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Pngtopam User Manual
pngtopam
Updated: 22 July 2008
Table Of Contents
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.
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 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 -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 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 ALSO
pnmtopng,
pnmtopng,
ptot,
pnmgamma,
pnm
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 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
pnmtopng package, most of it just to improve maintainability.
AUTHORS
Copyright (C) 1995-1997 by Alexander Lehmann and Willem van Schaik.
Table Of Contents
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