.TH IM_HEQ 3 "10 May 1991"
.SH NAME
im_heq, im_lhisteq, im_lhisteq_raw, im_hsp \- process an image using grey level transformations
.SH SYNOPSIS
#include <vips/vips.h>

int im_heq( in, out, bandno )
.br
IMAGE *in, *out;
.br
int bandno;

int im_lhisteq( in, out, xw, yw )
.br
IMAGE *in, *out;
.br
int xw, yw;

int im_hsp( in, ref, out )
.br
IMAGE *in, *ref, *out;

.SH DESCRIPTION
.B im_heq(3)
histogram equalises the unsigned char image held by the IMAGE descriptor
in.  The result is written to the IMAGE descriptor out.
If bandno is -1 then all input bands are equalised independently.  In all
other cases the input image is equalised using the histogram of bandno only.
The latter processing produces better results.

.B im_hsp(3)
maps in to out with histogram specified by the ref.  All images should be
unsigned char.  Each band of the output image is specified according to the
distribution of grey levels of the reference image according to
im_histspec(3).

.B im_lhisteq(3)
histogram equalises the one channel unsigned char image pointed to by the
Image descriptor in.  The result is written to the IMAGE descriptor out.
The histogram equalisation is based on a window of size xw by yw centered at
the current location of each input pixel. 

In the manner of 
.B im_conv(3)
the input image is expanded so that the output image is the same size as the
input.

.B im_lhisteq_raw(3)
is as above, but does not expand the input.

.SH RETURN VALUE
All functions returns 0 on success and -1 on error.
.SH SEE ALSO
im_histgr(3), im_histplot(3), im_histspec(3), im_lineprof(3),
im_stdif(3).
.SH COPYRIGHT
1991--1996 The National Gallery and Birkbeck College