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Pngtopam User Manual(0)                                Pngtopam User Manual(0)



NAME
       pngtopam - convert a PNG image into a Netpbm image


SYNOPSIS
       pngtopam  [-verbose]  [-alphapam  |  -alpha | -mix] [-background=color] [-gamma=value] [-text=filename] [-time]
       [pngfile]

       Minimum unique abbreviation of option is acceptable.  You may use double hyphens instead of  single  hyphen  to
       denote options.  You may use white space in place of the equals sign to separate an option name from its value.


DESCRIPTION
       This program is part of Netpbm(1).

       pngtopam reads a PNG image (Portable Network Graphics) as input and produces a Netpbm  image  as  output.   The
       type  of  the  output  file depends on the input file - if it's black & white, pngtopam creates a PBM file.  If
       it's grayscale, pngtopam creates a PGM file.  Otherwise, it creates a PPM file.  Except that with the -alphapam
       option,  it  always  creates  a  PAM  file.  That file has tuple type GRAYSCALE_ALPHA or RGB_ALPHA depending on
       whether the input has color or not.


OPTIONS
       -verbose
              Display various information about the input PNG image and the conversion process.

              If you want even more information about the PNG image, use pngcheck (not part of Netpbm).


       -alphapam
              Produce a single output image containing the main image (foreground) and the  alpha  channel  or  trans-
              parency  mask.   This  image is in the PAM format with tuple type of either GRAYSCALE_ALPHA (which has a
              depth of 2 channels) or RGB_ALPHA (which has a depth of 4 channels).

              You can specify only one of -alphapam, -alpha, and -mix.  With none of them, pngtopam produces an  image
              of the foreground of the input image and discards transparency information.

              This option was new in Netpbm 10.44 (September 2008).


       -alpha Output  the  alpha  channel  or transparency mask of the image. The result is either a PBM file or a PGM
              file, depending on whether different levels of transparency appear.

              pngtopam discards the main image (the foreground).

              You can specify only one of -alphapam, -alpha, and -mix.  With none of them, pngtopam produces an  image
              of the foreground of the input image and discards transparency information.


       -mix   Compose  the  image  with  the transparency or alpha mask against a background.  The background color is
              determined by the bKGD chunk in the PNG, except that you can override it with -background.  If  the  PNG
              has no bKGD chunk and you don't specify -background, the background color is white.

              You  can specify only one of -alphapam, -alpha, and -mix.  With none of them, pngtopam produces an image
              of the foreground of the input image and discards transparency information.


       -background=color
              This option specifies the background color with which to mix the image when you specify -mix.

              color is as described for the argument of the ppm_parsecolor() library routine .

              Examples:



       ?      -background=rgb:01/ff/80

       ?      -background=rgbi:1/255/128


              If you don't specify -background, the background color is what is specified in the PNG image, and if the
              PNG doesn't specify anything, white.

              You  cannot  specify  -background  unless  you also specify -mix.  Before Netpbm 10.27 (March 2005), you
              could specify -background without -mix and it was just ignored.  (This caused a usability problem).



       -gamma=value
              Converts the image to a new display-gamma value.  If a gAMA chunk is present in the  png-file,  pngtopam
              uses  the  specified image-gamma value.  If not, pngtopam considers the image-gamma to be 1.0.  Based on
              the image-gamma and the display-gamma given with this option, pngtopam adjusts the colors written to the
              pnm-file.

              Because  the  gammas  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.

              One oddity to be aware of when using -gamma on an image with transparency: The PNG image specifies  that
              a  certain  color  is transparent, i.e. every pixel in the image of that color is transparent.  But png-
              topam interprets this as applying to the gamma-corrected space, and there may be less precision in  that
              space than in the original, which means multiple uncorrected colors map to the same corrected color.  So
              imagine that the image contains 3 shades of white and specifies that one of them is transparent.   After
              gamma  correction,  those  three shades are indistinguishable, so pngtopam considers pixels of all three
              shades to be transparent.

              If this is not what you want, don't use gamma.  Instead, use pnmgamma on the output.



       -text=file
              Writes the tEXt and zTXt chunks to a file, in a format as described in the pnmtopng user manual.   These
              chunks contain text comments or annotations.


       -time  Prints the tIME chunk to stderr.




SEE ALSO
       pnmtopng(1), pnmtopng(1), ptot, pnmgamma(1), pnm(1)

       For information on the PNG format, see http://schaik.com/png .


NOTE
       A  PNG  image  contains  a lot of information that can't be represented in Netpbm formats.  Therefore, you lose
       information when you convert to another format with "pngtopam | pnmtoxxx".  If there is a specialized converter
       that  converts  directly  to the other format, e.g. ptot to convert from PNG to TIFF, you'll get better results
       using that.


LIMITATIONS
       There could be an option to include PNG comment chunks in the output image as PNM comments instead  of  putting
       them in a separate file.

       The  program  could  be  much  faster, with a bit of code optimizing.  As with any Netpbm program, speed always
       takes a back seat to quick present and future development.


HISTORY
       pngtopam was new in Netpbm 10.44, as a replacement for pngtopnm.  The main improvement over pngtopnm  was  that
       it  could generate a PAM image with a transparency channel, whereas with pngtopnm, you would have to extrac the
       transparency channel as a separate file, in a separate run.

       pngtopnm was new in Netpbm 8.1 (March 2000), the first big change to the package in Netpbm's  renaissance.   It
       and  pnmtopng  were  simply copied from the  pnmtopng package (1) by Greg Roelofs.  Those were based on simpler
       reference  applications  by   by   Alexander   Lehmann   <alexAThal.de>   and   Willem   van   Schaik
       <willemATschaik.com> and distributed with their PNG library.

       Nearly  all  of  the code has changed since it was copied from the pnmtopng package, most of it just to improve
       maintainability.



AUTHORS
       Copyright (C) 1995-1997 by Alexander Lehmann and Willem van Schaik.



netpbm documentation             22 July 2008          Pngtopam User Manual(0)