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<h1 class="chapter">Chapter 7. History of the Portable Network Graphics Format</h1>

<div class="htmltoc"><h4 class="tochead">Contents:</h4><p>
<a href="#png.ch07.div.1">7.1. Anatomy of an Internet Working Group</a><br />
<a href="#png.ch07.div.2">7.2. Implementation</a><br />
<a href="#png.ch07.div.3">7.3. MNG</a><br />
<a href="#png.ch07.div.4">7.4. Mainstream Support and Present Status</a><br />
</p></div>







<a name="INDEX-500.50-new" />
<p><i>Internet GIF tax,<br>
January '95.<br>
PNG to the rescue!</i></p>



<?x-size -1?><p>--Glenn Randers-Pehrson<?x-size 0?><a href="#FOOTNOTE-47">[47]</a>


</p><blockquote class="footnote">

<a name="FOOTNOTE-47" /><p>[47] Alternatively, ``Unisys bombshell, / Christmas
1994. / PNG to the rescue!''</p>

</blockquote>


<p><a name="INDEX-501" />
<a name="INDEX-502" />The Portable Network Graphics image format, or PNG for short, is the
first general-purpose image format to achieve wide, cross-platform
acceptance<a href="#FOOTNOTE-48">[48]</a>
since JPEG/JFIF arrived in the early 1990s.
Almost every major feature in PNG exists in other general-purpose
formats--specifically, GIF, JPEG, and TIFF--yet in January 1995, a
group of strangers felt compelled to band together and design another
image format from scratch. To understand why, it is necessary to
delve even further into history.</p><blockquote class="footnote">


<a name="FOOTNOTE-48" /><p>[48] The choice of adjectives is intentional:
there are other widely
accepted formats, such as Windows BMPs, but they're not cross-platform,
and there are cross-platform formats such as PostScript or the
astronomical FITS format, but they're not general-purpose.</p>


</blockquote>


<p><a name="INDEX-503" />
<a name="INDEX-504" />In 1977 and 1978, Israeli researchers Jacob Ziv and Abraham Lempel
published a pair of papers on a new class of lossless data-compression
algorithms in the journal <em class="emphasis">IEEE Transactions on Information Theory</em>.
<a name="INDEX-505" />
<a name="INDEX-506" />These algorithms, now collectively referred to as ``LZ77'' and ``LZ78,''
formed the basis for an entire industry of software, hardware, and
subsequent research papers. One of the follow-up papers was by Terry
<a name="INDEX-507" />Welch and was published in the June 1984 issue of <em class="emphasis">IEEE Computer</em>.
Entitled ``A Technique for High-Performance Data Compression,'' it
described his research at Sperry into a fast, efficient implementation
of LZ78 called LZW.</p>


<p><a name="INDEX-508" />By 1987, when CompuServe's Bob Berry was busy designing the GIF image
format, LZW was well established in the Unix world in the form of the
<em class="emphasis">compress</em> command, and in the PC world in the form of SEA's ARC.


As a fast algorithm with good compression and relatively low memory
requirements, LZW was ideally suited to the PCs of the day, and it became
Berry's choice for a GIF compression method, too. In turn, GIF became
the image format of choice on the Internet, particularly on the worldwide
discussion forum known as Usenet.</p>


<p>And so things remained largely unchanged until 1994. The introduction
(from a practical standpoint) of JPEG around 1992 or 1993 may have slowed
GIF's rising star slightly, but computational requirements and the
limitations of then-current graphics cards limited JPEG's acceptance for
several years. With the advent of graphical browsers for the World Wide Web
in 1992 and 1993,


GIF's popularity only increased: simple graphics with few colors
were the norm, and those were ideally suited to GIF's palette-based
<a name="INDEX-509" />format. With the release of Netscape Navigator 1.0 in 1994, progressive
rendering of images as they downloaded suddenly became widespread,
and GIF's interlacing scheme worked in its favor once more.<a href="#FOOTNOTE-49">[49]</a>
</p><blockquote class="footnote">


