DOKK / manpages / debian 10 / netpbm / pngtopnm.1.en
pngtopnm(1) General Commands Manual pngtopnm(1)

pngtopnm - convert a Portable Network Graphics file into portable anymap

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

Reads a Portable Network Graphics as input. Produces a portable anymap as output. The type of the output file depends on the input file - if it's black & white, a pbm file is written, else if it's grayscale a pgm file, else a ppm file.

Display the format of the input file and the type of the output file. If the chunks are part of the png-file, the alpha, transparency and gamma-values will be indicated.
Output the alpha channel or transparency mask of the image. The result is either a pbm file or pgm file, depending on whether different levels of transparency appear.
Compose the image with the transparency or alpha mask against a the background. When a background chunk is available that color is taken, else black will do.
If no background color chunck is present in the png-file, or when another color is required this parameter can be used to set the background color of images. This is especially useful for alpha-channel images or those with transparency chunks. The format, to specify the color in, is either (in the case of orange) "1.0,0.5,0.0", where the values are floats between zero and one, or with the syntax "#RGB", "#RRGGBB" or "#RRRRGGGGBBBB" where R, G and B are hexa-decimal numbers.
Converts the image to a new display-gamma value. When a gAMA chunk is present in the png-file, the image-gamma value will be used. When not, the image-gamma is considered to be 1.0. Based on the image-gamma and the display-gamma given with this option the colors written to the pnm-file will be adjusted.
Because the gamma's 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.
Writes the tEXt and zTXt chunks to a file, in a format as described in the pnmtopng man-page. These chunks contain text comments or annotations.
Prints the tIME chunk to stderr.

All flags can be abbreviated to their shortest unique prefix.

pnmtopng(1), ptot(1), pnmgamma(1), pnm(5)

Instead of pngtopnm|pnmtoxxx, a specific converter should be used, if available. E.g. ptot (PNG to TIFF 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