Pnmtopng User Manual(1) General Commands Manual Pnmtopng User Manual(1)
NAME
pnmtopng - convert a PNM image to PNG
SYNOPSIS
pnmtopng [-verbose] [-downscale] [-interlace] [-alpha=file] [-transpar-
ent=[=]color] [-background=color] [-palette=palettefile] [-gamma=value]
[-hist] [-text=file] [-ztxt=file] [-rgb='wx wy
rx ry gx gy bx by'] [-size='x y unit'] [-srgbintent=intent] [-mod-
time='[yy]yy-mm-dd
hh:mm:ss'] [-nofilter] [-sub] [-up] [-avg] [-paeth] [-compression=n]
[-comp_mem_level=n] [-comp_strategy={huffman_only|filtered}]
[-comp_method=deflated] [-comp_window_bits=n] [-comp_buffer_size=n]
[-force] [-libversion] [pnmfile]
OPTION USAGE
Obsolete options:
[-filter n]
Options available only in older versions:
[-chroma wx wy rx ry gx gy bx by] [-phys x y unit] [-time [yy]yy-mm-dd
hh:mm:ss]
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).
pnmtopng reads a PNM image as input and produces a PNG image as output.
Color component values in PNG files are either eight or sixteen bits
wide, so pnmtopng will automatically scale colors to have a maxval of
255 or 65535.
For a grayscale image, pnmtopng produces a PNG bit depth 1, 2, 4, 8 or
16. When the input image has a small maxval, the output PNG image has
a correspondingly small bit depth. But in mapping the PNM maxval to
the PNG maxval (which is by definition the maximum value that can be
represented in the number of bits), a fair amount of distortion happens
with these low maxvals. For example, with a PNM maxval of 5 and a PNG
maxval of 7, the input sample 2 becomes the output sample 3. The input
brightness is 2/5 = .40, while the output brightness is 3/7 = .43.
Note that this is not a problem if you view the maxval as a precision,
because in .4 and .43 are identical within the precision implied by
maxval 5. Indeed, if you convert this PNG back to a maxval 5 PGM, the
pixel's value will again be 2, exactly as it was originally. But if
you need precisely the same colors in the output PNG as in the input
PNM, make sure your input PNM has a maxval which is a power of two mi-
nus one. If you can't do that, then convert it with pamdepth to some-
thing with a large maxval that is a power of two minus one (255 and
65535 are good choices) to minimize the error.
OPTIONS
Note: Option Syntax of Older Versions
pnmtopng changed in Netpbm 10.30 (October 2005) to use the standard
Netpbm command line syntax. Before that, you could not use double hy-
phens to denote an option and could not use an equal sign to separate
an option name from its value. And the options had to come before the
non-option program arguments.
Furthermore, the options -chroma, -phys, and -time were replaced by
-rgb, -size, and -modtime, respectively. The only difference, taking
-phys/-size as an example, is that -phys takes multiple program argu-
ments as the option argument, whereas -size takes a single program ar-
gument which is composed of multiple words. E.g. The old shell com-
mand
pnmtopng -phys 800 800 0 input.pnm > output.png
is equivalent to the new shell command
pnmtopng -size "800 800 0" input.pnm > output.png
If you're writing a program that needs to work with both new and old ,
have it first try with the new syntax, and if it fails with "unrecog-
nized option," fall back to the old syntax.
Current Options
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see
Common Options ⟨index.html#commonoptions⟩ ), pnmtopng recognizes the
following command line options:
-verbose
This causes pnmtopng to display information about the format of
the output file.
-downscale
This enables pnmtopng to scale maxvalues of more then 65535 to
16 bits. Since this means loss of image data, pnmtopng does not
do it by default.
-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 contin-
ues 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 re-
ceiving 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 in-
stantly, but then the user is wasting time staring at a blank
space until the whole image arrives. With the standard non-in-
terlaced 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 in-
terlaced image, the displayer can start by showing a low-resolu-
tion version of the image, then gradually improve the display as
more data arrives.
