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Pamtopng User Manual(1) General Commands Manual Pamtopng User Manual(1)


pamtopng - convert a Netpbm image to PNG


pamtopng [-verbose] [-transparent=color] [-background=color] [-gamma=value] [-chroma='wx wy
rx ry gx gy bx by'] [-srgbintent=intent] [-time=[yy]yy-mm-dd
hh:mm:ss] [-text=file] [-ztxt=file] [-itxt=file] [-interlace] [pnmfile]


Minimum unique abbreviation of option is acceptable. You may use double hyphens instead of a single hyphen to denote options. You may use white space in place of the equals sign to separate an option name from its value.


This program is part of Netpbm(1).

pamtopng reads a Netpbm image as input and produces a PNG image as output.

Color component values in PNG files are either 8 or 16 bits wide, so where necessary pamtopng scales colors to have a maxval of 255 or 65535. In that case, it will add an sBIT chunk to indicated the original bit-depth.

pamtopng works only on images with maxval 1, 3, 15, 255, or 65535. You can use pamdepth to convert an image with some other maxval to one of these.

pamtopng produces a color PNG from a color PAM, even if the only colors in the image are shades of gray. To create a graycale PNG, from such an image (which might be slightly smaller), you can use other Netpbm programs to convert the input to grayscale.

Alternative: pnmtopng

Netpbm contains another program for generating PNG images: pnmtopng. pnmtopng is a much older program - it is in fact the first program in the world that could generate a PNG. pnmtopng is a complex, feature-laden program. It lets you control various arcane aspects of the conversion and create PNGs with various arcane features. It does various transformations on the image to create the greatest compression possible, to a degree that probably doesn't make any difference in the modern world.

The main advantage pamtopng has over pnmtopng is that the former can use the transparency channel of a PAM image to generate the transparency information in the PNG. In contrast, handling of the alpha channel is very cumbersome with pnmtopng.

One difference that does not exist, that some people might incorrectly infer from the names, is the possible input formats. Both programs can take PBM, PGM, PPM, and PAM input.

Because pnmtopng has been around virtually forever, programs and procedures that use it are more portable than those that use pamtopng. Its age and popularity also probably make it have fewer bugs.

pamtopng does not have any way to do what the following do in pnmtopng:

  • -palette
  • -history
  • -filter
  • -size
  • -paeth
  • -hist
  • -nofilter
  • -sub
  • -up
  • -avg
  • -force
  • -libversion
  • -compression
  • -comp_xxx

These are some of the other functions of pnmtopng that pamtopng lacks:

When you specify a transparent or background color that is not in the image, pnmtopng can optionally choose the closest one that is in the image. pamtopng always uses the exact color you specify.

Features that exist in both programs are controlled by largely the same command syntax. But there are these differences:

  • pnmtopng's -rgb option is -chroma in pamtopng. -chroma is a better name, and in fact was the name that pnmtopng used originally, but we had to change it when we had to change the syntax of the option value to conform to the rest of Netpbm.

  • pnmtopng's -modtime option is -time in pamtopng. The origin of -modtime is analogous to that of -rgb.


In addition to the options common to all programs based on libnetpbm (most notably -quiet, see
Common Options
), pamtopng recognizes the following command line options:

pamtopng marks the specified color as transparent in the PNG image -- Every pixel of this color is fully transparent. This causes pamtopng to include a tRNS chunk in the image identifying that color.

Specify the color (color) as described for the argument of the pnm_parsecolor() library routine . E.g. red or rgb:ff/00/0d.

This causes pamtopng 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.


This causes pamtopng 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.


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 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 pamtopng 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 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.

The same as -text, except the text string is compressed in the PNG output. pamtopng uses zTXt chunks instead of a tEXt chunks.


Similar to -text, but the text strings can be in a language other than English. The PNG image indicates what language that is and includes the text string key both in English and that language. pamtopng uses iTXt chunks instead of tEXt chunks.

For each record, you must specify the language and give the key both in English and in the text string language.


	Language        nl-NL  Taal             nl-NL

Title nl-NL Titel PNG file
Author nl-NL Auteur Pietje Puk
Description nl-NL Omschrijving Tekst in het Nederlands.

The language specification is based on the ISO 639-1 standard, see for the valid codes. The format is either a two character "nl" or an extended code like "en-US".

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 or four digit value.


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.

When you specify this option, pamtopng must hold the entire image in memory at once, whereas without it, the program holds only one raster row at a time. If you don't have enough memory for that, you might suffer extreme slowdowns or failure - not just in the process running pamtopng, but potentially throughout the system that shares memory with it. pnmtopng does not have this limitation (it holds only one row at a time in memory even when generating an interlaced PNG).

This option was new in Netpbm 10.86 (March 2019).

This causes the program to display various facts about the conversion.



pngtopam(1), pnmtopng(1), pam(1), pnm(1)

For information on the PNG format, see , , and .


pamtopng was new in Netpbm 10.70 (June 2015).

Before pamtopng, the two ways to create PNG images with Netpbm were pnmtopng and pamrgbatopng. The history of the former is discussed above. The latter was added to Netpbm in 2005 as a cheap way to fill a significant need that pnmtopng did not: the ability to turn the alpha channel in a PAM image into the alpha channel in a PNG image.

Handling of the alpha channel with pnmtopng is very cumbersome (as was dealing with alpha channels in general before the introduction of the PAM format). pamrgbatopng could do what people wanted with the alpha channel, but nothing else. It was a very small program with literally no command line options.

The goal in those days was eventually to expand pnmtopng to do the PAM alpha channel thing, rename it to pamtopng, and retire pamrgbatopng. But pnmtopng is such a complex program, because of its dizzying array of features and its need for backward compatibility, that adding that one capability to it was a daunting task and for ten years nobody attempted it.

In 2015, one of the authors of the original pnmtopng (from before it was even part of Netpbm -- a program that shared essentially no lines of code with pnmtopng of 2015) decided to go in a different direction. While many features of pnmtopng were pretty important and easy to implement, many others were probably of no use in the modern world or at least not important enough to justify the complexity they lent to the code. (The features thought to be outdated were ones that were intended to make the PNG output slightly smaller - something considerably less important with the declining cost of computer resources).

And there was an opportunity to drop those features: We could use the new name 'pamtopng' for a new program, keep the existing program under the name 'pnmtopng', and avoid most backward compatibility trouble.

Therefore, Willem van Schaik wrote an intermediate level program that had all the most important features of pnmtopng, plus the alpha channel handling of pamrgbatopng, with nice, simple code. That was pamtopng.

Because pamrgbatopng had no options, pamtopng was backward compatible with it without even trying. Therefore, as soon as we added pamtopng to Netpbm, we removed pamrgbatopng and recommended that pamrgbatopng be installed as an alias for pamtopng.


Copyright (C) 1995-1997 by Alexander Lehmann and Willem van Schaik. Copyright (C) 2015 by Willem van Schaik.


This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation is at
13 March 2019 netpbm documentation