<a name="FOOTNOTE-49" /><p>[49] Progressive capability had for quite some time been part of the JPEG
specification, too, but since the Independent JPEG Group's free library
didn't support the progressive mode until August 1995, neither did
any applications--including web browsers.</p>


</blockquote>


<p>Then, three days after Christmas 1994, CompuServe quietly dropped a small
bombshell on an unsuspecting world: henceforth, all GIF-supporting software
would require royalties. In fact, the announcement was apparently the
culmination of more than a year of legal wrangling with Unisys, which had
<a name="INDEX-510" />inherited the Welch LZW patent in the 1986 merger of Sperry and Burroughs,
and which had by 1993 become considerably more aggressive about enforcing
its patent in software-only applications.


</p>


<p>In any case, shortly after the holidays ended, word of the announcement
reached the Internet--specifically, the ever-volatile Usenet community.
As one might expect, the results were spectacular: within days, a full-fledged
conflagration of bluster, whining, flaming, vitriol, and general-purpose
noise had engulfed several of the Usenet newsgroups, among them
<em class="emphasis">comp.compression</em> and <em class="emphasis">comp.graphics</em>. But mixed in with the
noise was the genesis of an informal Internet working group led by Thomas
Boutell.
<a name="INDEX-511" />


Its purpose was to design not only a replacement
for the GIF format, but also a successor to it: better, smaller, more extensible,
and <em class="emphasis">free</em>.</p>


<div class="sect1"><a name="png.ch07.div.1" />
<h2 class="sect1">7.1. Anatomy of an Internet Working Group</h2>


<p><a name="INDEX-512" />What would become known as the ``PNG Group'' or ``PNG Development Group''
began as many such groups do--as a collection of participants in a
Usenet newsgroup. When the discussion became both more detailed
and considerably more verbose, it became a mailing list with an associated CompuServe
<a name="INDEX-513" />forum. Tom Boutell posted the very first PNG draft--then known as ``PBF,''
for Portable Bitmap Format--to <em class="emphasis">comp.graphics</em>,<a href="#FOOTNOTE-50">[50]</a>
<em class="emphasis">comp.compression</em>, and
<em class="emphasis">comp.infosystems.www.providers</em> on
Wednesday, 4&nbsp;January 1995. It had a 3-byte signature, chunk numbers
rather than chunk names, a maximum pixel depth of 8 bits, and no specified
compression method, but even at that stage it had more in common with today's
PNG than with any other existing format.</p><blockquote class="footnote">


<a name="FOOTNOTE-50" /><p>[50] Also known by some as the Peanut Butter Format, a.k.a. Chunky GIF.</p>


</blockquote>


<p>Within one week, most of the major features of PNG had been proposed, though
by no means yet accepted:  delta filtering for improved compression
<a name="INDEX-514" />
(Scott Elliott and
<a name="INDEX-515" />
Mark Adler),
<a name="INDEX-516" />
deflate compression (Tom Lane,
<a name="INDEX-516.01-new" />
the Info-ZIP Group and many others),
24-bit support (many folks),
<a name="INDEX-517" />
the PNG name itself (Oliver Fromme),
<a name="INDEX-517.01-new" />
internal CRCs (Greg Roelofs),
<a name="INDEX-518" />
gamma chunk (Paul Haeberli),
<a name="INDEX-519" />
and 48- and 64-bit support (Jonathan Shekter). That week also saw the
first proto-PNG mailing list set up, Tom Boutell's release of the second draft
of the specification, and Greg's posting of some test results that showed
a 10% improvement in compression if GIF's LZW method were simply replaced
with the deflate (LZ77) algorithm.
<a name="INDEX-520" />
<a name="INDEX-521" /></p>


<p>One of the real strengths of the PNG group was its ability to weigh the
pros and cons of various issues in a (mostly) rational manner, reach some
sort of consensus, and then move on to the next issue without prolonging
discussion on ``dead'' topics indefinitely. In part this was probably due
to the fact that the group was relatively small, yet possessed of a
sufficiently broad range of graphics and compression expertise that no one
felt unduly shut out when a decision went against him.<a href="#FOOTNOTE-51">[51]</a>
In part it was also due to a frequently updated ``scorecard,'' which listed
the accepted and rejected features and summarized any issues that were
still undecided.
</p><blockquote class="footnote">


