Provided by: tcllib_1.21+dfsg-1_all 
NAME
png - PNG querying and manipulation of meta data
SYNOPSIS
package require Tcl 8.2
package require crc32
package require png ?0.3?
::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
::png::getPixelDimension file
::png::image file
::png::write file data
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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, compression, 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 (international)
comment consists of 4 elements: the human readable keyword, 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.
::png::getPixelDimension file
Returns a dictionary with keys ppux, ppuy and unit if the information is present.
Otherwise, it returns the empty string.
The values of ppux and ppuy return the pixel per unit value in X or Y direction.
The allowed values for key unit are meter and unknown. In the case of meter, the
dpi value can be calculated by multiplying with the conversion factor 0.0254.
::png::image file
Given a PNG file returns the image in the list of scanlines format used by
Tk_GetColor.
::png::write file data
Takes a list of scanlines in the Tk_GetColor format and writes the represented
image to file.
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 Trackers
[http://core.tcl.tk/tcllib/reportlist]. Please also report any ideas for enhancements you
may have for either package and/or documentation.
When proposing code changes, please provide unified diffs, i.e the output of diff -u.
Note further that attachments are strongly preferred over inlined patches. Attachments can
be made by going to the Edit form of the ticket immediately after its creation, and then
using the left-most button in the secondary navigation bar.
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>