## NAME

autocorr -- Compute the autocorrelation of a 1- or 2-dimensional image.

## USAGE

`autocorr input output`

## DESCRIPTION

This task computes the autocorrelation of a 1- or 2-dimensional image; the autocorrelation is similar to cross-correlation, except that it correlates an image with itself instead of with a second image.

For 2-D transforms, this task has the option of using scratch images for intermediate results. Using scratch images may take longer, but it allows the task to function even with limited memory.

In contrast to cross-correlation, the coordinate information in the input image is not used to adjust the phase of the Fourier transform because phase information is lost when computing the amplitude.

## PARAMETERS

- input = "" [file name]
- The input 1- or 2-dimensional image file.
If both real and imaginary parts are to be read, the letters "r" and "i" will be appended to

`input`to form the names of the images for real and imaginary parts respectively.

- output = "" [file name]
- Output image created by
`autocorr`.The "r" character is not appended to this output image because

`autocorr`produces no imaginary part.

- (inreal = yes) [boolean]
- Use the real part of the input data file?

- (inimag = no) [boolean]
- Use the imaginary part of the input data file?

- (center = yes) [boolean]
- Shift the coordinate origin to the middle of the output image?

- (inmemory = yes) [boolean]
- For a two-dimensional input image,
if
`inmemory = yes`the image will be read into a complex array, the computations will be performed on that array in-memory, and the array will be written to the output image. This requires one complex word for each pixel. The complex array must fit entirely in memory (i.e. no paging) because when performing the Fourier transforms the array is accessed both by rows and by columns. If`inmemory = no`, see the description of`len_blk`.For 1-D images,

`inmemory`is ignored.

- (len_blk = 256) [integer]
- Size of a block to be transposed.
For 2-dimensional input images, if

`inmemory = no`this task transposes each image into scratch images before computing the forward and inverse Fourier transforms of the second axis. This parameter is the length of the side of a square region that is transposed in one step. If you get out-of-memory errors, you should flush the process cache (`flprcache`), reduce the size of`len_blk`and try again.This parameter is ignored for 1-D images or if

`inmemory = yes`.

- (verbose = yes) [boolean]
- Print input and output image names?
Setting

`verbose=yes`shows you the actual names of the image headers, including the "r" and "i" suffixes for real and imaginary parts.

## EXAMPLES

1. Compute the autocorrelation of images "civr" and "civi" (these are real and imaginary parts respectively). The output autocorrelation will be stored in an image called "p".

fo> autocorr civ p inimag=yes

## BUGS

## SEE ALSO

Type "help fourier option=sys" for a higher-level
description of the `fourier` package.