<a name="FOOTNOTE-51" /><p>[51] All of the PNG authors were male. Most of them still are. No doubt
there's a dissertation in there somewhere.</p>


</blockquote>


<p>But the most important factor in the group's progress was the position of
Benevolent Dictator, held by Tom Boutell. As with the very successful Linux
development model, in which Linus Torvalds is trusted with the
final say on anything having to do with the Linux kernel, so Tom, as
the initiating force behind the PNG project, was granted this power. When
consensus was impossible, Tom would make a decision, and that would settle
the matter. On one or two rare occasions he might 
<?x-need 10?>later have been persuaded
to reverse the decision, but this generally happened only if new information
came to light.</p>


<p>In any case, the development model worked: by the beginning of February
1995, seven drafts had been produced, and the PNG format was settling
down. (The PNG name was adopted in Draft 5, after a great deal of fuss;
GIF's indeterminate pronunciation<a href="#FOOTNOTE-52">[52]</a>
was the prime motivating factor, but the allure of an unofficial
recursive acronym--PNG's Not GIF--was what decided the matter.)
The next month was mainly spent working out the details: chunk-naming
conventions, CRC size and placement, choice of filter types, palette
ordering, specific flavors of transparency and alpha-channel support,
interlace method, and so on. CompuServe was impressed enough by the
design that on February 7, 1995, they announced support for PNG as the
designated successor to GIF, supplanting what they had initially
referred to as the GIF24 development project.


By the beginning of March, PNG Draft 9 was released and the
specification was officially frozen--just over two months from its
inception. Although further drafts followed, they merely added
clarifications, some recommended behaviors for encoders and decoders,
<a name="INDEX-522" />and a tutorial or two. Indeed, Glenn Randers-Pehrson has kept some
so-called ``paleo PNGs'' that were created at the time of Draft 9;
they are still readable by any PNG decoder today.
</p><blockquote class="footnote">


<a name="FOOTNOTE-52" /><p>[52] The author of the GIF specification pronounces it with a soft G, as ``jif.''</p>


</blockquote>


<p><a href="#png.ch07.tbl.1">Table 7-1</a> is a time line listing many of the major events in PNG's history.</p>


<a name="INDEX-522.01-new" />	<!-- Greg Roelofs -->
<a name="INDEX-522.02-new" />	<!-- pnglib library -->
<a name="INDEX-522.03-new" />	<!-- zlib library -->
<a name="INDEX-522.04-new" />	<!-- libpng library -->
<a name="png.ch07.tbl.1" />
<div class="table" align="center">

<p>
<table width="502" border="0">
  <tr>
    <td>
      <b class="emphasis-bold">Table 7-1.</b>
      <i>PNG Time Line</i>
    </td>
  </tr>
</table>
</p>

<p>
<table width="502" border="1">
  <tr>
    <td>

<table width="100%" border="0">

<tr>
<th align="right"><b class="emphasis-bold">Date</b></th>
<th> &nbsp; </th>
<th align="left"><b class="emphasis-bold">Event</b></th>
</tr>

<tr>
<td align="right">4 Jan 1995</td>
<td> &nbsp; </td>
<td>PBF Draft 1 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">4 Jan 1995</td>
<td> &nbsp; </td>
<td>Delta filtering (Scott Elliott, Mark Adler)</td>
</tr>

<tr>
<td align="right">4 Jan 1995</td>
<td> &nbsp; </td>
<td>Deflate compression (Tom Lane and others)</td>
</tr>

<tr>
<td align="right">4 Jan 1995</td>
<td> &nbsp; </td>
<td>24-bit support (many)</td>
</tr>

<tr>
<td align="right">5 Jan 1995</td>
<td> &nbsp; </td>
<td>TeleGrafix LZHUF proposal</td>
</tr>

