.TH IM_INVERTLUT 3 "June 2001"
.SH NAME
im_invertlut \- turn a set of greyscale measurements into a gamma-correcting
LUT
.SH SYNOPSIS
#include <vips/vips.h>

int
.br
im_invertlut( DOUBLEMASK *input, IMAGE *output, int lut_size )

.SH DESCRIPTION
Given a mask of target values and real values, generate a LUT which
will map reals to targets. Handy for linearising images from
measurements of a colour chart. All values in [0,1].
Extrapolate head and tail to 0 and 1.

Eg. input like:

  4 3
  0.1  0.2  0.3  0.1
  0.2  0.4  0.4  0.2
  0.7  0.5  0.6  0.3

means a patch with 10% reflectance produces an image with 20% in
channel 1, 30% in channel 2, and 10% in channel 3. A patch with 20%
reflectance makes an image with 40% red, 40% green and 20% blue, and so on.

Inputs don't need to be sorted (we do that). Generate any precision
LUT ... typically ask for 256 elements.

It won't work too well for non-monotonic camera responses (should fix this).

Interpolation is simple piecewise linear; ought to do something better really.

.SH RETURN VALUE
-1 on error, otherwise 0
.SH SEE ALSO
im_histgr(3), im_hsp(3), im_heq(3), im_identity(3).
.SH COPYRIGHT
2001, National Gallery