Normalised cross correlation

[Tony Yu]

On Sat, Mar 24, 2012 at 8:29 PM, Mike Sarahan <msarahan at gmail.com> wrote:

> Hi Tony,
>
> Glad you figured this out.




> I'm also not a huge fan of the Matlab-style padding.  I guess
> theoretically, it gives your match greater range - beyond the bounds
> of the image.  In practice, I found that such behavior in the presence
> of noise often gave me a peak outside the image, when I knew I wanted
> my peak inside the image.
>
> I'm mixed on the input padding.  Intuitively, I think the high value
> at template center is easier to understand for a human looking at the
> cross correlation result image.  I know that this confused me a lot
> when I first encountered cross correlation using OpenCV.  However, any
> subsequent analysis (peak finding, for example) would have to account
> for the difference, and I think this would be more of a headache than
> a benefit to people.  I can imagine a lot of careless analyses with
> offsets of exactly half the template dimensions.
>
> Your proposed method for the padding makes more sense to me than
> Matlab's.  I would recommend defaulting to no padding, and also
> recommend a little documentation that warns the user about the offset
> they will have to account for if using the pad option.
>
> I can't think of a use case for padding the output with zeros.  If
> anyone else knows one, I'll be happy to learn, though.
>

Hey Mike,

I guess my rationale for padding with zeros was the following: I want the
output hotspot to match the image coordinates, but I don't want to have
matches where the template exceeds the image boundaries.

With the match centered on the template-origin, this padding actually makes
no difference since you're padding only the higher indices, but with the
match centered on the template-center, there would be a difference. Also,
there's a (usually-insignificant) computational improvement compared to
padding the input.

I'm just making this stuff up as I go along, so this might not actually be
a desirable use case.

-Tony



> Best,
> Mike
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/scikit-image/attachments/20120325/3a9d8bbf/attachment.html>