Ubuntu Manpage: pnmtopng - convert a PNM image to PNG

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={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 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 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 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 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 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 argument, 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 ⟨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 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.

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

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 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 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, 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 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 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.

-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 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 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 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 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 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 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 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 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 usage (and minimum speed) to 9 for maximum memory usage (and speed).

The default is the default of the zlib library.