Subpixelshift by Phase correlation

Hi all
I am presently working on subpixel shift detection using phase
correlation .The algorithm uses the shifting property of the FT , to
find the shifts between the images .
Based on the algorithm the IFFT( phase correlation), should yield a
delta function at the displacement required to register the two
images.
Problem:
I am dividing the images into subblocks and estimating the shift
individually, Now instead of single delta function (peak) at the
displacement required to register the two images , I do get a sharp
dominant peak at (0,0) frequency and other random peaks.
This problem occur only with real time images, with simulated images
the dominant peak at (0, 0) is suppressed and the algorithm works
correctly.
Can anybody throw some light on the why there is a dominant peak at
the (0,0) frequency.

.

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