::png::validatefile
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::isPNGfile
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::imageInfofile
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::getTimestampfile
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::setTimestampfiletime
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::getCommentsfile
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::removeCommentsfile
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::addCommentfilekeywordtext
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::addCommentfilekeywordlangkeyword2text
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::getPixelDimensionfile
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::imagefile
Given a PNG file returns the image in the list of scanlines format used by Tk_GetColor.
::png::writefiledata
Takes a list of scanlines in the Tk_GetColor format and writes the represented image to file.
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