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| int | vips_maplut () | 
| int | vips_percent () | 
| int | vips_stdif () | 
| int | vips_hist_cum () | 
| int | vips_hist_norm () | 
| int | vips_hist_equal () | 
| int | vips_hist_plot () | 
| int | vips_hist_match () | 
| int | vips_hist_local () | 
| int | vips_hist_ismonotonic () | 
| int | vips_hist_entropy () | 
Histograms and look-up tables are 1xn or nx1 images, where n is less than 256 or less than 65536, corresponding to 8- and 16-bit unsigned int images. They are tagged with a VipsInterpretation of VIPS_INTERPRETATION_HISTOGRAM and usually displayed by user-interfaces such as nip2 as plots rather than images.
These functions can be broadly grouped as things to find or build 
histograms (vips_hist_find(), vips_buildlut(), vips_identity()), 
operations that 
manipulate histograms in some way (vips_hist_cum(), vips_hist_norm()), 
operations to apply histograms (vips_maplut()), and a variety of utility 
operations.
A final group of operations build tone curves. These are useful in pre-press work for adjusting the appearance of images. They are designed for CIELAB images, but might be useful elsewhere.
int vips_maplut (VipsImage *in,VipsImage **out,VipsImage *lut,...);
Optional arguments:
band
: apply one-band lut
 to this band of in
Map an image through another image acting as a LUT (Look Up Table). The lut may have any type and the output image will be that type.
The input image will be cast to one of the unsigned integer types, that is, VIPS_FORMAT_UCHAR, VIPS_FORMAT_USHORT or VIPS_FORMAT_UINT.
If lut
 is too small for the input type (for example, if in
 is
VIPS_FORMAT_UCHAR but lut
 only has 100 elements), the lut is padded out
by copying the last element. Overflows are reported at the end of 
computation.
If lut
 is too large, extra values are ignored. 
If lut
 has one band and band
 is -1 (the default), then all bands of in
 
pass through lut
. If band
 is >= 0, then just that band of in
 passes 
through lut
 and other bands are just copied. 
If lut
has same number of bands as in
, then each band is mapped
separately. If in
 has one band, then lut
 may have many bands and
the output will have the same number of bands as lut
.
See also: vips_hist_find(), vips_identity().
int vips_percent (VipsImage *in,double percent,int *threshold,...);
vips_percent() returns (through the threshold
 parameter) the threshold 
above which there are percent
 values of in
. If for example percent=10, the
number of pels of the input image with values greater than threshold
will correspond to 10% of all pels of the image.
The function works for uchar and ushort images only. It can be used to threshold the scaled result of a filtering operation.
See also: vips_hist_find(), vips_profile().
int vips_stdif (VipsImage *in,VipsImage **out,int width,int height,...);
Optional arguments:
a
: weight of new mean
m0
: target mean
b
: weight of new deviation
s0
: target deviation
vips_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 equation:
| 1 2 | 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 width
, height
 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:
| 1 | vips stdif $VIPSHOME/pics/huysum.v fred.v 0.5 128 0.5 50 11 11 | 
The operation 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.
See also: vips_hist_local().
int vips_hist_cum (VipsImage *in,VipsImage **out,...);
Form cumulative histogram.
See also: vips_hist_norm().
int vips_hist_norm (VipsImage *in,VipsImage **out,...);
Normalise histogram ... normalise range to make it square (ie. max == number of elements). Normalise each band separately.
See also: vips_hist_cum().
int vips_hist_equal (VipsImage *in,VipsImage **out,...);
Optional arguments:
band
: band to equalise
Histogram-equalise in
. Equalise using band bandno
, or if bandno
 is -1,
equalise bands independently.
See also:
int vips_hist_plot (VipsImage *in,VipsImage **out,...);
Plot a 1 by any or any by 1 image file as a max by any or any by max image using these rules:
unsigned char max is always 256
other unsigned integer types output 0 - maxium 
value of in
.
signed int types min moved to 0, max moved to max + min.
float types min moved to 0, max moved to any (square output)
int vips_hist_match (VipsImage *in,VipsImage *ref,VipsImage **out,...);
Adjust in
 to match ref
. If in
 and ref
 are normalised
cumulative histograms, out
 will be a LUT that adjusts the PDF of the image
from which in
 was made to match the PDF of ref
's image. 
See also: vips_maplut(), vips_hist_find(), vips_hist_norm(),
vips_hist_cum().
int vips_hist_local (VipsImage *in,VipsImage **out,int width,int height,...);
Performs local histogram equalisation on in
 using a
window of size xwin
 by ywin
 centered on the input pixel. 
The output image is the same size as the input image. The edge pixels are created by copy edge pixels of the input image outwards.
See also: vips_hist_equal().
int vips_hist_ismonotonic (VipsImage *in,gboolean *out,...);
Test in
 for monotonicity. out
 is set non-zero if in
 is monotonic.
int vips_hist_entropy (VipsImage *in,double *out,...);
Estimate image entropy from a histogram. Entropy is calculated as:
| 1 | -sum( p * log2( p ) ) | 
where p is histogram-value / sum-of-histogram-values.