**PURPOSE**- Computes the cross-correlation function of two images/volumes by using the Fourier transform relationship. The dimensions of the two pictures must be the same. Example.

**SEE ALSO****CC N**[Cross Correlation - Normalized ||] **CN**[Convolution ||] **CN N**[Convolution - Normalized ||] **AC**[Auto Correlation ||] **AC N**[Auto Correlation - Normalized ||] **CC C**[Cross Correlation Coefficient] **FT**[Fourier Transform ||]

**USAGE**- .OPERATION: CC

- .INPUT FILE: PIC001

[Enter the name of the first picture. (Header positions from this file are transferred to output.)].REFER FILE: REF001

[Enter the name of the second picture, which is used as the reference.].OUTPUT FILE: CCF001

[Enter name of the file which will contain cross-correlation coefficients. It can be the same as the input file. In this case INPUT FILE will be replaced by the cross correlation.]

**NOTES**

- The type of the input data can be real or Fourier in any
mixed form.
- The cross-correlation data are not scaled. (If values are
very, very large then maybe you should reduce the FMAX
of the input files first?)
- The origin of the CCF is at (NX/2 + 1, NY/2 + 1, NZ/2 + 1).
- The CCF contains artifacts from wraparound overlaps
implicit in the Fourier computation.
However, the CCF is artifact-free within

-L <= K <= +L

-M <= I <= +M

if the pictures are surrounded by frames containing the average, where the frame width is M/2 and the frame height is L/2. This can be achieved by use of the 'PD' operation.

The complete artifact-free CCF is obtained for the widths M=NY/2, L=NX/2. In this case, the padded pictures are just twice as large in each dimension as the original pictures. - The use of 'CC' with identical input and reference
files is equivalent to the 'AC' (Auto-Correlation)
operation. The input sequence:

CC

PIC001

PIC001

has the same effect as:

AC

PIC001

- If the data cannot fit into the memory, use the 'FT' and 'MU'
commands to calculate the CCF:

FT

INPUT1

FOUR1

FT

INPUT2

FOUR2

MU M

FOUR1

FOUR2

FT

FOUR2

CCF12

**WARNING**: The origin of*CCF12*calculated in this way is in (1,1,1).

**SUBROUTINES**- FMRS_2, FMRS_1, FMRS_3, CCRS, CCRD_2, CCRD_3

**CALLER**- CORR1

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