Ubuntu Manpage: pamtopng - convert a Netpbm image to PNG

NAME

       pamtopng - convert a Netpbm image to PNG

SYNOPSIS

       pamtopng  [-verbose]  [-transparent=color] [-background=color] [-gamma=value] [-chroma='wx
       wy
         rx ry gx gy bx by'] [-srgbintent=intent] [-time=[yy]yy-mm-dd
         hh:mm:ss] [-text=file] [-ztxt=file] [-itxt=file] [-interlace] [pnmfile]

OPTION USAGE

       Minimum unique abbreviation of option is acceptable.  You may use double  hyphens  instead
       of a 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).

pamtopng reads a Netpbm image as input and produces a PNG image as output.

Color component values in PNG files are either 8 or 16 bits wide, so where necessary
pamtopng scales colors to have a maxval of 255 or 65535. In that case, it will add an
sBIT chunk to indicated the original bit-depth.

pamtopng works only on images with maxval 1, 3, 15, 255, or 65535. You can use pamdepth
to convert an image with some other maxval to one of these.

pamtopng produces a color PNG from a color PAM, even if the only colors in the image are
shades of gray. To create a graycale PNG, from such an image (which might be slightly
smaller), you can use other Netpbm programs to convert the input to grayscale.

Alternative: pnmtopng
Netpbm contains another program for generating PNG images: pnmtopng. pnmtopng is a much
older program - it is in fact the first program in the world that could generate a PNG.
pnmtopng is a complex, feature-laden program. It lets you control various arcane aspects
of the conversion and create PNGs with various arcane features. It does various
transformations on the image to create the greatest compression possible, to a degree that
probably doesn't make any difference in the modern world.

The main advantage pamtopng has over pnmtopng is that the former can use the transparency
channel of a PAM image to generate the transparency information in the PNG. In contrast,
handling of the alpha channel is very cumbersome with pnmtopng.

One difference that does not exist, that some people might incorrectly infer from the
names, is the possible input formats. Both programs can take PBM, PGM, PPM, and PAM
input.

Because pnmtopng has been around virtually forever, programs and procedures that use it
are more portable than those that use pamtopng. Its age and popularity also probably make
it have fewer bugs.

pamtopng does not have any way to do what the following do in pnmtopng:

• -palette

• -history

• -filter

• -size

• -paeth

• -hist

• -nofilter

• -sub

• -up

• -avg

• -force

• -libversion

• -compression

• -comp_xxx

These are some of the other functions of pnmtopng that pamtopng lacks:

• When you specify a transparent or background color that is not in the image,
pnmtopng can optionally choose the closest one that is in the image. pamtopng
always uses the exact color you specify.

Features that exist in both programs are controlled by largely the same command syntax.
But there are these differences:

• pnmtopng's -rgb option is -chroma in pamtopng. -chroma is a better name, and in
fact was the name that pnmtopng used originally, but we had to change it when we
had to change the syntax of the option value to conform to the rest of Netpbm.

• pnmtopng's -modtime option is -time in pamtopng. The origin of -modtime is
analogous to that of -rgb.

OPTIONS

In addition to the options common to all programs based on libnetpbm (most notably -quiet,
see
Common Options ⟨index.html#commonoptions⟩ ), pamtopng recognizes the following command
line options:

-transparent=color
pamtopng marks the specified color as transparent in the PNG image -- Every pixel
of this color is fully transparent. This causes pamtopng to include a tRNS chunk
in the image identifying that color.

Specify the color (color) as described for the argument of the pnm_parsecolor()
library routine ⟨libnetpbm_image.html#colorname⟩ . E.g. red or rgb:ff/00/0d.

-background=color
This causes pamtopng to create a background color chunk in the PNG output which can
be used for subsequent transparency channel or transparent color conversions.
Specify color the same as for -transparent.

-gamma=value
This causes pamtopng to create a gAMA chunk. This information helps describe how
the color values in the PNG must be interpreted. Without the gAMA chunk, whatever
interprets the PNG must get this information separately (or just assume something
standard). If your input is a true PPM or PGM image, you should specify
-gamma=.45. But sometimes people generate images which are ostensibly PPM except
the image uses a different gamma transfer function than the one specified for PPM.
A common case of this is when the image is created by simple hardware that doesn't
have digital computational ability. Also, some simple programs that generate
images from scratch do it with a gamma transfer in which the gamma value is 1.0.

-chroma=chroma_list
This option specifies how red, green, and blue component values of a pixel specify
a particular color, by telling the chromaticities of those 3 primary illuminants
and of white (i.e. full strength of all three).

The chroma_list value is a blank-separated list of 8 floating point decimal
numbers: the CIE-1931 X and Y chromaticities (in that order) of each of white, red,
green, and blue, in that order.

This information goes into the PNG's cHRM chunk.

In a shell command, make sure you use quotation marks so that the blanks in
chroma_list don't make the shell see multiple command arguments.

-srgbintent=intent
This asserts that the input is a pseudo-Netpbm image that uses an sRGB color space
(unlike true Netpbm) and indicates how you intend for the colors to be rendered.
It causes pamtopng to include an sRGB chunk in the PNG image that specifies that
intent, so see the PNG documentation for more information on what this really
means.

intent is one of:

• perceptual

• relativecolorimetric

• saturation

• absolutecolorimetric

-text=filename
This option lets you include arbitrary text strings in the PNG output, as tEXt
chunks.

filename is the name of a file that contains your text strings.

