In images aligning, how to exclude some areas from xcorr2 execution?

Hi,

I'm trying to align two images using the procedure described into the "Align Two Images Using Cross-Correlation" example, from the documentation:

http://it.mathworks.com/help/signal/ref/xcorr2.html

In few words, I subtract the mean value and then I perform the cross-correlation. In my case, the result is good but not precise enough.

In details, I have two similar photos to align. The background in the pictures is similar, but not the same, because it is slightly shifted, so the need to align. However, one image presents an occlusion due to a small object.

So, I would like to perform xcorr2 excluding the pixels corresponding to the object, i.e., to perform the alignment only considering the background. How can I do this?

Thanks for your answers.