NAME

png - PNG querying and manipulation of meta data

Table Of Contents

• Table Of Contents

• Synopsis

• Description

• COMMANDS

• Bugs, Ideas, Feedback

• Keywords

• Category

• Copyright

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

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. 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 © 2004, Code: Aaron Faupell
Copyright © 2004, Doc: Andreas Kupries