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, 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.

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 http://sourceforge.net/tracker/?group_id=12883. Please also report any ideas for enhancements you may have for either package and/or documentation.

KEYWORDS

png, image, comment, timestamp