Pngtopam User Manual(0) Pngtopam User Manual(0)NAMEpngtopam - convert a PNG image into a Netpbm image
SYNOPSISpngtopam [-verbose] [-alphapam | -alpha | -mix] [-background=color]
[-gamma=value] [-text=filename] [-time] [pngfile]
Minimum unique abbreviation of option is acceptable. You may use dou‐
ble 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 (fore‐
ground) and the alpha channel or transparency 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 -back‐
ground. 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 ⟨libppm.html#colorname⟩ .
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 pngtopam 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 ALSOpnmtopng(1), pnmtopng(1), ptot, pnmgamma(1), pnm(1)
For information on the PNG format, see http://schaik.com/png
⟨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 con‐
vert 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.
HISTORYpngtopam 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
<alex@hal.rhein-main.de> and Willem van Schaik <willem@schaik.com> and
distributed with their PNG library.
Nearly all of the code has changed since it was copied from the pnm‐
topng 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)