<tr>
<td align="right">6 Jan 1995</td>
<td> &nbsp; </td>
<td>PNG name (Oliver Fromme)</td>
</tr>

<tr>
<td align="right">7 Jan 1995</td>
<td> &nbsp; </td>
<td>PBF Draft 2 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">7 Jan 1995</td>
<td> &nbsp; </td>
<td>ZIF early results (Greg Roelofs)</td>
</tr>

<tr>
<td align="right">7 Jan 1995</td>
<td> &nbsp; </td>
<td>Internal CRC(s) (Greg Roelofs)</td>
</tr>

<tr>
<td align="right">8 Jan 1995</td>
<td> &nbsp; </td>
<td>Gamma chunk (Paul Haeberli)</td>
</tr>

<tr>
<td align="right">8 Jan 1995</td>
<td> &nbsp; </td>
<td>48-, 64-bit support (Jonathan Shekter)</td>
</tr>

<tr>
<td align="right">9 Jan 1995</td>
<td> &nbsp; </td>
<td>FGF proposal, implementation (Jeremy Wohl)</td>
</tr>

<tr>
<td align="right">10 Jan 1995</td>
<td> &nbsp; </td>
<td>First NGF/PBF/proto-PNG mailing list (Jeremy Wohl)</td>
</tr>

<tr>
<td align="right">15 Jan 1995</td>
<td> &nbsp; </td>
<td>PBF Draft 3 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">16 Jan 1995</td>
<td> &nbsp; </td>
<td>CompuServe announces GIF24 development (Tim Oren)</td>
</tr>

<tr>
<td align="right">16 Jan 1995</td>
<td> &nbsp; </td>
<td>Spec available on WWW (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">16 Jan 1995</td>
<td> &nbsp; </td>
<td>PBF Draft 4 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">23 Jan 1995</td>
<td> &nbsp; </td>
<td>PNG Draft 5 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">24 Jan 1995</td>
<td> &nbsp; </td>
<td>PNG Draft 6 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">26 Jan 1995</td>
<td> &nbsp; </td>
<td>Final 8-byte signature (Tom Lane)</td>
</tr>

<tr>
<td align="right">1 Feb 1995</td>
<td> &nbsp; </td>
<td>PNG Draft 7 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">2 Feb 1995</td>
<td> &nbsp; </td>
<td>Adam7 interlacing scheme (Adam Costello)</td>
</tr>

<tr>
<td align="right">7 Feb 1995</td>
<td> &nbsp; </td>
<td>CompuServe drops GIF24 in favor of PNG (Tim Oren)</td>
</tr>

<tr>
<td align="right">13 Feb 1995</td>
<td> &nbsp; </td>
<td>PNG Draft 8 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">7 Mar 1995</td>
<td> &nbsp; </td>
<td>PNG Draft 9 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">11 Mar 1995</td>
<td> &nbsp; </td>
<td>First working PNG viewer (Oliver Fromme)</td>
</tr>

<tr>
<td align="right">13 Mar 1995</td>
<td> &nbsp; </td>
<td>First valid PNG images posted (Glenn Randers-Pehrson)</td>
</tr>

<tr>
<td align="right">1 May 1995</td>
<td> &nbsp; </td>
<td>pnglib 0.6 released (Guy Eric Schalnat)</td>
</tr>

<tr>
<td align="right">1 May 1995</td>
<td> &nbsp; </td>
<td>zlib 0.9 released (Jean-loup Gailly, Mark Adler)</td>
</tr>

<tr>
<td align="right">5 May 1995</td>
<td> &nbsp; </td>
<td>PNG Draft 10 (Thomas Boutell)</td>
</tr>

<tr>
<td align="right">13 Jun 1995</td>
<td> &nbsp; </td>
<td>PNG web site (Greg Roelofs)</td>
</tr>

<tr>
<td align="right">27 Jul 1995</td>
<td> &nbsp; </td>
<td>NCSA X Mosaic 2.7b1 with native PNG support (Dan Pape)</td>
</tr>