-alpha=filename
This specifies the transparency (alpha) channel of the image.
You supply the transparency channel as a standard PGM transpar-
ency mask (see the PGM(1) specification. pnmtopng does not nec-
essarily represents the transparency information as a transpar-
ency channel in the PNG format. If it can represent the trans-
parency information through a palette, it will do so in order to
make a smaller PNG file. pnmtopng even sorts the palette so it
can omit the opaque colors from the transparency part of the
palette and save space for the palette.
-transparent=color
pnmtopng marks the specified color as transparent in the PNG im-
age.
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.
If the color you specify is not present in the image, pnmtopng
selects instead the color in the image that is closest to the
one you specify. Closeness is measured as a Cartesian distance
between colors in RGB space. If multiple colors are equidis-
tant, pnmtopng chooses one of them arbitrarily.
However, if you prefix your color specification with "=", e.g.
-transparent =red
only the exact color you specify will be transparent. If that
color does not appear in the image, there will be no transpar-
ency. pnmtopng issues an information message when this is the
case.
-background=color
Causes pnmtopng 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.
-palette=palettefile
This option specifies a palette to use in the PNG. It forces
pnmtopng to create the paletted (colormapped) variety of PNG --
if that isn't possible, pnmtopng fails. If the palette you
specify doesn't contain exactly the colors in the image, pnm-
topng fails. Since pnmtopng will automatically generate a
paletted PNG, with a correct palette, when appropriate, the only
reason you would specify the -palette option is if you care in
what order the colors appear in the palette. The PNG palette
has colors in the same order as the palette you specify.
You specify the palette by naming a PPM file that has one pixel
for each color in the palette.
Alternatively, consider the case that have a palette and you
want to make sure your PNG contains only colors from the pal-
ette, approximating if necessary. You don't care what indexes
the PNG uses internally for the colors (i.e. the order of the
PNG palette). In this case, you don't need -palette. Pass the
Netpbm input image and your palette PPM through pnmremap.
Though you might think it would, using -palette in this case
wouldn't even save pnmtopng any work.
-gamma=value
Causes pnmtopng 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=.52. But sometimes people generate images which are os-
tensibly 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.
-hist Use this parameter to create a chunk that specifies the fre-
quency (or histogram) of the colors in the image.
-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 consecu-
tive lines of text. The first line of an entry starts in the
first column (i.e. the first column is not white space) and ev-
ery 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. pnmtopng uses zTXt chunks instead of a tEXt chunks,
unless the key for the text string starts with 'A' or 'T'. This
odd exception exists for backward compatibility; we don't know
why the program was originally designed this way, except that
the distinction was meant to roughly identify the keys 'Author'
and 'Title'.
-rgb=chroma_list
This option specifies how red, green, and blue component values
of a pixel specify a particular color, by telling the chromatic-
ities 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 or-
der.
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 com-
mand arguments.
This option was new in Netpbm 10.30 (October 2005). Before
that, the option -chroma does the same thing, but with slightly
different syntax.
-size="x y unit"
This option determines the aspect ratio of the individual pixels
of your image as well as the physical resolution of it.
unit is either 0 or 1. When it is 1, the option specifies the
physical resolution of the image in pixels per meter. For exam-
ple, -size="10000 15000 1" means that when someone displays the
image, he should make it so that 10,000 pixels horizontally oc-
cupy 1 meter and 15,000 pixels vertically occupy one meter. And
even if he doesn't take this advice on the overall size of the
displayed image, he should at least make it so that each pixel
displays as 1.5 times as high as wide.
When unit is 0, that means there is no advice on the absolute
physical resolution; just on the ratio of horizontal to vertical
physical resolution.
This information goes into the PNG's pHYS chunk.
When you don't specify -size, pnmtopng creates the image with no
pHYS chunk, which means square pixels of no absolute resolution.
This option was new in Netpbm 10.30 (October 2005). Before
that, the option -phys does the same thing, but with slightly
different syntax.
-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 pnmtopng to in-
clude 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
This option was new in Netpbm 10.71 (June 2015). Before that,
pnmtopng never generates an sRGB chunk.
