provide the appearance of full motion. To use video in a multimedia application, the developer has to capture, digitize and edit the video segments using special video production hardware and software. Video can also be captured directly in digital format using a digital video camera. Due to the size of video files, incorporating video into a multimedia application is often a challenge. Files require large amounts of storage space, therefore they are often compressed. Video compression works by recognizing that only a small portion of the video image changes from frame to frame, and after storing the first reference frame only changes from one frame to the next are stored. The Motion Pictures Experts Group has defined a standard for video and audio compression and de-compression, called MPEG. MPEG compression can reduce the size of video files up to 95 percent, while retaining near TV quality. Video compression has allowed video to play a more important role in multimedia applications. Technologies such as streaming video made video a viable part of multimedia on the Web. As with streaming audio, streaming video allows the user to view longer or live video images as they are downloaded to the computer. The standard used for transmitting video data on the Internet is RealVideo, which is a component of RealPlayer supported by most current Web browsers. Streaming video also allows conducting Internet videoconferences that work like Internet telephony. A video camera, videoconferencing software and video capture card digitize and compress the images and sounds. After they are sent over the Internet, equipment and software at the receiving end assemble and decompress the data presenting the images and sound as video. The Synchronized Multimedia Integration Language (SMIL, pronounced "smile") enables simple authoring of interactive audiovisual presentations. SMIL is typically used for "rich media/multimedia" presentations, which integrate streaming audio and video with other media type. SMIL is an HTML-like language and many SMIL presentations are written using a simple text-editor.
Another important application of multimedia is to create simulations, which are computer-based models of real-life situations. Multimedia simulations often replace costly and sometimes dangerous demonstrations and training, such as in chemistry, nuclear physics, aviation and medicine. Also, multimedia simulations are used in the game industry. Virtual reality is the simulation of a real or imagined environment that appears as a three-dimensional (3-D) synthetic space that has dynamic properties specified by software. On the Web, virtual reality involves the display of 3-D images that the user can explore and manipulate interactively. Most Web-based virtual reality applications are developed using virtual reality modeling language (VRML), which is a language that describes the geometry of the scene. Using VRML, a developer can create an entire 3-D site, called a virtual reality world containing infinite space and depth. Virtual reality has many practical applications in science, education, advertising, design and other fields.
Some of the multimedia on the Web is developed in Java, which is a programming language specifically designed for use on the Internet. Developers use Java to create stand-alone applications or programs called applets that can be downloaded and run in the window of any Java-enabled browser. An applet is a short program executed inside of another program that runs on the user's computer.
Most Internet browsers have the capability of displaying basic multimedia elements on a Web page. Sometimes the browser needs an additional program called a plug-in or helper application, which extends the capability of the browser. A plug-in runs multimedia elements within the browser window, while a helper application runs multimedia elements in a window separate from the browser. Plug-ins and helper applications can be downloaded or copied at no charge from many sites on the Web. Usually, Web pages that use multimedia include links to Web sites that contain the required plug-in or helper. Some browsers include commonly used plug-ins, such as Shockwave. To view the virtual world, the user needs a VRML browser or a VRML plug-in to a Web browser.
Combining media for Web applications brings Web pages to life, increasing the types of information available on the Web, expanding the Web's potential uses and making the Internet a more entertaining place to explore.
| Editor's Note: The original Mini-Tutorial on the Internet by Katarzyna J. Macura, M.D., Ph.D., was published in the AAWR Newsletter Focus. Dr. Macura updated her series for RSNA News. |
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