History of the Portable Network Graphics (PNG) Format

PNG is a new, awesome graphics format designed to take the place of GIF—here's how it came about.
Oy, My Head Hurts

But specifying a format is one thing; implementing it is quite another. Although the original intent was to create a “lightweight” format—and, compared to TIFF or even JPEG, PNG is fairly lightweight—even a completely orthogonal feature set can introduce substantial complications. For example, consider progressive display of an image in a web browser. First comes straight decoding of the compressed data—no problems. Then any line filtering must be inverted to get the actual image data. Oops, it's an interlaced image: pixels are appearing here and there within each 8x8 block, so they must be rendered appropriately and possibly buffered. The image also has transparency and is being overlaid on a background image, adding a bit more complexity. At this point we're not much worse off than we would be with an interlaced, transparent GIF; the line filters and 2D interlacing scheme are pretty straightforward extensions to what programmers have already dealt with. Even adding gamma correction to the foreground image isn't too much trouble.

However, it's not just simple transparency; we have an alpha channel. And we don't want a sparse display—we like the replicating progressive method that Netscape Navigator uses. Now things are tricky: each replicated pixel-block has some percentage of the fat foreground pixel mixed in with complementary amounts of the background pixels in the block. And just because the current fat pixel is 65% transparent (or, even worse, completely opaque) doesn't mean later ones in the same block will be, too; thus, we have to remember all of the original background pixel-values until their final foreground pixels are composited and overlaid. Toss in the ability to render all of this nicely on an 8-bit, color-mapped display, and most programmers' heads will explode.

Make It So

Of course, some of these things are application (presentation or front-end) issues, not general PNG-decoding (back-end) issues. Nevertheless, a good PNG library should allow for the possibility of such applications—which is another way of saying that it should be general enough not to place undue restrictions on any programmer who wants to implement such things.

Once Draft 9 was released, many people set about writing PNG encoders and/or decoders. The true glory is really reserved for three people, however: Info-ZIP's Jean-loup Gailly and Mark Adler (both also of gzip fame), who originally wrote Zip's deflate() and UnZip's inflate() routines and then, for PNG, rewrote them as a portable library called zlib (Reference 5), and Guy Eric Schalnat of Group 42, who almost single-handedly wrote the libpng reference implementation (originally “pnglib”) from scratch ([Reference 6). The first truly usable versions of the libraries were released two months after Draft 9, on the first of May, 1995. Although both libraries were missing some features required for full implementation, they were sufficiently complete to be used in various freeware applications. Draft 10 of the specification was released at the same time, with clarifications resulting from these first implementations.

Fast-Forward to the Present

The pace of subsequent developments slowed at that point. This was partly due to the fact that, after four months of intense development and dozens of e-mail messages every day, everyone was burned out; partly because Guy controlled libpng's development and became busy with other things at work; and partly because of the perception that PNG was basically “done.” The latter point was emphasized by a CompuServe press release to that effect in mid-June. A press release, I might add, in which their PR guys claimed much of the credit for PNG's development (sigh).

Nevertheless, progress continued. In June of 1995 I set up the PNG home page, now grown to roughly a dozen pages (Reference 7), and Kevin Mitchell officially registered the “PNGf” Macintosh file ID with Apple Computer. In August, Alexander Lehmann and Willem van Schaik released a fine pair of additions to the NetPBM image manipulation suite, particularly handy under Linux: pnmtopng and pngtopnm version 2.0. And in December at the Fourth International World Wide Web Conference, the World Wide Web Consortium (W3C) released the PNG Specification version 0.92 as an official standards-track Working Draft.

1996 saw the February release of version 0.95 as an Internet Draft by the Internet Engineering Task Force (IETF), followed in July by the Internet Engineering Steering Group's (IESG) approval of version 1.0 as an official Informational RFC. However, the IETF secretary still hasn't issued the actual RFC number at the time of this writing, six months later (sigh). The Virtual Reality Modeling Language (VRML) Architecture Group in early August adopted PNG as one of the two required image formats for minimal VRML 2.0 conformance (Reference 8). Meanwhile the W3C promoted the spec to Proposed Recommendation status in July and then to full Recommendation status on the first of October (Reference 9). Finally, in mid-October the Internet Assigned Numbers Authority (IANA) formally approved “image/png” as an official Internet media type, joining image/gif and image/jpeg as non-experimental image formats for the Web. Much of this standardization would not have happened nearly as quickly without the tireless efforts of Tom Lane and Glenn Randers-Pehrson, who took over editing duties of the spec from Thomas Boutell.

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