-modtime="[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 digit or four digit value.
This option was new in Netpbm 10.30 (October 2005). Before
that, the option -time does the same thing, but with slightly
different syntax.
-filter=n
This option is obsolete. Before Netpbm 10.22 (April 2004), this
was the only way to specify a row filter. It specifies a single
type of row filter, by number, that pnmtopng must use on each
row.
Use -nofilter, -sub, -up, -avg, and -paeth in current Netpbm.
-nofilter
-sub
-up
-avg
-paeth Each of these options permits pnmtopng to use one type of row
filter. pnmtopng chooses whichever of the permitted filters it
finds to be optimal. If you specify none of these options, it
is the same as specifying all of them -- pnmtopng uses any row
filter type it finds optimal.
These options were new with Netpbm 10.22 (April 2004). Before
that, you could use the -filter option to specify one permitted
row filter type. The default, when you specify no filter op-
tions, was the same.
-compression=n
This option sets set the compression level of the zlib compres-
sion. Select a level from 0 for no compression (maximum speed)
to 9 for maximum compression (minimum speed).
The default is the default of the zlib library.
-comp_mem_level=n
This option sets the memory usage level of the zlib compression.
Select a level from 1 for minimum memory usage (and minimum
speed) to 9 for maximum memory usage (and speed).
The default is the default of the zlib library.
This option was new in Netpbm 10.30 (October 2005).
-comp_strategy={huffman_only|filtered}
This options sets the compression strategy of the zlib compres-
sion. See Zlib documentation for information on what these
strategies are.
The default is the default of the zlib library.
This option was new in Netpbm 10.30 (October 2005).
-comp_method=deflated
This option does nothing. It is here for mathematical complete-
ness and for possible forward compatibility. It theoretically
selects the compression method of the zlib compression, but the
Z library knows only one method today, so there's nothing to
choose.
The default is the default of the zlib library.
This option was new in Netpbm 10.30 (October 2005).
-comp_window_bits=N
This option tells how big a window the zlib compression algo-
rithm uses. The value is the base 2 logarithm of the window
size in bytes, so 8 means 256 bytes. The value must be from 8
to 15 (i.e. 256 bytes to 32K).
See Zlib documentation for details on what this window size is.
The default is the default of the zlib library.
This option was new in Netpbm 10.30 (October 2005).
-comp_buffer_size=N
This option determines in what size pieces pnmtopng does the
zlib compression. One compressed piece goes in each IDAT chunk
in the PNG. So the bigger this value, the fewer IDAT chunks
your PNG will have. Theoretically, this makes the PNG smaller
because 1) you have less per-IDAT-chunk overhead, and 2) the
compression algorithm has more data to work with. But in real-
ity, the difference will probably not be noticeable above about
8K, which is the default.
The value n is the size of the compressed piece (i.e. the com-
pression buffer) in bytes.
This option was new in Netpbm 10.30 (October 2005).
-force When you specify this, pnmtopng limits its optimizations. The
resulting PNG output is as similar to the Netpbm input as possi-
ble. For example, the PNG output will not be paletted and the
transparency channel will be represented as a full transparency
channel even if the information could be represented more suc-
cinctly with a transparency chunk.
-libversion
This option causes pnmtopng to display version information about
itself and the libraries it uses, in addition to all its normal
function. Do not confuse this with the Netpbm common option
-version, which causes the program to display version informa-
tion about the Netpbm library and do nothing else.
You can't really use this option in a program that invokes pnm-
topng and needs to know which version it is. Its function has
changed too much over the history of pnmtopng. The option is
good only for human eyes.
SEE ALSO
pngtopam(1), pamtopng(1), pnmremap(1), pnmgamma(1), pnm(1)
For information on the PNG format, see http://schaik.com/png
⟨http://schaik.com/png⟩ .
AUTHOR
Copyright (C) 1995-1997 by Alexander Lehmann and 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/pnmtopng.html
netpbm documentation 13 March 2019 Pnmtopng User Manual(1)
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