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image.twopointcorrelation - Function

	Package	general
	Module	images
	Tool	image

Compute two point correlation function from the image

Synopsis
twopointcorrelation(outfile, region, mask, axes, method, overwrite)

Arguments

outfile	in	Output image file name
		Allowed:	String
		Default:	unset
region	in	The region of interest
		Allowed:	Region tool
		Default:	Whole image
mask	in	OTF mask
		Allowed:	Boolean LEL expression or mask region
		Default:	None
axes	in	The pixel axes to compute structure function over
		Allowed:	Vector of integers
		Default:	Sky or first two
method	in	The method of computation
		Allowed:	String from 'structurefunction'
		Default:	structurefunction
overwrite	in	Overwrite (unprompted) pre-existing output file ?
		Allowed:	T or F
		Default:	F

Returns
T or fail

Description

This function (short-hand name is tpc) computes two-point auto-correlation functions from an image.

By default, the auto-correlation function is computed for the Sky axes. If there is no sky in the image, then the first two axes are used. Otherwise you can specify which axes the auto-correlation function lags are computed over with the axes argument (must be of length 2).

Presently, only the Structure Function is implemented. This is defined as :

S(lx, ly) = < (data(i, j) - data(i + lx, j + ly))² >

where lx, ly are integer lags in the x (first axis) and y (second axis) directions. The ensemble average is over the all values at the same lag pair. This process is extremely compute intensive and so you may have to be patient.

In an auto-correlation function image there are some symmetries. The first and third quadrants are symmetric, and the second and fourth are symmetric. So in principle, all the information is in the top or bottom half of the image. We just write it all out to look nice. The long lags don't have a lot of contributing values of course.

Example

 
- im := imagemaketestimage();        # Output image is virtual
- im2 := im.twopointcorrelation()    # Output image is virtual