Ubuntu Manpage: ppmhist - print a histogram of the colors in a PPM image

NAME

       ppmhist - print a histogram of the colors in a PPM image

SYNOPSIS

       ppmhist [-hexcolor | -float | -colorname | -map] [-nomap] [-noheader] [-sort={frequency,rgb}] [-forensic]
       [ppmfile]

DESCRIPTION

This program is part of Netpbm(1).

ppmhist reads a PPM image as input and generates a histogram of the colors in the image, i.e. a list of
all the colors and how many pixels of each color are in the image.

Output Format
The output is in one of two basic formats: a report for humans and a PPM image for use by programs. The
PPM image is actually quite readable by humans too.

Human Report

You get this format by specifying (or defaulting to) the -nomap option.

The format is one line for each color in the input image.

By default, there are two lines of column header and a summary at the top. Use the -noheader option to
suppress those lines.

The summary tells you whether black or white are present and how many shades of gray and color are
present. The summary was new in Netpbm 10.82 (March 2018).

In each line, ppmhist identifies the color by red, green, and blue components. By default, it lists each
of these in decimal, using the exact values that are in the PPM input. So if the image has a maxval of
255, the numbers in the listing range from 0 to 255. With the -hexcolor option, you can change these
numbers to hexadecimal. With the -float option, the numbers are fractional, adjusted to a maxval of 1.

Each line lists the luminosity of the color. It is in decimal on the same scale as the rgb values (see
above).

Each line lists the number of pixels in the image that have the color. This is in decimal.

PPM Output

You get this format with the -map option.

The output file is a genuine PPM image, but it is PPM Plain format and contains comments so that it is
not a lot different from the human report described above.

As a PPM image, it can be useful as input to other programs that need some kind of palette. The image is
a single row with one column for each distinct color in the image.

The function of PPM output is essentially the same as the output of pnmcolormap all. ppmhist is much
older than pnmcolormap.

OPTIONS

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

-sort={frequency,rgb}
The -sort option determines the order in which the colors are listed in the output. rgb means to
sort them first by the intensity of the red component of the color, then of the green, then of the
blue, with the least intense first. frequency means to list them in order of how many pixels in
the input image have the color, with the most represented colors first. Among colors with the
same frequency, the order is the same as with rgb.

The default is frequency.

Before Netpbm 10.88 (September 2019), with -sort=frequency, the order of colors that have the same
frequency is arbitrary.

-hexcolor
Print the color components in hexadecimal. See output format .

You may not specify this option along with -float or map.

-float Print the color components and the luminosity as floating point numbers in the range [0,1]. See
output format .

You may not specify this option along with -hexcolor or map.

This option was added in Netpbm 10.19 (November 2003).

-map Generates a PPM file of the colormap for the image, with the color histogram as comments. See
output format .

You may not specify this option along with -float or hexcolor.

-nomap Generates the histogram for human reading. This is the default.

-colorname
Add the color name to the output. This is the name from the system color dictionary . If the
exact color is not in the color dictionary, it is an asterisk followed by the name of the closest
color that is in the dictionary. If you don't have a system color dictionary, the program fails.

This option was added in Netpbm 10.10 (October 2002).

-noheader
Do not print the column headings.

-forensic

With this option, ppmhist works on images that contain invalid sample values. Normally, like most
Netpbm programs, ppmhist fails if it encounters a sample value greater than the maxval that the
image declares. The presence of such a value means the image is invalid, so the pixels have no
meaning. But with -forensic, ppmhist produces a histogram of the actual sample values without
regard to maxval. It issues messages summarizing the invalid pixels if there are any.

One use for this is to diagnose the problem that caused the invalid Netpbm image to exist.

There is a small exception to the ability of ppmhist to process invalid pixels even with
-forensic: it can never process a sample value greater than 65535. Note that in the rarely used
Plain PPM format, it is possible for a number greater than that to appear where a sample value
belongs.

This option was new in Netpbm 10.66 (March 2014). But Netpbm older than 10.66 does not properly
reject invalid sample values, so the effect is very similar to -forensic.

AUTHOR

       Copyright (C) 1989 by Jef Poskanzer.

DOCUMENT SOURCE