1IM_STDIF(3)                Library Functions Manual                IM_STDIF(3)
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NAME

6       im_stdif, im_stdif_raw - statistical differentiation of an image
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SYNOPSIS

9       #include <vips/vips.h>
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11       int im_stdif(in, out, a, m0, b, s0, xw, yw)
12       IMAGE *in, *out;
13       double a, m0, b, s0;
14       int xw, yw;
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16       int im_stdif_raw(in, out, a, m0, b, s0, xw, yw)
17       IMAGE *in, *out;
18       double a, m0, b, s0;
19       int xw, yw;
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DESCRIPTION

23       im_stdif()  preforms  statistical differencing according to the formula
24       given in page 45 of the book "An Introduction to Digital Image Process‐
25       ing"  by Wayne Niblack. This transformation emphasises the way in which
26       a pel differs statistically from  its  neighbours.  It  is  useful  for
27       enhancing  low-contrast  images  with  lots  of  detail,  such as X-ray
28       plates.
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30       At point (i,j) the output is given by the eqn:
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32         vout(i,j) = a*m0 + (1-a)*meanv +
33                 (vin(i,j) - meanv) * (b*s0) / (s0+b*stdv)
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35       Values a, m0, b and s0 are entered, while meanv and stdv are the values
36       calculated  over a moving window of size xw, yw centred on pixel (i,j).
37       m0 is the new mean, a is the weight given to it. s0 is the new standard
38       deviation, b is the weight given to it. Try:
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40         im_stdif $VIPSHOME/pics/huysum.v fred.v 0.5 128 0.5 50 11 11
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42       The  opreation  works  on one-band UCHAR images only, and writes a one-
43       band UCHAR image as its result. The output image has the same  size  as
44       the input - a black border is added to mark uncomputable pixels.
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46       im_stdif_raw() behaves exactly as im_stdif(), but does not add the bor‐
47       der of black pixels.
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RETURN VALUE

51       All functions returns 0 on success and -1 on error.
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SEE ALSO

54       im_lhisteq(3), im_histgr(3), im_hsp(3), im_heq(3).
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57       1991-1996, The National Gallery and Birkbeck College
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61                                  10 May 1991                      IM_STDIF(3)
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