**PURPOSE**- Computes the normalized auto-correlation function of image/volume by using the Fourier transform relationship. Example.

**SEE ALSO****AC**[Auto Correlation ||] **AC MS**[Auto Correlation - Masked and normalized ||] **CC**[Cross Correlation ||] **CC N**[Cross Correlation - Normalized ||] **CN**[Convolution ||] **CN N**[Convolution - Normalized ||] **FT**[Fourier Transform ||]

**USAGE**- .OPERATION: AC N

- .INPUT FILE: PIC001

[Enter the name of the picture to be auto-correlated.].OUTPUT FILE: ACF001

[Enter name of the file which will contain the auto-correlation coefficients.]

**NOTES**

- On completion of this operation, the output file will contain
the auto-correlation function. The origin is at:
(NX/2 + 1, NY/2 + 1, NZ/2 + 1).
- The input data must be real.
- If the data cannot fit into the memory, use the 'FT' and 'SQ'
commands to calculate the ACF:

FT

INPUT

FOUR

SQ

FOUR

MODFT

FT

MODFT

ACF

**WARNING**: The orgin of the ACF calculated in this way is in (1,1,1).

**SUBROUTINES**- NORM3, FMRS_2, FMRS_2R, FFTMCF, FMRS_1, FMRS_3, FMRS_3R, ACRS_2, ACRS_3

**CALLER**- CORR1

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