pnmhisteq - Online in the Cloud

PROGRAM:

NAME

pnmhisteq - histogram equalise a portable anymap

SYNOPSIS

pnmhisteq [-gray] [-rmap pgmfile] [-wmap pgmfile] [-verbose] [pnmfile]

DESCRIPTION

pnmhisteq increases the contrast of a portable graymap or pixmap through the technique of
histogram equalisation[1]. A histogram of the luminance of pixels in the map is computed,
from which a transfer function is calculated which spreads out intensity levels around
histogram peaks and compresses them at troughs. This has the effect of using the
available levels of intensity more efficiently and thereby increases the detail visible in
the image.

Mathematically, if N[i] is the number of pixels of luminosity i in the image and T is the
total number of pixels, luminosity j is replaced by:

j
---

> N[i] / T

---
i=0

If you're processing a related set of images, for example frames of an animation, it's
generally best to apply the same intensity map to every frame, since otherwise you'll get
distracting frame-to-frame changes in the brightness of objects. pnmhisteq's -wmap option
allows you to save, as a portable graymap, the luminosity map computed from an image
(usually a composite of the images you intend to process created with pnmcat). Then, you
can subsequently process each of the individual images using the luminosity map saved in
the file, supplied with the -rmap option.

OPTIONS

-gray When processing a pixmap, only gray pixels (those with identical red, green, and
blue values) are included in the histogram and modified in the output image.
This is a special purpose option intended for images where the actual data are
gray scale, with colour annotations you don't want modified. Weather satellite
images that show continent outlines in colour are best processed using this
option. The option has no effect when the input is a graymap.

-rmap mapfile
Process the image using the luminosity map specified by the portable graymap
mapfile. The graymap, usually created by an earlier run of pnmhisteq with the
-wmap option, contains a single row with number of columns equal to the maxval
(greatest intensity) of the image. Each pixel in the image is transformed by
looking up its luminosity in the corresponding column in the map file and
changing it to the value given by that column.

-wmap mapfile
Creates a portable graymap, mapfile, containing the luminosity map computed from
the histogram of the input image. This map file can be read on subsequent runs
of pnmhisteq with the -rmap option, allowing a group of images to be processed
with an identical map.

-verbose Prints the histogram and luminosity map on standard error.

All flags can be abbreviated to their shortest unique prefix.

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