pnmtopng

Updated: July 2008
Table Of Contents

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"] [-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]

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.

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

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 -rgb 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 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.

-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 specification. pnmtopng does not necessarily represents the transparency information as an alpha 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 ppm_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 alpha 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=.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 frequency (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 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.

-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 represents 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 "continuation 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 character 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 starting 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 compressed.

-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).

-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).