The output contains a distinct tEXt chunk for each entry in the file.

Here is an example of a text string file:

Title PNG file
Author John Doe
Description how to include a text chunk
PNG file
"Creation Date" 2015-may-11
Software pamtopng

The file is divided into entries, each entry comprising consecutive lines of text.
The first line of an entry starts in the first column (i.e. the first column is not
white space) and every other line has white space in the first column. The first
entry starts in the first line, so it is not valid for the first line of the file
to have white space in its first column.

The first word in an entry is the key of the text string (e.g. 'Title'). It begins
in column one of the line and continues up to, but not including, the first
delimiter character or the end of the line, whichever is first. You can enclose
the key in double quotes in which case the key can consists of multiple words. The
quotes are not part of the key. The text string per se begins after the key and
any delimiter characters after it, plus the text in subsequent continuation lines.

There is no limit on the length of a file line or entry or key or text string.
There is no limit on the number of entries.

-ztxt=filename
The same as -text, except the text string is compressed in the PNG output.
pamtopng uses zTXt chunks instead of a tEXt chunks.

-itxt=filename
Similar to -text, but the text strings can be in a language other than English.
The PNG image indicates what language that is and includes the text string key both
in English and that language. pamtopng uses iTXt chunks instead of tEXt chunks.

For each record, you must specify the language and give the key both in English and
in the text string language.

Example:

Language nl-NL Taal nl-NL
Title nl-NL Titel PNG file
Author nl-NL Auteur Pietje Puk
Description nl-NL Omschrijving Tekst in het Nederlands.

The language specification is based on the ISO 639-1 standard, see
http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes for the valid codes. The
format is either a two character "nl" or an extended code like "en-US".

-time='[yy]yy-mm-dd hh:mm:ss'
This option allows you to specify the modification time value to be placed in the
PNG output. You can specify the year parameter either as a two or four digit
value.

-interlace
This causes the PNG file to be interlaced, in Adam7 format. The interlaced format
is one in which the raster data starts with a low-resolution representation of the
entire image, then continues with additional information for the entire image, then
even more information, etc. In Adam7 in particular, there are seven such passes of
the whole image. This is useful when you are receiving the image over a slow
communication line as someone is waiting to see it. The simplest thing to do in
that case is wait for the entire image to arrive and then display it instantly, but
then the user is wasting time staring at a blank space until the whole image
arrives. With the standard non-interlaced format, the data arrives row-by-row
starting at the top, so the displayer could display each row of the image as it
arrives and gradually paint down to the bottom. But with an interlaced image, the
displayer can start by showing a low-resolution version of the image, then
gradually improve the display as more data arrives.

When you specify this option, pamtopng must hold the entire image in memory at
once, whereas without it, the program holds only one raster row at a time. If you
don't have enough memory for that, you might suffer extreme slowdowns or failure -
not just in the process running pamtopng, but potentially throughout the system
that shares memory with it. pnmtopng does not have this limitation (it holds only
one row at a time in memory even when generating an interlaced PNG).

This option was new in Netpbm 10.86 (March 2019).

-verbose
This causes the program to display various facts about the conversion.

HISTORY

pamtopng was new in Netpbm 10.70 (June 2015).

Before pamtopng, the two ways to create PNG images with Netpbm were pnmtopng and
pamrgbatopng. The history of the former is discussed above. The latter was added to
Netpbm in 2005 as a cheap way to fill a significant need that pnmtopng did not: the
ability to turn the alpha channel in a PAM image into the alpha channel in a PNG image.

Handling of the alpha channel with pnmtopng is very cumbersome (as was dealing with alpha
channels in general before the introduction of the PAM format). pamrgbatopng could do
what people wanted with the alpha channel, but nothing else. It was a very small program
with literally no command line options.

The goal in those days was eventually to expand pnmtopng to do the PAM alpha channel
thing, rename it to pamtopng, and retire pamrgbatopng. But pnmtopng is such a complex
program, because of its dizzying array of features and its need for backward
compatibility, that adding that one capability to it was a daunting task and for ten years
nobody attempted it.

In 2015, one of the authors of the original pnmtopng (from before it was even part of
Netpbm -- a program that shared essentially no lines of code with pnmtopng of 2015)
decided to go in a different direction. While many features of pnmtopng were pretty
important and easy to implement, many others were probably of no use in the modern world
or at least not important enough to justify the complexity they lent to the code. (The
features thought to be outdated were ones that were intended to make the PNG output
slightly smaller - something considerably less important with the declining cost of
computer resources).

And there was an opportunity to drop those features: We could use the new name 'pamtopng'
for a new program, keep the existing program under the name 'pnmtopng', and avoid most
backward compatibility trouble.

Therefore, Willem van Schaik wrote an intermediate level program that had all the most
important features of pnmtopng, plus the alpha channel handling of pamrgbatopng, with
nice, simple code. That was pamtopng.

Because pamrgbatopng had no options, pamtopng was backward compatible with it without even
trying. Therefore, as soon as we added pamtopng to Netpbm, we removed pamrgbatopng and
recommended that pamrgbatopng be installed as an alias for pamtopng.

AUTHOR

       Copyright (C) 1995-1997 by Alexander Lehmann and Willem van Schaik.  Copyright (C) 2015 by
       Willem van Schaik.

DOCUMENT SOURCE

       This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The  master
       documentation is at

              http://netpbm.sourceforge.net/doc/pamtopng.html