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       im_ifthenelse - use an unsigned char image to join two images together


       #include <vips/vips.h>

       int im_ifthenelse(c, a, b, out)
       IMAGE *c, *a, *b, *out;

       int im_blend(c, a, b, out)
       IMAGE *c, *a, *b, *out;


       im_ifthenelse  builds  an  output image which uses some pels from a and
       some from b: if the conditional image c is non-zero at that point,  the
       pel comes from a; if it is zero, the pel comes from b.

       The  conditional  image  c can have either 1 band, in which case entire
       pels come either from a or b, or n bands, where  n  is  the  number  of
       bands  in  both  a  and  b,  in which case individual band elements are
       chosen from a and b.

       Images a and b must match in size, type and number of bands.

       im_blend(3) works just as  im_ifthenelse(3),  except  that  instead  of
       selecting  between  a  and b, values in the condition image are used to
       softly blend between the two. 255 means a only, 0  means  b  only,  128
       means 50:50.


       0 on success and -1 on error.


       im_equal(3), im_and(3).


       National Gallery


       J. Cupitt

                                30 October 1992