1.1.1.0 image.twopointcorrelation

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image.twopointcorrelation - Function

1.1.1 Compute two point correlation function from the image
Description

This function 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 :

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

where lx,ly are integer lags in the x (0-axis) and y (1-axis) directions. The ensemble average is over all the 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.

Arguments


Inputs
outfile	Output image file name. Default is unset.
	allowed:	string
	Default:
region	The region of interest. Default is whole image.
	allowed:	any
	Default:	record
mask	OTF mask, Boolean LEL expression or mask region. Default is none.
	allowed:	any
	Default:	variant
axes	The pixel axes to compute structure function over. The default is sky or first two axes.
	allowed:	intArray
	Default:	-1
method	The method of computation. String from ’structurefunction’.
	allowed:	string
	Default:	structurefunction
overwrite	Overwrite (unprompted) pre-existing output file?
	allowed:	bool
	Default:	false

Returns
bool

Example

"""
#
print "\t----\t twopointcorrelation Ex 1 \t----"
ia.maketestimage(); # Output image is virtual
ia.twopointcorrelation() # Output image is virtual
#
"""

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