**NAME**

im_fastcor, im_spcor - correlate two images

**SYNOPSIS**

**#include** **<vips/vips.h>**
**int** **im_fastcor(in,** **ref,** **out)**
**IMAGE** ***in,** ***ref,** ***out;**
**int** **im_spcor(in,** **ref,** **out)**
**IMAGE** ***in,** ***ref,** ***out;**

**DESCRIPTION**

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
input.
**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
ushort.
**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.

**BUGS**

The functions do not check for integer overflow.

**RETURN** **VALUE**

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

**SEE ALSO**

im_conv(3), im_lrmosaic(3).

**COPYRIGHT**

The National Gallery and Birkbeck College, 1990-1997.
14 May 1991 IM_FASTCOR(3)