Netscape Navigator

Netscape's Navigator browser, which originally shipped standalone but more recently has been bundled as part of the Communicator suite, supplanted NCSA Mosaic late in 1994 as the standard browser by which all others were measured. Version 1.1N was released in the spring of 1995, at roughly the same time as the frozen PNG specification, but despite the hopes and efforts of the PNG developers, the first Navigator 2.0 betas shipped later that year with animated GIF support rather than PNG. Navigator 2.0 did offer the possibility of platform-specific, third-party PNG support via Netscape's new plug-in interface, but only for Windows and Macintosh. Alas, even that was fatally flawed from an image-support perspective: Navigator's native image-handling code (via the HTML IMG tag) had no provision for handing off unknown image types to plug-ins. That meant that even if PNG plug-ins were written for both supported platforms, and even if a majority of users downloaded and installed a plug-in, it would be useless for standard HTML--only pages using Netscape's proprietary EMBED tag would invoke the custom code. Moreover, Navigator 2.0 plug-ins were given no access to the existing page background, which meant that PNG transparency would be completely ignored.

The Navigator 3.0 betas in 1996 extended plug-in support to include Unix platforms as well, but they fixed none of the fundamental problems in the plug-in API.[13] The interface was considerably revamped in 1997 for the 4.0 betas, however, finally allowing transparency support via something called a windowless plug-in--though only for the Windows and Macintosh platforms. Support was also added for images referenced via the new HTML OBJECT tag. But the basic lack of a connection between plug-ins and the native IMG-tag code persisted, and this barrier extended to the new OBJECT-handling code as well--even a JPEG or GIF image in an OBJECT tag would fail unless an appropriate plug-in were found. Should the outer OBJECT happen to be a PNG, Navigator would fail to render even the inner GIF or JPEG in the absence of a PNG plug-in. Unlike IMG tags, Navigator required OBJECT tags to include the otherwise optional HEIGHT and WIDTH attributes to invoke a plug-in. In at least one version, the browser would ignore not only an undimensioned OBJECT but also all subsequent dimensioned ones.

[13] Applications Programming Interface, the means by which one piece of code (in this case, the plug-in) talks to another (in this case, the browser). APIs are also how programs request services from the operating system or the graphical windowing system.

But in November 1997, a year after the World Wide Web Consortium (W3C) officially recommended PNG for web use, Netscape released Navigator 4.04 with native PNG support--that is, it was at last capable of displaying PNG images referenced in HTML IMG tags without the need for a third-party plug-in. Unfortunately, versions 4.04 through 4.51 had no support for any type of transparency, nor did they support gamma correction, and their handling of OBJECT tags remained broken. At least a few of these releases, including 4.5, had a bug that effectively caused any PNG image served by Microsoft Internet Information Servers to be rendered as the dreaded broken-image icon. (I'll come back to this in the server section later in this chapter, but the bug is fixed in Navigator 4.51.) But the 4.x versions did support progressive display of interlaced PNGs, at least.

Concurrent with the later Communicator 4.0 releases, on March 31, 1998, Netscape released most of the source code to its development version of Communicator, nominally a pre-beta version ``5.0.'' Developers around the world promptly dug into the code to fix their favorite bugs and add their pet features. One nice surprise was that the so-called Mozilla sources already contained a minimal level of transparency support. There were two main problems with it, however: the transparency mask for all but the final pass of interlaced images was scaled incorrectly--a minor bug, hardly unexpected given the early stage of development--and the transparency was either fully off or fully on for any given pixel, regardless of whether multilevel transparency information (an alpha channel) was present. The latter problem proved to be more serious than it sounded. Because of the way Mozilla's layout engine worked, at any given moment the code had no idea what the background looked like; instead, it depended on the local windowing system to composite partly transparent foreground objects with the background image(s). In other words, adding full support for alpha transparency was not something that could be done just once in the image-handling code, but instead required modifying the ``front end'' code for each windowing system supported: at a minimum, Windows, Macintosh, and Unix's X Window System, plus any new ports that got added along the way.

Difficult as it may sound, fixing Mozilla's (and therefore Navigator's) support for PNG alpha channels is by no means an insurmountable challenge. But in one of life's little ironies, the person who initially volunteered to fix the code, and who thereafter nominally became responsible for it, also somehow agreed to write this book. Alas, when push came to shove, the book is what got the most attention. :-) But all is not lost; by the time this text reaches print, full alpha support should be well on its way into Mozilla and then into Navigator 5.0 as well.

Table 2-1 summarizes the status and level of PNG support in all of the major releases of Netscape's browser to date. The latest public releases, Navigator 4.08 and 4.51, are available for Windows 3.x, Windows 95/98/NT, Macintosh 68k and PowerPC, and more than a dozen flavors of Unix; the web page is at http://home.netscape.com/browsers/. Version 4.04 for OS/2 Warp is available only from IBM's site, http://www.software.ibm.com/os/warp/netscape/.