NAME¶

pngtopam - convert a PNG image into a Netpbm image

SYNOPSIS¶

pngtopam [-verbose] [-alphapam | -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(1).

pngtopam reads a PNG image (Portable Network Graphics) as input and produces a Netpbm image as output. The type of the output file depends on the input file - if it's black & white, pngtopam creates a PBM file. If it's grayscale, pngtopam creates a PGM file. Otherwise, it creates a PPM file. Except that with the -alphapam option, it always creates a PAM file. That file has tuple type GRAYSCALE_ALPHA or RGB_ALPHA depending on whether the input has color or not.

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).

-alphapam

Produce a single output image containing the main image (foreground) and the alpha channel or transparency mask. This image is in the PAM format with tuple type of either GRAYSCALE_ALPHA (which has a depth of 2 channels) or RGB_ALPHA (which has a depth of 4 channels).

You can specify only one of -alphapam, -alpha, and -mix. With none of them, pngtopam produces an image of the foreground of the input image and discards transparency information.

This option was new in Netpbm 10.44 (September 2008).

-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.

pngtopam discards the main image (the foreground).

You can specify only one of -alphapam, -alpha, and -mix. With none of them, pngtopam produces an image of the foreground of the input image and discards transparency information.

-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.

You can specify only one of -alphapam, -alpha, and -mix. With none of them, pngtopam produces an image of the foreground of the input image and discards transparency information.

-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 without -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, pngtopam uses the specified image-gamma value. If not, pngtopam considers the image-gamma to be 1.0. Based on the image-gamma and the display-gamma given with this option, pngtopam 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.

One oddity to be aware of when using -gamma on an image with transparency: The PNG image specifies that a certain color is transparent, i.e. every pixel in the image of that color is transparent. But pngtopam interprets this as applying to the gamma-corrected space, and there may be less precision in that space than in the original, which means multiple uncorrected colors map to the same corrected color. So imagine that the image contains 3 shades of white and specifies that one of them is transparent. After gamma correction, those three shades are indistinguishable, so pngtopam considers pixels of all three shades to be transparent.

If this is not what you want, don't use gamma. Instead, use pnmgamma on the output.

-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.

SEE ALSO¶

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

For information on the PNG format, see http://schaik.com/png .

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 "pngtopam | 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 include PNG comment chunks in the output image as PNM comments instead of putting them in 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.

HISTORY¶

pngtopam was new in Netpbm 10.44, as a replacement for pngtopnm. The main improvement over pngtopnm was that it could generate a PAM image with a transparency channel, whereas with pngtopnm, you would have to extrac the transparency channel as a separate file, in a separate run.

pngtopnm was new in Netpbm 8.1 (March 2000), the first big change to the package in Netpbm's renaissance. It and pnmtopng were simply copied from the pnmtopng package (1) by Greg Roelofs. Those were based on simpler reference applications by by Alexander Lehmann <alex@hal.rhein-main.de> and Willem van Schaik <willem@schaik.com> and distributed with their PNG library.

Nearly all of the code has changed since it was copied from the pnmtopng package, most of it just to improve maintainability.

AUTHORS¶

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