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Manual: pnmtopng

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] [-transparent=[=]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] [-modtime='[yy]yy-mm-dd
hh:mm:ss'] [-nofilter] [-sub] [-up] [-avg] [-paeth] [-compression=n] [-comp_mem_level=n] [-comp_strategy={huff‐
man_only|filtered}] [-comp_method=deflated] [-comp_window_bits=n] [-comp_buffer_size=n] [-force] [-libversion] [pnm‐
file]

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 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).

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 col‐
ors 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 hap‐
pens 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 minus one. If you can't do that, then convert it with pamdepth to
something with a large maxval that is a power of two minus one (255 and 65535 are good choices) to minimize the er‐
ror.

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 hyphens 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 arguments as the option argu‐
ment, whereas -size takes a single program argument which is composed of multiple words. E.g. the old shell command

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 "unrecognized 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 ), pn‐
mtopng 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 continues with additional informa‐
tion 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 im‐
age 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.

-alpha=filename
This specifies the transparency (alpha) channel of the image. You supply the transparency channel as a stan‐
dard PGM transparency mask (see the PGM(1) specification. pnmtopng does not necessarily represents the trans‐
parency information as a transparency channel in the PNG format. If it can represent the transparency infor‐
mation 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 image.

Specify the color (color) as described for the argument of the pnm_parsecolor() library routine . 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 equidistant, 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 transparency. 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 trans‐
parency 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, pnmtopng 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 palette, 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 im‐
age 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 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 hard‐
ware 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 frequency (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 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 af‐
ter 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 num‐
ber 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 dis‐
tinction 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 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.

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 resolu‐
tion 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 example, -size="10000 15000 1" means that when someone displays the image, he should make it so
that 10,000 pixels horizontally occupy 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 hori‐
zontal 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 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

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 per‐
mitted 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 spec‐
ify one permitted row filter type. The default, when you specify no filter options, was the same.

-compression=n
This option sets set the compression level of the zlib compression. 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 us‐
age (and minimum speed) to 9 for maximum memory usage (and speed).

The default is the default of the zlib library.