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NAME
pgmhist - print a histogram of the values in a PGM image

SYNOPSIS
pgmhist

[-median, -quartile, -decile]

[-forensic]

[-machine]

[pgmfile]

DESCRIPTION
This program is part of Netpbm(1).

pgmhist reads a PGM image as input and prints a histogram of the gray
values or other gray value distribution metrics.

If you specify none of -median, -quartile, or -decile, pgmhist prints a
complete histogram showing how many pixels of each possible gray value
exist in the image. Along with each gray value, it tells you how many
pixels are at lest as black as it and how many are at least as white.

-median, -quartile, and -decile options cause pgmhist instead to print
the indicated quantiles. Each quantile is a gray value that actually
appears in the image (as opposed to fractional values that are sometimes
used for quantiles). The 3rd quartile is the least gray value for which
at least 75% of the pixels are as dark or darker than it. The 4th quar-
tile is the brightest gray value that appears in the image.

OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see ]8;;index.html#commonoptions\ Common Options]8;;\ ), pgmhist recognizes the fol-
lowing command line options:

You may specify at most one of -median, -quartile, and -decile. If none
of these is specified pgmhist prints a histogram of gray values.

-median

This option causes pgmhist to print the median gray value.

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

-quartile

This option causes pgmhist to print the four quartile gray val-
ues.

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

-decile

This option causes pgmhist to print the ten decile gray values.

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

-forensic

With this option, pgmhist works on images that contain invalid
gray values. Normally, like most Netpbm programs, pgmhist fails
if it encounters a gray 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,
pgmhist produces a histogram of the actual gray 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 in-
valid Netpbm image to exist.

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

-machine

This option causes pgmhist to print the information in a way eas-
ily digestible by a machine as opposed to a human.

For the quantiles, there is one line per quantile, in quantile
order, and it consists of the gray value of the quantile in deci-
mal with no leading zeroes.

For the full histogram output, it consists of one line per possi-
ble gray value (whether that value appears in the image or not),
in order of the gray values. The line consists of two tokens
separated by a space. The first is the gray value; the second is
the number of pixels in the image that have that gray value.
Both are decimal numbers without leading zeroes.

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