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Manual: pamrecolor

Pamrecolor User Manual(1) General Commands Manual Pamrecolor User Manual(1)

NAME
pamrecolor - alter colors without affecting luminance

SYNOPSIS
pamrecolor [--colorspace=name] [--rmult=fraction] [--gmult=fraction] [--bmult=fraction] [--targetcolor=color] [--col‐
orfile=file] [-randomseed=integer]

[infile]

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).

pamrecolor changes an image's colors to be as close as possible to given target colors but with the constraint that
the luminance not be modified. That is, the original image and the target image will look identical if both are con‐
verted to grayscale (e.g. with ppmtopgm(1)). You can have pamrecolor select target colors randomly, specify a single
hue for the entire image, or take the target colors from a target image.

In addition to real Netpbm images, pamrecolor works on pseudo-Netpbm images based on arbitrary color spaces. You can
define the color space explicitly or choose one of many that pamrecolor knows by name.

The output is a PAM image on standard output. Options control the exact format of the PAM. If you want a PNM (PBM,
PGM, or PPM) image, use pamtopnm(1) on the output. There is no need to convert if you will use the image as input to
a current Netpbm program, but many other programs don't know what a PAM is.

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

--colorspace=name
Designate the color space to use for determining the contribution to luminance of each of the red, green, and
blue color channels. For example, in the SMPTE-C color space an RGB color is converted to grayscale by multi‐
plying the red channel by 0.2124132, the green channel by 0.7010437, and the blue channel by 0.0865432 and
summing the resulting three products.

When you use this option, the input and output images are not true Netpbm images, because the Netpbm image
format specifies a particular color space. Instead, you are using a variation on the format in which the sam‐
ple values in the raster have different meaning. Many programs that ostensibly use Netpbm images actually use
a variation with a different color space. For example, GIMP uses sRGB internally and if you have GIMP gener‐
ate a Netpbm image file, it really generates a variation of the format that uses sRGB.

pamrecolor knows the following color spaces (name values):

adobe

Adobe RGB (1998) with a D65 reference white

apple

Apple RGB with a D65 reference white

cie

CIE with an Illuminant E reference white

ntsc

NTSC RGB with an Illuminant C reference white

pal

PAL/SECAM with a D65 reference white

smpte-c

SMPTE-C with a D65 reference white

srgb

sRGB with a D65 reference white

wide

Wide-gamut RGB with a D50 reference white

The default is "ntsc" because this is the color space that the Netpbm formats and many graphics utilities use.
As a counterexample, GIMP uses sRGB as its native color space.

The luminance values pamrecolor uses for each of the above come from Bruce Lindbloom's Computing RGB-to-XYZ
and XYZ-to-RGB matrices" (1) page.

--rmult=fraction

--gmult=fraction

--bmult=fraction
Instead of selecting a color space by name, you can specify explicitly the contribution of each color channel
to the overall luminance as red, green, and blue multipliers. These three options must be used together, and
the three fraction values must sum to 1.0. For example, you can specify the ProPhoto (ROMM) RGB color space
with "--rmult=0.2880402 --gmult=0.7118741 --bmult=0.0000857".

--targetcolor=color
Designate color as the target color for the image. pamrecolor will make each pixel as close as possible to
color subject to the constraint that the luminance must stay the same as in the original image. Specify color
as in the argument of the pnm_parsecolor() library routine (e.g., "hotpink" or "#ff69b4").

If you specify neither --targetcolor nor --colorfile, pamrecolor will randomly select a target color for each
pixel of the input image.

You may not specify both -targetcolor and -colorfile.

--colorfile=file
Take per-pixel target colors from Netpbm file file instead of using a single target color for the entire im‐
age. file should be a PPM or color PAM image. If the image in the file wider or taller than the input image,
pamrecolor uses only the left and top part of it. If the image is narrower or shorter, pamrecolor considers
the image to be repeated in a tile pattern.

If you specify neither --targetcolor nor --colorfile, pamrecolor will randomly select a target color for each
pixel of the input image.

You may not specify both -targetcolor and -colorfile.

-randomseed=integer
This is the seed for the random number generator that generates the pixels.

Use this to ensure you get the same image on separate invocations.

By default, pamrecolor uses a seed derived from the time of day and process ID, which gives you fairly uncor‐
related results in multiple invocations.

This option was new in Netpbm 10.61 (December 2012).

EXAMPLES
This command tints an image yellow:

pamrecolor --targetcolor=yellow colorpic.pam > yellowpic.pam

This command takes the colors from colorpicture.ppm and applies them to graypicture.pgm:

pamrecolor --colorfile=colorpic.ppm graypic.pgm > colorizedpic.pam

The grayscale version of colorizedpic.pam will look just like graypic.pgm. Note that if you use a non-Netpbm tool to
do the conversion to grayscale, you may additionally need to specify an appropriate --colorspace value for your con‐
version tool.

NOTES
Here are a couple of fun special effects you can produce with pamrecolor:

• Specify a color file that is identical to the input image but with some large, colored text added to it. The
text will "magically" vanish when the image is converted to grayscale.

• Provide a low-contrast grayscale image - perhaps a secret message written in similar shades of gray - as the
input file and a colorful but completely different image as the color file. If done carefully, the grayscale
image can be hidden by the colorful image. Only people who know to convert the result to grayscale can re‐
cover the original grayscale image.

• Use --targetcolor=tan to make an image look like an old-timey photograph (or, more precisely, a sepia-toned
photograph of the late 1800s).

HISTORY
Scott Pakin wrote pamrecolor in July 2010.

pamrecolor was new in Netpbm 10.52 (September 2010).

Manual: pamrecolor

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