<tr>
<td align="right">20 Sep 1995</td>
<td> &nbsp; </td>
<td>Arena 0.98b with native PNG support (Dave Beckett)</td>
</tr>

<tr>
<td align="right">8 Dec 1995</td>
<td> &nbsp; </td>
<td>PNG spec 0.92 released as W3C Working Draft</td>
</tr>

<tr>
<td align="right">23 Feb 1996</td>
<td> &nbsp; </td>
<td>PNG spec 0.95 released as IETF Internet Draft</td>
</tr>

<tr>
<td align="right">28 Mar 1996</td>
<td> &nbsp; </td>
<td>Deflate and zlib approved as Informational RFCs (IESG)</td>
</tr>

<tr>
<td align="right">22 May 1996</td>
<td> &nbsp; </td>
<td>Deflate and zlib released as Informational RFCs (IETF)</td>
</tr>

<tr>
<td align="right">17 Jun 1996</td>
<td> &nbsp; </td>
<td>libpng 0.89c released (Andreas Dilger)</td>
</tr>

<tr>
<td align="right">1 Jul 1996</td>
<td> &nbsp; </td>
<td>PNG spec 1.0 released as W3C Proposed Recommendation</td>
</tr>

<tr>
<td align="right">11 Jul 1996</td>
<td> &nbsp; </td>
<td>PNG spec 1.0 approved as Informational RFC (IESG)</td>
</tr>

<tr>
<td align="right">24 Jul 1996</td>
<td> &nbsp; </td>
<td>zlib 1.0.4 released (Jean-loup Gailly, Mark Adler)</td>
</tr>

<tr>
<td align="right">4 Aug 1996</td>
<td> &nbsp; </td>
<td>VRML 2.0 spec released with PNG as requirement (VAG)</td>
</tr>

<tr>
<td align="right">1 Oct 1996</td>
<td> &nbsp; </td>
<td>PNG spec 1.0 approved as W3C Recommendation</td>
</tr>

<tr>
<td align="right">14 Oct 1996</td>
<td> &nbsp; </td>
<td><b class="emphasis-bold">image/png</b> approved (IANA)</td>
</tr>

<tr>
<td align="right">6 Nov 1996</td>
<td> &nbsp; </td>
<td>sRGB chunk registered (PNG Development Group)</td>
</tr>

<tr>
<td align="right">9 Dec 1996</td>
<td> &nbsp; </td>
<td>sPLT chunk registered (PNG Development Group)</td>
</tr>

<tr>
<td align="right">15 Jan 1997</td>
<td> &nbsp; </td>
<td>PNG spec 1.0 released as Informational RFC 2083 (IETF)</td>
</tr>

<tr>
<td align="right">28 Jan 1997</td>
<td> &nbsp; </td>
<td>pCAL chunk registered (PNG Development Group)</td>
</tr>

<tr>
<td align="right">5 Apr 1997</td>
<td> &nbsp; </td>
<td>libpng 0.95b released (Andreas Dilger)</td>
</tr>

<tr>
<td align="right">1 Oct 1997</td>
<td> &nbsp; </td>
<td>Internet Explorer 4.0 with native PNG support (Microsoft)</td>
</tr>

<tr>
<td align="right">11 Nov 1997</td>
<td> &nbsp; </td>
<td>Navigator 4.04 with native PNG support (Netscape)</td>
</tr>

<tr>
<td align="right">28 Feb 1998</td>
<td> &nbsp; </td>
<td>MHEG-5 UK profile for digital TV released (UK DTG)</td>
</tr>

<tr>
<td align="right">9 Mar 1998</td>
<td> &nbsp; </td>
<td>libpng 1.0 released (Glenn Randers-Pehrson)</td>
</tr>

<tr>
<td align="right">9 Jul 1998</td>
<td> &nbsp; </td>
<td>zlib 1.1.3 released (Jean-loup Gailly, Mark Adler)</td>
</tr>

<tr>
<td align="right">17 Aug 1998</td>
<td> &nbsp; </td>
<td>iCCP chunk registered (PNG Development Group)</td>
</tr>

