pnmtopng Man page

pnmtopng General Commands Manual pnmtopng


pnmtopng – convert a portable anymap into a Portable Network Graphics


pnmtopng [-verbose] [-downscale] [-interlace] [-alpha file] [-transparent [=]color] [-background color] [-gamma value] [-hist] [-chroma wx wy rx ry gx gy bx by] [-phys x y unit] [-text file] [-ztxt file] [-time [yy]yy-mm-dd hh:mm:ss] [-filter type] [-compression level] [-force] [pnmfile]


Reads a portable pixmap as input. Produces a Portable Network Graphics
file as output.

Color 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. Grayscale files will be produced with bit depths 1, 2, 4, 8 or
16. An extra pnmdepth step is not necessary.


Display the format of the output file.

Enables scaling of maxvalues of more then 65535 to 16 bit. Since
this means loss of image data, the step is not performed by

Creates an interlaced PNG file (Adam7).

-alpha file
The alpha channel of pixel (or image) specifies the transparency
of a pixel. To create this fourth pixel value a separate .pbm-
or .pgm-file is needed. In this file black (0) stands for fully
transparant and white (1) will become opaque. The sizes of both
pbm/pgm/ppm-files must be the same. If the information con‐
tained in the alpha mask can also be represented as a trans‐
parency index, it will be used, since this should result in a
smaller image file.

-transparent color
ppmtogif marks the specified color as transparent in the PNG

You specify the color as in ppmmake.E.g. red or
rgb:ff/00/0d. If the color you specify is not present in the
image, pnmtopnm 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, pnmtopnm chooses one of them arbitrar‐

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
To create a background color chunck in the png-file, which can
be used for subsequent alpha-channel or transparent-color con‐
versions. See -transparent for format of color.

-gamma value
Creates an gAMA chunk. By providing the gamma-value of the pnm-
file the software that lateron will display the png-file will be
able to do the necessary gamma-corrections. A good rule-of-thumb
is that when the file is created by a software program (like a
CAD-program or a ray-tracer) the value is probably 1.0. When the
pnm-file looks good on a non-gamma corrected PC display (which
has itself a gamma-value of 2.2 – 2.8), a value of 0.45 should
be given.

-hist Use this parameter to create a chunk that specifies the fre‐
quency (or histogram) of the colors in the image.

-chroma white point X and Y, red X and Y, green X and Y, and blue X and
To specify the white point and rgb values following the CIE-1931

-phys x y unit
When your image should not be displayed with square but with
rectangular pixels this option should be used to create a pHYS
chunk. When the unit-value is 0 the x and y only gives the ratio
of pixel width and height. When it is 1 the x and y specify the
number of pixels per meter.

-text file
Allows to include comments in the text-chunk of the png-file.
The format of the text-file is as follows: when the first column
does not contain a blank or a tab, the first word is considered
to be the keyword. For keywords to contain spaces, enclose them
in double-quotes.
When the first character on a line is a blank or tab, the rest
of the line is a new line of the current comment. Note that the
initial spaces are not considered to be part of the comment

Here is an example:
Title PNG-file
Author your name
Description how to include a text-chunk
into a PNG file
“Creation date” 3-feb-1987
Software pnmtopng

-ztxt file
The same as -text, but now the text will be compressed.

-time yy-mm-dd hh:mm:ss or -time yyyy-mm-dd hh:mm:ss
This option allows you to specify the (modification)time. The
year parameter can be given as a two- or a four-digit value.

-filter type
When the types of filters must be restricted you can specify
here which filter you want to use. Allowed values are: 0 (none),
1 (sub), 2 (up), 3 (avg) and 4 (paeth).

-compression level
To explicitly set the compression level of zlib use this parame‐
ter. Select a level between 0 for no compression (max speed) and
9 for maximum compression.

-force When set, -force limits the optimizations of pnmtopng. A png-
file similar to the pnm-input is as much as possible enforced.
For example no paletted files will be created and alpha-channel
images will not be converted to images with a transparency

All flags can be abbreviated to their shortest unique prefix.


pngtopnm, gif2png(1), pnmgamma, pnm(5)

Instead of xxxtopnm|pnmtopng, a specific converter should be used, if
available. E.g. gif2png (GIF conversion), etc.


There could be an option to read the comment text from pnm comments
instead of a separate file.

The program could be much faster, with a bit of code optimizing.

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

6 January 1997 pnmtopng

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