Pnmtopng User Manual(0) Pnmtopng User Manual(0)NAMEpnmtopng - convert a PNM image to PNG
SYNOPSISpnmtopng [-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'] [-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 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
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.
OPTIONSpnmtopng 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 argu‐
ments as the option argument, whereas -rgb takes a single program argu‐
ment 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
pnmtopng, have it first try with the new syntax, and if it fails with
'unrecognized option,' fall back to the old syntax.
-verbose
Display the format of the output file.
-downscale
Enables scaling of maxvalues of more then 65535 to 16 bit.
Since
this means loss of image data, pnmtopng does not do it by
default..TP -interlace
Creates an interlaced PNG file (Adam7).
-alpha=filename
This specifies the transparency (alpha channel) of the image.
You supply the alpha channel as a standard PGM alpha mask (see
the PGM(1)specification.pnmtopngdoesnot necessarily represents
the transparency information as an alpha channel in the PNG for‐
mat. 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
ppm_parsecolor() library routine ⟨libppm.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 arbitrar‐
ily.
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 trans‐
parency. 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 alpha channel or trans‐
parent color conversions. Specify color the same as for -trans‐
parent.
-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=.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.
-hist Use this parameter to create a chunk that specifies the fre‐
quency (or histogram) of the colors in the image.
-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
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 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
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.
-text=filename
This option lets you include comments in the text chunk of the
PNG output. file is the name of a file that contains your text
comments.
Here is an example of a comment file:
Title PNG file
Author Bryan Henderson
Description how to include a text chunk
PNG file
"Creation date" 3-feb-1987
Software pnmtopng
The format of the file is as follows: The file is divided into
lines, delimited by newline characters. The last line need not
end with a newline character. A group of consecutive lines rep‐
resents a comment.
A "delimiter character" is a blank or tab or null character.
The first line representing a comment must not start with a
delimiter character. Every other line in the group is a "con‐
tinuation line" and must start with a delimiter character.
The first line representing a comment consists of a keyword and
the first line of comment text. The keyword begins in Column 1
of the file line and continues up to, but not including, the
first delimiter character, or the end of the line, whichever is
first. Exception: you can enclose the keyword in double quotes
and spaces and tabs within the double quotes are part of the
keyword. The quotes are not part of the keyword. A NUL charac‐
ter is not allowed in a keyword.
The first line of the comment text is all the text in the file
line beginning after the keyword and any delimiter characters
after it. immediately after the delimiter character that marks
the end of the keyword.
A continuation line defines a subsequent line of the comment.
The comment line is all the text on the continuation line start‐
ing with the first non-delimiter character.
There is one newline character between every two comment lines.
There is no newline character after the last line of comment
text.
There is no limit on the length of a file line or keyword or
comment text line or comment text. There is no limit on the
number of comments or size of or number of lines in the file.
-ztxt=filename
The same as -text, except pnmtopng considers the text com‐
pressed.
-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 compres‐
sion. Select a level from 0 for no compression (maximum speed)
to 9 for maximum compression (minimum speed).
-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).
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.
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.
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.
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
alpha channel will be represented as a full alpha channel even
if the information could be represented more succinctly 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
only good for human eyes.
SEE ALSOpngtopam(1), pamrgbatopng(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.
netpbm documentation July 2008 Pnmtopng User Manual(0)
