.TH IM_HISTGR 3 "10 May 1991"
.SH NAME
im_histcum, 
im_histeq, 
im_histgr, 
im_histnD, 
im_histnorm, 
im_histspec, 
im_identity,
im_identity_ushort, 
im_ismonotonic 
\- create, display and process histograms or luts
.SH SYNOPSIS
#include <vips/vips.h>

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

int im_histnD( in, out, bins )
.br
IMAGE *in, *out;
.br
int bins;

int im_histcum(hist, lut)
.br
IMAGE *hist, *lut;

int im_histnorm(hist, lut)
.br
IMAGE *hist, *lut;

int im_histeq(hist, lut)
.br
IMAGE *hist, *lut;

int im_histspec(histin, histref, lut)
.br
IMAGE *histin, *histref, *lut;

int im_identity(lut, bands)
.br
IMAGE *lut;
.br
int bands;

int im_identity_ushort(lut, bands, sz)
.br
IMAGE *lut;
.br
int bands;
.br
int sz;

int im_ismonotonic( IMAGE *lut, int *out )

.SH DESCRIPTION
.B im_histgr(3) 
writes the histogram of image in to image out. If bandno is -1,
then out will have the same number of bands as in, and each band of out will
have the histogram of the corresponding band of in. Otherwise, bandno selects
the band of the image for which the histogram will be found, numbering from
zero.

Image in may be either FMTUCHAR or FMTUSHORT. If in is uchar, then out will
have 256 elements, one for each possible pixel value. If in is ushort, then
.B im_histgr(3) 
finds the maximum of in, and outputs a histogram with max + 1
elements. 

For example, suppose you have an image from a 12-bit camera, where
each pixel is in the range [0,4095]. Calling 
.B im_histgr(3) 
for this image will
make a histogram with at most 4096 elements. If the histogram is smaller than
this, then it means that the right hand end of the histogram was all zero, and
has not been generated.

Also check 
.B im_histnD(3) 
below for another way to make histograms.

.B im_histnD(3) 
makes a n-dimensional histogram from an n-band image (1, 2 and 3 bands only).
Because 3D histograms can get very large very quickly, the 
.B bins
parameter sets the length of each dimension, that is, the number of bins the
possible numeric range of the image is divided into.

Unsigned 8 and 16 bit images only.

Use 
.B im_histplot(3) 
to graph the histogram for visualisation. See the separate manpage.

.B im_histcum(3)
forms a cumulative histogram. 

.B im_histnorm(3) 
normalises a histogram. The maximum histogram value becomes equal to the
number of pixels in the histogram. In effect, the histogram 
becomes 'square'. Each channel is normalised separately.

.B im_histeq(3)
takes as input a histogram held by the IMAGE descriptor hist and creates an
unsigned char look up table (held by the IMAGE descriptor lut) which when
applied on the original image, (with histogram held by hist) histogram
equalises it. Histogram equalisation is carried out for each band of hist
individually. 

.B im_histspec(3)
creates a lut for transforming an image with histogram held by histin
according to the pdf (probability density function) of a reference image with
histogram held by histref. histin and histref should have the same number
of bands.  

.B im_identity(3)
creates an look-up-table  with Xsize=256, Ysize=1, Bands=bands, Bbits=BBBYTE,
BandFmt=FMTUCHAR, Type=LUT, which is held by the IMAGE descriptor lut.  The
created image consist a bands linear (ramp) lut and is the basis for building
look-up tables.  

.B im_identity_ushort(3)
creates an look-up-table  with Xsize=sz, Ysize=1, Bands=bands, Bbits=BBSHORT,
BandFmt=FMTUSHORT, Type=LUT, which is held by the IMAGE descriptor lut.  The
created image consist of a linear (ramp) lut and is the basis for building
look-up tables.  

.B im_ismonotonic(3)
sets out to non-zero if the look-up table (or histogram) in lut is monotonic,
that is, if it's slope is always >0.

.SH RETURN VALUE
All functions return 0 on success and -1 on error.
.SH SEE ALSO
im_hist(3), im_hsp(3), im_heq(3), im_maplut(3), im_tone_map(3),
im_histplot(3).
.SH COPYRIGHT
The National Gallery and Birkbeck College, 1991 \- 1994.