pngtopnm

Updated: 24 March 2005
Table Of Contents

NAME

pngtopnm - convert a PNG image into a PNM image

SYNOPSIS

pngtopnm [-verbose] [-alpha | -mix] [-background=color]
[-gamma=value] [-text=filename] [-time] [pngfile]

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.

pngtopnm reads a PNG image (Portable Network Graphics) as input and produces a PPM image as output. The type of the output file depends on the input file - if it's black & white, pngtopnm creates a PBM file. If it's grayscale, pngtopnm creates a PGM file. Otherwise, it creates a PPM file.

OPTIONS

-verbose

Display various information about the input PNG image and the conversion process.

If you want even more information about the PNG image, use pngcheck (not part of Netpbm).

-alpha

Output the alpha channel or transparency mask of the image. The result is either a PBM file or a PGM file, depending on whether different levels of transparency appear.

-mix

Compose the image with the transparency or alpha mask against a background. The background color is determined by the bKGD chunk in the PNG, except that you can override it with -background. If the PNG has no bKGD chunk and you don't specify -background, the background color is white.

-background=color

This option specifies the background color with which to mix the image when you specify -mix.

color is as described for the argument of the ppm_parsecolor() library routine.

Examples:

-background=rgb:01/ff/80
-background=rgbi:1/255/128

If you don't specify -background, the background color is what is specified in the PNG image, and if the PNG doesn't specify anything, white.

You cannot specify -background unless you also specify -mix. Before Netpbm 10.27 (March 2005), you could specify -background with -mix and it was just ignored. (This caused a usability problem).

-gamma= value

Converts the image to a new display-gamma value. If a gAMA chunk is present in the png-file, pngtopnm uses the specified image-gamma value. If not, pngtopnm considers the image-gamma to be 1.0. Based on the image-gamma and the display-gamma given with this option, pngtopnm adjusts the colors written to the pnm-file.

Because the gammas of uncompensated monitors are around 2.6, which results in an image-gamma of 0.45, some typical situations are: when the image-gamma is 0.45 (use -verbose to check) and the picture is too light, your system is gamma-corrected, so convert with "-gamma 1.0". When no gAMA chunk is present or the image-gamma is 1.0, use 2.2 to make the picture lighter and 0.45 to make the picture darker.

-text=file

Writes the tEXt and zTXt chunks to a file, in a format as described in the pnmtopng user manual. These chunks contain text comments or annotations.

-time

Prints the tIME chunk to stderr.

NOTE

A PNG image contains a lot of information that can't be represented in Netpbm formats. Therefore, you lose information when you convert to another format with "pngtopnm | pnmtoxxx". If there is a specialized converter that converts directly to the other format, e.g. ptot to convert from PNG to TIFF, you'll get better results using that.

LIMITATIONS

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. As with any Netpbm program, speed always takes a back seat to quick present and future development.