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       im_fastcor, im_spcor - correlate two images


       #include <vips/vips.h>

       int im_fastcor(in, ref, out)
       IMAGE *in, *ref, *out;

       int im_spcor(in, ref, out)
       IMAGE *in, *ref, *out;


       These  functions  calculate  spatial  correlation  between two one-band
       images held by the image descriptors in and  ref.   The  sizes  of  ref
       should be smaller than the sizes of in.  The correlation is carried out
       by overlapping ref on the top left corner of in and moving it over  in.

       The  output  image  is  the same size as the input. The edge pixels are
       calculated by expanding the input image using  im_embed(3)  in  mode  1
       (replicating  edge pixels) just enough so that the output can match the

       im_spcor(3) calculates the spatial correlation between in and ref using
       the  correlation  coefficient  from Niblack "An Introduction to Digital
       Image Processing,", Prentice/Hall, pp 138.  The resultant  coefficients
       are written as float numbers in out.  The images must be char, short or

       im_fastcor(3) simply returns the sum of squares of differences  between
       in  and  ref.   This  is  much  faster,  but less useful. The resultant
       coefficients are written as unsigned int numbers in  out  which  has  a
       size of in.


       The functions do not check for integer overflow.


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


       im_conv(3), im_lrmosaic(3).


       The National Gallery and Birkbeck College, 1990-1997.

                                  14 May 1991                    IM_FASTCOR(3)