This was posted to Slashdot on 27 June 2000, supposedly in conjunction with Mozilla's Milestone 17 release, with a followup message a few hours later that clarified a few things and corrected some errors. This version already reflects those changes. [Click here for the previous version of this article, from early 1999.]

PNG, MNG, JNG and Mozilla M17

Greg Roelofs
26 June 2000

PNG support in Mozilla has improved greatly over the last few releases ("milestones"), and with each milestone comes a corresponding Slashdot posting and a lot of discussion. Unfortunately, not all of the discussion is entirely accurate, so here's a preemptive posting that attempts to update folks on the status of PNG support in Mozilla and other apps and to clear up some of the more common misconceptions. (This seems to be an annual event...thanks to Jamie McCarthy for the suggestion this time around.)

Home Page

First of all, the PNG home page got booted off of in early March, and in early May it settled into what should be its absolutely final home:

This is currently hosted on, Walnut Creek CD-ROM's new site for free software (quel surprise!), but if something should ever happen to Walnut Creek, will be redirected appropriately. (On a related note, the new zlib URL is, which is also currently hosted on (Subsequent note, June 2001: zlib at last has its own domain:, currently hosted by France-Teaser.)

PNG Features for the Web

Insofar as this is ostensibly a Mozilla posting, let's have a brief rundown of the PNG features that are most useful to web designers:

PNG Extensions and the Future

PNG is extensible. PNG is lossless. PNG is a single-image, raster (bitmap) format. One of its overriding design goals was backward compatibility. As a result, don't expect to see any sort of lossy compression methods (JPEG is doing a fine job of that, with the exception of transparency--but see JNG, below). Also don't expect to see any vector-based extensions--SVG with gzip content-encoding has that covered. Indeed, don't expect to see any new, incompatible compression methods for quite a while. Until there are lossless methods that can, on average, halve the size of PNG images, the cost in software compatibility is far too great. (Keep in mind that there still browsers that don't support progressive JPEG, and that was a relatively trivial change! And let's not even talk about JPEG 2000...)

PNG is also not going to become an animated format. Leaving multiple-image support out of PNG was a conscious design decision by the PNG development group, and it's still the right decision. Overloading a still image format with animation or video features merely confuses users and web browsers, which have no way to distinguish still images from animations without prying into the data streams (which usually means downloading them first). Developers who prefer to program monolithically can always program for MNG instead; it's architecturally identical to PNG, and PNG is a pure subset of MNG.

Related Formats

MNG: As the previous paragraph suggests, the animated version of PNG is called MNG, for Multiple-image Network Graphics. It supports looping (including nested loops), clipping, deltas, and other features, plus everything PNG supports--including alpha transparency, of course. The home page is here:

Since this spring, a free reference library, libmng, has been under development by Gerard Juyn; its home page is at:

Note that the MIME type is video/x-mng; it has not yet been registered with the IETF. Undoubtedly there will be many misconfigured web servers in coming years...

JNG: JNG is short for JPEG Network Graphics and is a proper subset of MNG, just as PNG is, but it's worth a separate mention. The idea is to combine the best of both worlds: JPEG's excellent compression and PNG's incredibly spiffy alpha transparency and color correction. JNG is almost identical to PNG, but in addition to standard IDAT chunks (which in JNG contain the alpha channel), there are also JDAT chunks that contain a standard JPEG/JFIF stream (suitable for handing off to libjpeg). From a developer's standpoint, if you've got support for both PNG alpha and ordinary JPEG/JFIF, adding JNG is a breeze. Of course, JNG is also supported by recent libmng betas. Its MIME type is image/x-jng.

Browser Status

Most browsers have supported PNG since at least late 1997 (when Netscape Navigator and Microsoft Internet Explorer finally did), but almost without exception, their support for alpha transparency has been abominable. Amazingly enough, it seems that 2000 may be the year that browsers finally support it, more or less ubiquitously. In April alone there were three newcomers, with another in May; so far this year, the total has more than doubled. Here's the current list of browsers that at least attempt to do alpha transparency correctly, with their supported platforms indicated in italics. If screen shots of the PNG alpha-transparency test page are available, they're linked to the browser name:

Honorable Mention goes to Siegel & Gale's PNG Live plug-in for Netscape, which was the only plug-in ever to manage alpha transparency (in Windows only). It died before ever getting out of beta, though, and plug-ins in general are useless for PNG. So is the HTML 4.0 OBJECT tag, but don't get me started...

Other Apps, Libs, etc.

I currently list some 500 distinct PNG-supporting packages (more if you break things like Microsoft Office into their constituent parts) in 8 categories (soon to be 9 or 10), not to mention a dozen pieces of hardware. PNG has now reached the point where even freeware authors generally don't bother to tell me when they've added support; it's largely taken for granted. (I do occasional Freshmeat sweeps, but I usually don't have time, and many entries don't mention PNG even if it's supported.) Quite a number of the apps include full source code, by the way--which is the way it should be, of course. ;-)

Within the libraries-and-toolkits category, there are a surprising number of independent PNG implementations (either encoders or decoders or both), including ones in C, C++, Java, JavaScript, Pascal, and even Ada95. PNG is now a standard part of Java 2 SE 1.3 (but not Tcl/Tk as I originally claimed), and it is the main image format in the popular gd library and all of its Perl-based derivatives. In turn, this has led to its online use in areas as diverse as server statistics, chemical diagrams, computer-generated mazes, and weather maps.

Even better, PNG is the native, internal image format for a number of major applications (including Macromedia Fireworks and Microsoft Office), and it's becoming a popular icon format for advanced GUIs. It also ships as a standard part of BeOS, via the Translation Kit, and it's supported natively in the Windows Me shell (and possibly in Windows 2000 Professional).


Ordinarily I'd mumble something about how PNG has finally achieved massive studliness and will soon be taking over the world, but what the hell--it has, it is, and if it's not obvious from what I've already written, another couple of lines won't make any difference. Go forth, visit the web site, write code, make lots of PNGs, etc., etc.

And Microsoft, pleeeeease get on the ball with Internet Explorer for Windows and Unix...

[primary site hosted by SourceForge] Last modified 14 March 2009.

Copyright © 2000-2009 Greg Roelofs.