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

6       im_invertlut  -  turn a set of greyscale measurements into a gamma-cor‐
7       recting LUT
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SYNOPSIS

10       #include <vips/vips.h>
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12       int
13       im_invertlut( DOUBLEMASK *input, IMAGE *output, int lut_size )
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DESCRIPTION

17       Given a mask of target values and real values,  generate  a  LUT  which
18       will  map  reals to targets. Handy for linearising images from measure‐
19       ments of a colour chart. All values in  [0,1].   Extrapolate  head  and
20       tail to 0 and 1.
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22       Eg. input like:
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24         4 3
25         0.1  0.2  0.3  0.1
26         0.2  0.4  0.4  0.2
27         0.7  0.5  0.6  0.3
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29       means  a patch with 10% reflectance produces an image with 20% in chan‐
30       nel 1, 30% in channel 2, and  10%  in  channel  3.  A  patch  with  20%
31       reflectance makes an image with 40% red, 40% green and 20% blue, and so
32       on.
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34       Inputs don't need to be sorted (we do that). Generate any precision LUT
35       ... typically ask for 256 elements.
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37       It  won't  work too well for non-monotonic camera responses (should fix
38       this).
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40       Interpolation is simple piecewise linear; ought to do something  better
41       really.
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RETURN VALUE

45       -1 on error, otherwise 0
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SEE ALSO

48       im_histgr(3), im_hsp(3), im_heq(3), im_identity(3).
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51       2001, National Gallery
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55                                   June 2001                   IM_INVERTLUT(3)
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