Re: image edges with convolution

Oof wrote: 
Hi there,

I'm fiddling with simple convolution filters, and i was wondering how to
treat the edges of the image.
now i'm just ignoring the first row and column of the image, aswell as the
bottom row and last column. ( with  a 3x3 kernel )

Q: how can i apply the filter on the whole image incl the outermost
boundries of the image


What you are doing now is the only correct way.
However, has you are discovering, producing an
output image which is a different size from the
input image is inconvenient.

There are several ways you can handle the
boundaries; they are all wrong.  They include:

1.  Mirror the edge pixels beyond the edge.
2.  Replicate the edge pixel beyond the edge.
3.  Assume the data is cyclical, and so
    encroach into the opposite edge.
4.  Pad with zeros (or some other constant
    value).
5.  Pad with data values that are statistically
    the same has those close to the edge.

Method 1. seems to be the most popular.  I use
2. because it happens to be convenient for me.

--
Regards,
Martin Leese
E-mail: please@xxxxxxxxxxxxxxxxxxxxxxxxxx
Web: http://members.tripod.com/martin_leese/
.