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 ⟨index.html#commonoptions⟩ ), 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 ⟨#output⟩ .

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 ⟨#output⟩ .

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 ⟨#output⟩ .

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 ⟨libppm.html#dictionary⟩ . If the exact color is not in the color
dictionary, it is the closest color that is in the dictionary and is preceded by a
'*'. 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