IM_INVERTLUT

Section: C Library Functions (3)
Updated: June 2001
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NAME

im_invertlut - turn a set of greyscale measurements into a gamma-correcting LUT

SYNOPSIS

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

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.

RETURN VALUE

-1 on error, otherwise 0

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