# Re: efficient kernel or masking algorithm ?

```WOW, I need to look at these equations over about a dozen times to see what
is going on ?

I have been struggling with the variance of an nxn window of data, INCLUDING
central pixel

;mean of the neighboring pixels (including central)
mean=smooth(arr,n)
;square deviation from that mean
sqdev=(arr-mean)^2
;variance of an nxn window of data, INCLUDING central pixel
var=(smooth(sqdev,n)*n^2-sqdev)/(n^2-1)

This doesn't seem correct with test samples ? (Only difference is mean and
division by n^2-1 ??)

Thanks JD

Rich

J.D. Smith <jdsmith@astro.cornell.edu> wrote in message
3A25758E.A83B10CA@astro.cornell.edu">news:3A25758E.A83B10CA@astro.cornell.edu...
>
> Oh my this is a common topic lately.  See my recent posts in a thread
> with title "Array Manipulations".  Here's the good stuff:
>
> ; the nxn window total
> total=smooth(arr,n)*n^2
> ; the nxn window total not including central pixel
> neighbors=smooth(arr,n)*n^2-arr
> ; the mean of the neighboring pixels (excluding central)
> neighmean=(smooth(arr,n)*n^2-arr)/(n^2-1)
> ; the square deviation from that mean
> sqdev=(arr-neighmean)^2
> ; the variance of an nxn window of data, excluding central pixel
> imvar=(smooth(sqdev,n)*n^2-sqdev)/(n^2-2)
>
> Take a look at the "EDGE*" keywords too, if you care about what happens
> near the borders.
>
> JD
>
> --
>  J.D. Smith                  |   WORK: (607) 255-6263
>  Cornell Dept. of Astronomy  |         (607) 255-5842
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```