.TH IM_FASTCOR 3 "14 May 1991"
.SH NAME
im_fastcor, im_spcor \- correlate two images
.SH SYNOPSIS
.B #include <vips/vips.h>

.B int im_fastcor(in, ref, out)
.br
.B IMAGE *in, *ref, *out;

.B int im_spcor(in, ref, out)
.br
.B IMAGE *in, *ref, *out;
.SH DESCRIPTION
These functions calculate spatial correlation between two 
one-band images held
by the image descriptors 
.B in 
and 
.B ref.
The sizes of 
.B ref 
should be smaller than
the sizes of 
.B in.
The correlation is carried out by overlapping 
.B ref 
on the top
left corner of 
.B in 
and moving it over 
.B in.

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

.B im_spcor(3) 
calculates the spatial correlation between 
.B in
and 
.B 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 
.B out. 
The images must be char, short or ushort.

.B im_fastcor(3) 
simply returns the sum of squares of differences between 
.B in
and 
.B 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.

.SH BUGS
The functions do not check for integer overflow.
.SH RETURN VALUE
The function returns 0 on success and -1 on error.
.SH SEE ALSO
im_conv(3), im_lrmosaic(3).
.SH COPYRIGHT
The National Gallery and Birkbeck College, 1990-1997.