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PNG Extensions list and the need for an 'inFO' chunk

From: Adrian Custer <acuster@gmail.com>
Date: Tue, 12 Jun 2012 12:51:53 -0400
Message-ID: <1339519913.18694.1.camel@debdown>
To: png-group@w3.org
Hello everyone,

Thanks for the great work on the PNG spec! 


I am working as part of the Open Geospatial Consortium (OGC) on a web
service standard which exchanges PNG datastreams which are cartographic
maps (or tiles thereof). As part of that work, I am having to establish
how a PNG decoder determines the spatial referencing of a PNG image,
both the initial orientation (where I rely on the orientation of the
'reference image') and the various geospatial referencing elements. For
the latter, I claim that PNG provides *no* information but this forces
me to dismiss the 'pHYs' chunk of the standard as descriptive and also,
possibly, to address currently registered extensions such as 'oFFs'.
Unfortunately, I cannot find an authoritative list of registered
extensions. 

This email therefore is asking you for the canonical reference to the
registered extensions. Also, I ask for initial comments for a new chunk
type 'inFO' which would act much like the iTXt chunk except that it
would not be 'safe-to-copy'. I would like such a chunk to define the
four elements needed for  


My first question is where one can find the authoritative list of the
'chunks' which are currently registered in the registry given in section
4.9 of the standard: 
  http://www.w3.org/TR/PNG/#4Concepts.Registration
The closest source I have found is the document:
  Register of public PNG chunks and keywords, Version 1.4.5
  ftp://ftp.simplesystems.org/pub/png/documents/pngreg-1.4.5-pdg.html
Is that the formal list?


A second question regards the need for a 'chunk' for textual metadata
which is not 'safe-to-copy'. 

If I understand the standard correctly, I conceive of the need for an
'inFO' chunk that would act like the newest chunk for text (iTXt) but
would explicitly *not be* safe-to-copy. My lack of certainty about the
need for this element comes from being unsure as to whether the
modifications allowed for 'safe-to-copy' chunks include cropping the
image which would invalidate the information I hope to store in the
'inFO' chunk. 

For geospatial referencing, I would aim to define several keywords for
the spatial referencing concepts needed: 
      * the 'graphic CRS' which establishes a coordinate reference
        system for the graphic by describing the anchoring of a
        coordinate system (usually 'i' increasing to the right, 'j'
        increasing downwards) to the 'reference image' where the
        critical semantic element is whether the origin of the
        coordinate system is at the center or the outer corner of the
        top left array element,
      * the 'spatial CRS' which gives the name of, and possibly
        parameters for, a geographic projection, and
      * a transform, usually an affine, between the coordinate system of
        the graphic CRS and the coordinate system of the projection, so
        the image could be placed, say, in central Africa.
Ideally, the 'inFO' chunk and the three keywords would allow us to
define a GeoPNG extension standard in which a PNG could have simple
georeferencing parameters attached. (GeoTIFF is a similar extension for
TIFF files.)

Has the working group every tackled a proposal for an information chunk
like 'inFO' which needed this 'unsafe-to-copy' attribute? Would any of
you have any comments on the merits of my idea?


Thanks for any pointers or comments. Hoping this list has not long gone
dead,

~Adrian Custer
Received on Tuesday, 12 June 2012 19:47:58 GMT

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