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png(3tcl)		      Image manipulation		     png(3tcl)

______________________________________________________________________________

NAME
       png - PNG querying and manipulation of meta data

SYNOPSIS
       package require Tcl  8.2

       package require crc32

       package require png  ?0.1.2?

       ::png::validate file

       ::png::isPNG file

       ::png::imageInfo file

       ::png::getTimestamp file

       ::png::setTimestamp file time

       ::png::getComments file

       ::png::removeComments file

       ::png::addComment file keyword text

       ::png::addComment file keyword lang keyword2 text

_________________________________________________________________

DESCRIPTION
       This  package  provides	commands  to  query and modify PNG images. PNG
       stands  for   Portable	Network	  Graphics   and   is	specified   at
       http://www.libpng.org/pub/png/spec/1.2.

COMMANDS
       ::png::validate file
	      Returns a value indicating if file is a valid PNG file. The file
	      is checked for PNG signature, each chunks checksum is  verified,
	      existence	 of  a	data chunk is verified, first chunk is checked
	      for header, last chunk is checked for ending. Things not checked
	      for  are:	 validity  of  values  within a chunk, multiple header
	      chunks, noncontiguous data chunks, end chunk before actual  eof.
	      This procedure can take lots of time.

	      Possible return values:

	      OK     File is a valid PNG file.

	      SIG    no/broken PNG signature.

	      BADLEN corrupt chunk length.

	      EOF    premature end of file.

	      NOHDR  missing header chunk.

	      CKSUM  crc mismatch.

	      NODATA missing data chunk(s).

	      NOEND  missing end marker.

       ::png::isPNG file
	      Returns  a boolean value indicating if the file file starts with
	      a PNG signature. This is a much faster and less intensive	 check
	      than  ::png::validate  as	 it  does not check if the PNG data is
	      valid.

       ::png::imageInfo file
	      Returns a dictionary with keys width, height, depth, color, com‐
	      pression,	 filter,  and interlace. The values are the associated
	      properties of the PNG image in file.  Throws an error if file is
	      not  a  PNG  image, or if the checksum of the header is invalid.
	      For information on interpreting  the  values  for	 the  returned
	      properties    see	   http://www.libpng.org/pub/png/spec/1.2/PNG-
	      Chunks.html.

       ::png::getTimestamp file
	      Returns the epoch time if a timestamp chunk is found in the  PNG
	      image contained in the file, otherwise returns the empty string.
	      Does not attempt to verify the checksum of the timestamp	chunk.
	      Throws an error if the file is not a valid PNG image.

       ::png::setTimestamp file time
	      Writes  a	 new  timestamp	 to the file, either replacing the old
	      timestamp, or adding one just before the data  chunks  if	 there
	      was no previous timestamp. time is the new time in the gmt epoch
	      format.  Throws an error if file is not a valid PNG image.

       ::png::getComments file
	      Currently supports only uncompressed comments. Does not  attempt
	      to  verify  the  checksums of the comment chunks. Returns a list
	      where each element is a comment. Each comment is itself a	 list.
	      The  list	 for  a plain text comment consists of 2 elements: the
	      human readable keyword, and the text data. A  unicode  (interna‐
	      tional)  comment consists of 4 elements: the human readable key‐
	      word, the language identifier, the translated keyword,  and  the
	      unicode  text  data.  Throws an error if file is not a valid PNG
	      image.

       ::png::removeComments file
	      Removes all comments from the PNG image in file. Beware  -  This
	      uses  memory  equal to the file size minus comments, to hold the
	      intermediate result.  Throws an error if file is not a valid PNG
	      image.

       ::png::addComment file keyword text
	      Adds  a plain text comment to the PNG image in file, just before
	      the first data chunk. Will throw an error if no  data  chunk  is
	      found. keyword has to be less than 80 characters long to conform
	      to the PNG specification.

       ::png::addComment file keyword lang keyword2 text
	      Adds a unicode (international) comment to the PNG image in file,
	      just before the first data chunk. Will throw an error if no data
	      chunk is found. keyword has to be less than 80  characters  long
	      to  conform to the PNG specification. keyword2 is the translated
	      keyword, in the language specified by  the  language  identifier
	      lang.

BUGS, IDEAS, FEEDBACK
       This  document,	and the package it describes, will undoubtedly contain
       bugs and other problems.	 Please report such in the category png of the
       Tcllib  SF  Trackers  [http://sourceforge.net/tracker/?group_id=12883].
       Please also report any ideas for enhancements you may have  for	either
       package and/or documentation.

KEYWORDS
       comment, image, png, timestamp

CATEGORY
       File formats

COPYRIGHT
       Copyright (c) 2004, Code: Aaron Faupell <afaupell@users.sourceforge.net>
       Copyright (c) 2004, Doc:	 Andreas Kupries <andreas_kupries@users.sourceforge.net>

png				     0.1.2			     png(3tcl)
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