Plug-ins are a powerful mechanism for extending the capabilities of Netscape's web browser. Using plug-ins, the browser can display files in formats that were not even conceived of when the browser was developed, such as multimedia files embedded within larger web pages. This allows web pages to be designed with maximum visual impact. Like any powerful technology, plug-ins can easily be misunderstood or misused. This article will explore when Netscape plug-ins might be appropriate to use, explain how to install and remove plug-ins, suggest where to locate useful plug-ins, and provide insight into implementing your own plug-ins.
It might be useful to compare and contrast plug-ins with helper applications. Plug-ins and helper applications share many features and may be used interchangeably in many circumstances. Helper applications are older and may be more familiar to users than plug-ins. Netscape has supported helper applications since version 1.0 of their browser software, but plug-in support did not show up until version 2.0 (version 3.0 for UNIX).
Both helper applications and plug-ins are supplementary software used by web browsers to handle specific file formats. Helper applications can be run independently of the web browser. Plug-ins, however, are integrated into the browser and can be run only within a browser. Potential helper applications may already be installed on your system. Some very useful helper applications, such as xv, were developed even before web browsers became popular.
Helper applications, being independent of the browser, must create their own window for a user interface. Plug-ins may use the window provided by the browser. Since they share the browser window, they can be used to display files that are embedded within larger HTML files. Helper applications cannot display embedded files. Plug-ins can have access to file contents while the file is being downloaded by the browser. Such plug-ins are called “streaming” plug-ins. Helper applications are “launched” only after the file is fully downloaded.
The MIME (multi-purpose Internet mail extensions) protocol, described in RFC-1521, allows applications to exchange different types of files on the Internet. A text header identifies the data format of the message body. Web browsers use MIME information to categorize files before displaying them. For example, if the MIME header indicates the body is of type “image/gif”, the browser will display the message body as a GIF (Graphics Interchange Format) file. If no MIME header is available, the browser assigns a MIME type to the file based on the file's extension. Once the MIME type of the file has been determined, the browser searches its internal tables for the plug-in or helper application assigned to that MIME type.
As a web surfer, you may encounter HTML pages that have other files embedded within them. Without an appropriate plug-in, your browser will not be able to display the embedded file. It might seem impolite for a web-page designer to create a page you cannot display without specialized software, but there are some reasonable excuses for such behavior. For example, certain non-proprietary MIME types are in common use on the Internet, and are supported by Netscape on the Mac OS and Windows, but not UNIX. UNIX users must install a plug-in for such MIME types. One example of this type of file is MIDI (musical instrument digital interface); these are often embedded within web pages to provide background music.
As a web designer, you may want to use a newly developed multimedia file format on your web page. The current MIME type may have been developed too recently to be supported by existing browsers. The MIME protocol is designed to be extensible. Third parties are constantly developing new MIME types with specialized functionality. Examples of such MIME types include document rendering (application/pdf), portable graphics (image/png), vector graphics (application/shockwave-flash) or streaming audio (audio/pn-realaudio-plugin). Often these MIME types are developed in the hope of selling authoring software. Polite third parties provide free plug-ins for as many platforms as possible. Unfortunately, UNIX support is often the first to be sacrificed.
It may be tempting to invent new MIME types, rather than using existing functionally equivalent MIME types, or to embed specialized file types on your web page, such as Microsoft PowerPoint files. However, your viewers might not have an appropriate plug-in or may be unable or unwilling to locate and install one. Your carefully designed web page may have dull gray rectangles where you expect flashy graphics. It is generally a good idea to use only the most popular Internet MIME types on your web pages. If your needs can be met only by using a more specialized MIME type, it is polite to first check for plug-in availability or even supply the necessary plug-ins yourself.