.TH IM_STDIF 3 "10 May 1991"
.SH NAME
im_stdif, im_stdif_raw \- statistical differentiation of an image
.SH SYNOPSIS
.B #include <vips/vips.h>

int im_stdif(in, out, a, m0, b, s0, xw, yw)
.br
IMAGE *in, *out;
.br
double a, m0, b, s0;
.br
int xw, yw;

int im_stdif_raw(in, out, a, m0, b, s0, xw, yw)
.br
IMAGE *in, *out;
.br
double a, m0, b, s0;
.br
int xw, yw;

.SH DESCRIPTION
im_stdif() preforms statistical differencing according to the formula
given in page 45 of the book "An Introduction to Digital Image Processing" by
Wayne Niblack. This transformation emphasises the way in which a pel differs
statistically from its neighbours. It is useful for enhancing low-contrast
images with lots of detail, such as X-ray plates.

At point (i,j) the output is given by the eqn:

  vout(i,j) = a*m0 + (1-a)*meanv + 
          (vin(i,j) - meanv) * (b*s0) / (s0+b*stdv)

Values a, m0, b and s0 are entered, while meanv and stdv are the values
calculated over a moving window of size xw, yw centred on pixel (i,j). m0 is the
new mean, a is the weight given to it. s0 is the new standard deviation, b is
the weight given to it. Try:

  im_stdif $VIPSHOME/pics/huysum.v fred.v 0.5 128 0.5 50 11 11

The opreation works on one-band UCHAR images only, and writes a one-band UCHAR
image as its result. The output image has the same size as the input - a black
border is added to mark uncomputable pixels.

im_stdif_raw() behaves exactly as im_stdif(), but does not add the border of
black pixels.

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