<tr>
<td align="right">23 Oct 1998</td>
<td> &nbsp; </td>
<td>PNG spec 1.1 approved (PNG Development Group)</td>
</tr>

<tr>
<td align="right">21 Dec 1998</td>
<td> &nbsp; </td>
<td>Opera 3.51 with native PNG support (Opera Software)</td>
</tr>

<tr>
<td align="right">31 Dec 1998</td>
<td> &nbsp; </td>
<td>PNG spec 1.1 released (PNG Development Group)</td>
</tr>

<tr>
<td align="right">14 Jan 1999</td>
<td> &nbsp; </td>
<td>libpng 1.0.3 released (Glenn Randers-Pehrson)</td>
</tr>

<tr>
<td align="right">9 Feb 1999</td>
<td> &nbsp; </td>
<td>iTXt chunk registered (PNG Development Group)</td>
</tr>

<tr>
<td align="right">22 Jun 1999</td>
<td> &nbsp; </td>
<td><em class="emphasis">PNG: The Definitive Guide</em> published</td>
</tr>

<?x-space 5p?>
</table>

    </td>
  </tr>
</table>
</p>
</div>



<p>Perhaps equally interesting are some of the proposed features and design
suggestions that ultimately were not accepted: the Amiga IFF format;
uncompressed bitmaps, either gzip'd or stored inside zipfiles; thumbnail
<a name="INDEX-523" />images and/or generic multi-image support; ``little-endian''<a href="#FOOTNOTE-53">[53]</a>
byte order; Unicode UTF-8 character set for text; YUV and other lossy
(nonlossless) image-encoding schemes; vector graphics; and so forth. Many
of these topics produced an amazing amount of discussion--in fact, the main
proponent of the zipfile idea was still arguing about it more than two years
later.


<a name="INDEX-525" /></p><blockquote class="footnote">


<a name="FOOTNOTE-53" /><p>[53] The name stems from a reference in <em class="emphasis">Gulliver's Travels</em> to opposing
factions of silly people, some of whom (Lilliputians) broke their
eggs at the little end before eating them and some of whom
(Blefuscudians) broke them at the big end.


<a name="INDEX-524" />The argument over PNG's byte order was almost equally silly, but in
the end (so to speak) big-endian was chosen for two reasons: it's
easier for humans to read and debug in a ``hex dump'' (a textual
rendering of a binary file), and it's the same as ``network byte
order,'' which is something of an Internet standard.</p>


</blockquote>
</div>

















</div>
<div class="sect1"><a name="png.ch07.div.2" />
<h2 class="sect1">7.2. Implementation</h2>


<p><a name="INDEX-526" />A frozen spec opens the door to implementations, and many people set about
writing PNG encoders and decoders as soon as Draft 9 appeared. The real
glory, however, is reserved for the handful of people who took it upon
themselves to write the free programming libraries supporting PNG: Jean-loup
<a name="INDEX-527" />
<a name="INDEX-528" />Gailly and Mark Adler, both of Info-ZIP and gzip fame, who rewrote the
deflate compression engine in a form suitable for general-purpose use and
<a name="INDEX-528.01-new" />
released it as <em class="emphasis">zlib</em>; and
<a name="INDEX-529" />
Guy Eric Schalnat of Group 42, who almost single-handedly wrote the
<a name="INDEX-529.01-new" />
initial version of <em class="emphasis">libpng</em> (then known as
<a name="INDEX-529.02-new" />
<em class="emphasis">pnglib</em>). The first truly usable versions of the
libraries were released
two months after Draft 9, on May 1, 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 and corrections resulting from these first implementations.</p>


<a name="INDEX-529.03-new" />
<p>The pace of development slowed at that point, at least to outward appearances.
Partly this was due to the fact that, after four straight months of intense
development and many megabytes of email, everyone was exhausted; partly it
was due to the fact that Guy controlled the development of libpng, and he
became busy with other things at work. Often overlooked is the fact that,
while writing the spec was a very focused effort and writing the reference
implementation was only slightly less so, once the library had been released
in a usable form there were literally hundreds of potential applications
pulling at developers' interests. And finally, there was the simple
perception that PNG was basically done--a point that was emphasized by
a CompuServe press release to that effect in June 1995.</p>


