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Re: Errata for PNG spec (tRNS chunk)

From: Chris Lilley <chris@w3.org>
Date: Fri, 26 Jun 2009 02:05:27 +0200
Message-ID: <1724072479.20090626020527@w3.org>
To: Glenn Randers-Pehrson <glennrp@gmail.com>
CC: "PNG/MNG discussion list" <png-mng-misc@lists.sourceforge.net>, <www-archive@w3.org>, <daduce@brookes.ac.uk>
On Thursday, June 25, 2009, 1:20:51 AM, Glenn wrote:

GRP> Chris,

GRP> In clause (tRNS chunk), the table entry for colour type 2 has
GRP> the green and blue samples in the wrong order.

GRP> It should read:

GRP>      Colour type 2

GRP>      Red sample value      2 bytes
GRP>      Green sample value  2 bytes
GRP>      Blue sample value     2 bytes


Thanks for the report.

The W3C process for errata management is described at

In accordance with that document I have updated the errata document
(this is the first reported erratum).

Since the original PNG specification had the correct order, and since
the order Red, Blue, Green is used nowhere else in the specification,
it looks as if implementations have silently corrected for the obvious
typo. Thus, this erratum does not alter conformance and I have
classified it accordingly.

Is anyone aware of any implementation which followed the letter of PNG
second edition here (and would thus become nonconformant)?

Please review the proposed erratum, even though the error is simple
and obvious.

I wonder if a simple test case, say a PNG image with two colors, one
of which is transparent if the erratum (or PNG first edition) is
followed and the other of which is transparent if PNG second edition
had been followed, would be a useful addition?

Since this specification was published as both a W3C Recommendation
and as an ISO Standard, I have copied David Duce since the proposed
correction (despite being trivial) needs to be reported to ISO. I
assume that they will issue a Technical Corrigendum, but welcome
guidance from David here.

 Chris Lilley                    mailto:chris@w3.org
 Technical Director, Interaction Domain
 W3C Graphics Activity Lead
 Co-Chair, W3C Hypertext CG
Received on Friday, 26 June 2009 00:05:40 UTC

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