We are trying to solve the current homework assignment using the sift, harris and find2chi functions provided in the email (on July 4th). Doing that, we have run into the following problem:
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??? Subscript indices must either be real positive integers or logicals.
Error in ==> sift at 36
dxb = dx(py(i)-7:py(i)+8, px(i)-7:px(i)+8);
This happens when we apply the given SIFT function, using the harris interest points also generated by the given harris.m functions. What happens here (I think), is that harris finds interest points on the image border, and trying to obtain the 16x16px window around them results in negative pixel coordinates (subscript indices). In our own solution for the previous exercise, we had a workaround for this problem; however, as a result, our SIFT function worked much slower than the reference implementation and is thus unsuitable for the current assignment (processing all interest points from all the training images would take forever).
How do we work around this problem?..