pngtopnm – convert a Portable Network Graphics file into portable
pngtopnm [-verbose] [-alpha | -mix] [-background color] [-gamma value] [-text file] [-time] [pngfile]
Reads a Portable Network Graphics as input. Produces a portable anymap
as output. The type of the output file depends on the input file – if
it’s black & white, a pbm file is written, else if it’s grayscale a pgm
file, else a ppm file.
Display the format of the input file and the type of the output
file. If the chunks are part of the png-file, the alpha, trans‐
parency and gamma-values will be indicated.
-alpha Output the alpha channel or transparency mask of the image. The
result is either a pbm file or pgm file, depending on whether
different levels of transparency appear.
-mix Compose the image with the transparency or alpha mask against a
the background. When a background chunk is available that color
is taken, else black will do.
If no background color chunck is present in the png-file, or
when another color is required this parameter can be used to set
the background color of images. This is especially useful for
alpha-channel images or those with transparency chunks. The for‐
mat, to specify the color in, is either (in the case of orange)
“1.0,0.5,0.0”, where the values are floats between zero and one,
or with the syntax “#RGB”, “#RRGGBB” or “#RRRRGGGGBBBB” where R,
G and B are hexa-decimal numbers.
Converts the image to a new display-gamma value. When a gAMA
chunk is present in the png-file, the image-gamma value will be
used. When not, the image-gamma is considered to be 1.0. Based
on the image-gamma and the display-gamma given with this option
the colors written to the pnm-file will be adjusted.
Because the gamma’s of uncompensated monitors are around 2.6,
which results in an image-gamma of 0.45, some typical situations
are: when the image-gamma is 0.45 (use -verbose to check) and
the picture is too light, your system is gamma-corrected, so
convert with “-gamma 1.0”. When no gAMA chunk is present or the
image-gamma is 1.0, use 2.2 to make the picture lighter and 0.45
to make the picture darker.
Writes the tEXt and zTXt chunks to a file, in a format as
described in the pnmtopng man-page. These chunks contain text
comments or annotations.
-time Prints the tIME chunk to stderr.
All flags can be abbreviated to their shortest unique prefix.
Instead of pngtopnm|pnmtoxxx, a specific converter should be used, if
available. E.g. ptot (PNG to TIFF conversion), etc.
There could be an option to read the comment text from pnm comments
instead of a separate file.
The program could be much faster, with a bit of code optimizing.
Copyright (C) 1995-1997 by Alexander Lehmann
and Willem van Schaik.
6 January 1997 pngtopnm