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       im_litecor - perform light correction


       #include <vips/vips.h>

       int im_litecor(in, white, out, clip, factor)
       IMAGE *in, *out;
       int clip;
       double factor;


       im_litecor(3)  performs light correction on the image held by the IMAGE
       descriptor in, with respect to a reference  white  image  held  by  the
       IMAGE   descriptor  white.   The  result  is  written  onto  the  IMAGE
       descriptor out.  The function works on byte one channel images only.

       The flag clip can take two values 0 and 1.  If clip is 1 then the input
       is  corrected  with  reference  to the maximum value of white (maxw) as

        pel_out = factor * pel_in * maxw / pel_white.

       If clip is 0 then the output is scaled with the maximum possible output
       set  to  255.   In this case factor is not used but it must be set to a
       dummy value.

       The basic reason for lighting correction is that the input  frame  does
       not  have  a  uniform  distribution  of  white light due to the optical
       response of the lens.  The function accepts a white image  which  is  a
       simple  multiple of the input image in size; for example it is possible
       that the white is a subsampled version of in; however the sizes  of  in
       must  be an exact multiple of the white.  If clip is set to 0, lighting
       correction is carried out and the result is scaled between 0  and  255.
       This can be used to correct individual frames.

       If multiband images are grabbed, then flag should be set to 1, since no
       scaling must be done.  In this case the factor can reduce the number of
       clipped pels if overshooting occurs in the brightest band.  The program
       prints the number of clipped pels with im_warning(3).


       The function returns 0 on success and -1 on error.


       clip==0 case not working too well.


       im_add(3), im_lintra(3), im_multiply(3).


       N. Dessipris


       N. Dessipris - 05/12/1991

                                5 December 1991