<p>Nevertheless, progress continued. June saw the genesis of the
PNG web site, which has now grown to more than two dozen pages,


<a name="INDEX-530" />and Kevin Mitchell officially registered the ``PNGf'' Macintosh file
<a name="INDEX-531" />
<a name="INDEX-532" />ID with Apple Computer. In August 1995, Alexander Lehmann and Willem
van Schaik released a fine pair of additions to the NetPBM
image-manipulation suite: 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.
<a name="INDEX-533" />
<a name="INDEX-534" /></p>


<p><a name="INDEX-535" />February 1996 saw the 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. (It was finally released by the IETF
as RFC 2083 


in January 1997.) In early August, the Virtual Reality Modeling Language
(VRML) Architecture Group adopted PNG as one of the two required
image formats for minimal VRML 2.0 conformance.


Meanwhile, the W3C promoted the spec to Proposed Recommendation status in
July and then to full Recommendation status on the first of October.


<a name="INDEX-536" />Finally, in mid-October 1996, 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
<a name="INDEX-537" />
<a name="INDEX-538" />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.
<a name="INDEX-539" /></p>


















</div>
<div class="sect1"><a name="png.ch07.div.3" />
<h2 class="sect1">7.3. MNG</h2>


<p><a name="INDEX-540" />
<a name="INDEX-541" />Also in 1996 came the revival of efforts to produce a multiple-image variant of
PNG suitable for slide shows, animations, and very efficient storage of
certain simple kinds of images. Multi-image support had
been left out of the PNG specification for several reasons: multi-image
capability in GIF was supported by virtually no one; multi-image GIFs were
indistinguishable from single-image GIFs (i.e., they had the same filename
extension); including multi-image support in PNG would have delayed both
its development and its acceptance in the marketplace, due to the burden of
extra complexity, and creating a separate, PNG-based multi-image format
not only would be a logical extension of PNG but also would be more
appropriate to a group with backgrounds in animation and multimedia. As
it happened, however, this latter group never materialized, and with the
<a name="INDEX-542" />early-1996 release of Netscape Navigator 2.0 with support for GIF
animations,<a href="#FOOTNOTE-54">[54]</a>
it became clear that the PNG Group needed to produce some sort of response.</p><blockquote class="footnote">


<a name="FOOTNOTE-54" /><p>[54] Alas, Netscape's support of GIF animations probably did more to ensure
the format's longevity than any other event in GIF's history.</p>


</blockquote>


<p>Unfortunately there was a fairly fundamental disagreement within the group
over whether the new format should be a very thin layer on top of PNG,
capable of duplicating GIF animations but not much more, or whether it should
be a full-fledged multimedia format capable of synchronizing images, sound, and
possibly video. Although the former would have been trivial (and fast) to
design and implement, proponents of the latter design held sway during the
early discussions in the summer of 1996. In the end, however, something
of a compromise was created--though possibly due more to attrition than 
consensus. Called Multiple-image Network Graphics, the MNG format design was largely
<a name="INDEX-543" />shaped by Glenn Randers-Pehrson and included simple but general operations
to manipulate sections of images, but no direct sound or video support. As
of November 1998 the MNG specification 
was close to being frozen, but was also quite large and still awaiting
implementation in the form of a reference library similar to libpng. Until
such time as either a reference library or some other form of complete
implementation exists, the MNG spec will not be approved as a standard,
nor is it likely that more than a handful of third-party developers
will offer support for it.


