phistogramequalization

Performs contrast stretching using histogram equalization.

Synopsis

phistogramequalization min max [-m mask] [im_in|-] [im_out|-]

Description

phistogramequalization performs a gray-levels transformation by forcing the gray-levels to spread over the entire intensity range between min and max. As a consequence, histogram equalization expands gray-levels where information is important and compresses gray-levels where the information is low.
This transform is not likely to work well on input images that have dark gray-levels distribution.

The new output bounds of the output gray-levels are given by the parameter min and max.

The equalization has the form:

Compute the cumulated histogram for im_in;
Normalize histogram between [0..1];

Build output image im_out with for each pixel 'p':

    im_out[p] = HC[im_in[p]]*(max-min);

For color and multispectral images, the transform uses the marginal approach: it is applied on each band individually.

Parameters

min and max specify the bounds of the output pixel value. They are related to the type of the input image.
Note: if min > max then min and max are set with the minimum and maximum values of the input image type; for example, 0 and 255 for Uchar images.

Inputs

im_in: an image.

Outputs

im_out: an image with the same properties as im_in.

Result

Returns SUCCESS or FAILURE in case of invalid parameter values.

Examples

Equalizes histogram with the new bounds [0,255]:

    phistogramequalization 20 200 tangram.pan a.pan

Equalizes histogram with the same bounds than the input image:

    phistogramequalization 0 -1 tangram.pan a.pan

C++ prototype

Errc PHistogramEqualization( const Img2duc &im_in, Img2duc &im_out, float min, float max );

Version française

Rehaussement de contraste par égalisation d'histogramme.

Author: Régis Clouard