The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly.

Concluding Remarks

The Portable Network Graphics format represents one more step in the evolution of portable, robust image formats. With good, ubiquitous support just around the corner in web browsers, and with support in image viewing and editing applications not only common but actually expected by customers, PNG's future is bright.

Of course, in the four years since PNG was created, we've learned a few lessons about what works and what doesn't. In the spirit of various publications' ``post-game analyses'' or ``postmortems,'' here is a quick look at some of the things we did right and some we did wrong, in no particular order.

Alpha transparency

Content developers are justifiably excited about the possibility of using variable transparency, including real anti-aliasing. The fact that PNG can do 8-bit (or smaller) RGBA-palettes is currently underappreciated and decidedly underimplemented, but it will come to be seen as one of PNG's greatest strengths in the next year or two.

Gamma and color correction

Despite rather spotty support in applications to date, gamma and color correction are features designers have been begging for, though not always by name. They will eventually come to be expected, but support in more browsers and image editors (correct support!) is necessary before users will begin to notice the difference. And while operating-system support for gamma and color correction isn't absolutely necessary, having it--as in recent releases of Unix/X and Mac OS and rumored future versions of Windows--makes the lives of application developers much easier.


The lack of a PNG-related animation format early on, and the subsequent delay in finishing and implementing a viable one, was perhaps PNG's greatest failing--certainly it is one of the most oft-heard criticisms. While there was no way the PNG Development Group could have known about Netscape's GIF-animation surprise late in 1995, in retrospect, it is obvious that the group should have begun development on a PNG-like alternative right away.

Allowing the early MPNG project to be swayed too far in the direction of a heavyweight multimedia format was also a mistake; the best course would have been to come up with something just a little better than animated GIF, with the option of extending it later to become more in line with today's MNG. A ``thin'' PNG animation spec, similar to the capability provided today by ImageMagick, could have been implemented easily by mid-1996 or even the end of 1995. (Starting small and working up is always easier!) Fortunately, recent drafts of the MNG specification have added the concept of ``simplicity profiles,'' so developers finally have the option of supporting a subset of the full PNG/MNG animation spec in a well-defined manner. Versions since 0.93 have actually extracted low complexity and very low complexity subsets into separate documents--MNG-LC and MNG-VLC, respectively--so ``starting small and working up'' is now not only possible but also actively encouraged.

Open Source-style development

It is difficult to zero in on one feature that counts as PNG's greatest success, but arguably the open, Internet-based development process was (and is) it. Even four years later, creating a robust, portable, extensible, well-specified image format from scratch in two months is nothing short of amazing.[108] The continued infusion of new blood and new ideas has been invaluable. New code is good, too.

[108] Or perhaps we are just now learning what university professors and Linux enthusiasts have always known: graduate-student-powered development is the way to go. Certainly the author of this book didn't get a whole lot done during the first two months of 1995, when PNG was being designed. (Actually, only about a quarter of the most active members of the PNG Development Group were students at the time, but the remainder achieved honorary grad-studenthood.)

Free reference code

When trying to promote the acceptance of a new format in existing applications, nothing succeeds so well as doing some of the developers' work! The availability of free and robust reference libraries (libpng and zlib), with minimal restrictions on reuse and redistribution, was clearly vital to PNG's success.

Decoupled compression engine

Separating the file format, as symbolized by libpng, from the compression engine, symbolized by zlib, probably made the format more palatable to programmers. If, for some reason, one were averse to using both libraries (perhaps due to code size), one could choose to implement only the PNG half--which is not nearly so intimidating as rewriting both the PNG library and the deflate algorithm. The fact that zlib's core compression code was already a trusted and familiar component of gzip and the Info-ZIP tools may have helped, too.

Slow browser support

The failure to get good PNG support into the Big Two browsers even four years after PNG's release--and the lack of any support for two-and-a-half years--must count as a strike against the PNG Group, even if it's still not apparent what could have been done differently. At the time, Netscape and Microsoft were in the midst of the so-called Browser War, and one more image format, even one that boasted alpha and gamma support, just wasn't flashy enough to show up on their proverbial radar screens. Personal contacts might have helped, but both companies were large enough that finding the right contact was close to impossible.

Nevertheless, that's (mostly) water under the bridge. As I noted way back in Chapter 1, "An Introduction to PNG", the 4.0 releases of both browsers have supported PNG files natively since late 1997, and the 5.0 releases are expected to fully support both alpha transparency and gamma correction. Once that happens, web designers can be expected to begin using (and demanding!) PNGs with alpha and gamma support on web pages within a year or so.


Pushing PNG as a standard (Recommendation) of the World Wide Web Consortium was probably key to getting PNG support into the Big Two by the end of 1997. And PNG's inclusion in the VRML97 ISO specification led to its status today as an ISO standards-track format, which is likely both to help speed its acceptance in areas outside the Web (such as medical imaging, perhaps) and to ensure its longevity as a common and useful image format.


As most implementors know, there are specifications, and then there are specifications. PNG's spec has been praised by a number of third parties as being one of the cleanest, most thorough, and least ambiguous image specifications ever written. Partly, this was due to the work of some very good editors, but it owes a lot to the Open Source process, too (the ``many eyeballs'' effect).


PNG's well-defined method for adding new features in a backward-compatible manner has already proven itself many times over. The addition of the iTXt chunk early in 1999 is the latest example; even 1995-era viewers can still display a PNG image with such a chunk in it. Of course, such a powerful tool cuts both ways, as became apparent when some users mistakenly tried to use PNG images containing Fireworks's huge editing extensions on web pages.

Internal consistency checks

The presence of cyclic redundancy check (CRC) values in every chunk is a positive thing and helps PNG's robustness, but one of the original aims--the ability to verify from a command-line prompt that a PNG image was downloaded properly--turned out not to be particularly useful. The advent of high-speed modems, ubiquitous Internet connections, and, above all, web browsers with smart downloading capabilities, all served to make the command-line feature obsolete before it was ever really put in place. The pngcheck utility discussed in Chapter 5, "Applications: Image Converters" was originally written for this purpose, but it has since evolved into more of a PNG conformance tester.

Overall, PNG has done quite well. Yes, there were a few missed turns, a few mistakes, and somewhat slower acceptance than many of us had hoped. But as Tom Lane has repeatedly reminded us, JPEG didn't catch on any faster, and even GIF took quite a while outside of CompuServe. The fact that PNG is currently one of only three accepted image formats on the Web is quite an achievement. May its next four years be equally exciting!

Sun sets over GIF.
With PNG on the horizon,
Web is dark no more.

--Michael N. Randers-Pehrson

Last Update: 2010-Nov-26