CImg image registration

Hi all,

One of the CImg examples given regards image registration/morphing of David's face between two viewpoints. Looking at the code, a motion vector field ("u") is calculated and used to estimate the motion between the two frames. *
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The optflow() function uses the get_norm_pointwise function - this seems to be the function to use when calculating how much estimated x,y,z motion there has been between frames. I am just wondering what the 1,2,-1 argument values of this function means?*
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By the way, is my interpretation correct? Is get_norm_pointwise "the" function to use in this regard?

How do I go about finding the translated x, y values of a pixel? Ultimately, I know that getting the norm of the pixel value will give this, but how do I determine the x and y values?