<a name="INDEX-544" /></p>


















</div>
<div class="sect1"><a name="png.ch07.div.4" />
<h2 class="sect1">7.4. Mainstream Support and Present Status</h2>


<p>
<a name="INDEX-545" />
<a name="INDEX-545.01-new" />	<!-- libpng library -->
If 1996 was the year of PNG's standardization, 1997 was the year of PNG
applications. After having taken over libpng development from Guy Eric Schalnat
in June 1996, Andreas Dilger shepherded it through versions 0.89 to 0.96,
adding numerous features and finding and fixing bugs; application
developers seemed not to mind the library's ``beta'' version number, and
increasingly employed it in their mainstream apps. With native support
<a name="INDEX-546" />
<a name="INDEX-547" />
<a name="INDEX-548" />
<a name="INDEX-549" />
<a name="INDEX-550" />
<a name="INDEX-551" />
in popular programs such as Adobe's Photoshop and Illustrator, Macromedia's
Freehand, JASC's Paint Shop Pro, Ulead's PhotoImpact, and Microsoft's
Office 97 suite, PNG's star was clearly rising. But perhaps the crowning
moment came in the autumn, with fresh versions of the Big Two web browsers.
Microsoft's Internet Explorer 4.0 in October and Netscape's Navigator 4.04
in November both included native, albeit somewhat limited, PNG support.
At last, the widespread use of PNG on the Web came within the realm of
possibility.</p>


<p>The theme for 1998 seems to have been <em class="emphasis">maturity</em>. Having been
handed the reins of principal libpng development at the beginning of
<a name="INDEX-552" />the year, Glenn Randers-Pehrson fixed many bugs, finished the
documentation and generally polished libpng into a stable release
worthy of a ``1.0'' version number by early March--three years to
the day, in fact, after the PNG specification was frozen. In February,
<a name="INDEX-553" />
<a name="INDEX-554" />
<a name="INDEX-555" />the UK Digital Television Group released the MHEG-5 UK Profile for
next-generation teletext on digital terrestrial television; the
profile included PNG as one of its bitmap formats, and as a result,
manufacturers such as Philips, Sony, Pace and Nokia were expected to
<a name="INDEX-556" />be shipping digital televisions and set-top boxes with built-in PNG
support by the time this book reaches print. At the very end of March 1998,
<a name="INDEX-557" />
<a name="INDEX-558" />
<a name="INDEX-559" />Netscape released Mozilla, the pre-alpha source code to Communicator
5.0, which allowed interested third parties (like the PNG Group) to
tinker with the popular browser and make it work as intended. In
October, the PNG Group approved some important additions and
clarifications to one of the more difficult technical aspects of the
PNG spec, namely, gamma and color correction; these changes defined
the PNG 1.1 specification--the first official revision in three and
a half years. And at roughly the same time, a joint committee of the
International Organization for Standardization (ISO) and the
<a name="INDEX-560" />
<a name="INDEX-561" />
<a name="INDEX-562" />International Electrotechnical Commission (IEC) began the yearlong
process to make Portable Network Graphics an official international
standard (to be known as ISO/IEC 15948 upon approval).
</p>


<p>But a history bereft of darker events is perhaps not so interesting...and,
sadly enough, for a brief period in April 1998, it appeared that things
might once again be percolating on the legal front. Specifically, there
<a name="INDEX-563" />
<a name="INDEX-563.01-new" />
were rumors that Stac, Inc., believed the deflate compression engine in zlib
(which is used by libpng) infringed on two of their patents. Careful
reading of the patents in question, United States patents 4,701,745 and
5,016,009, suggests that although it is possible to write an infringing
deflate engine, the one actually used in zlib does not do so.<a href="#FOOTNOTE-55">[55]</a>
Moreover, as this is written, a full year has passed with no public
claims from Stac, no further private contacts, and no confirmation of
the original rumors. However, until this is tested in court or Stac
makes a public announcement clearing zlib of suspicion, at least a
small cloud will remain over the Portable Network Graphics
format as a whole. The irony should be evident to one and all.







<a name="INDEX-564" />
<a name="INDEX-565" />
<a name="INDEX-566" />
</p><blockquote class="footnote">


<a name="FOOTNOTE-55" /><p>[55] It should go without saying--but lawyers like it to be said anyway--that
this is not official legal advice. Consult a patent attorney to be (more)
certain. But note that deflate is also being standardized into open Internet
protocols such as PPP.</p>


</blockquote>

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