Re: when should I pad image before filtering?
From: lucy (losemind_at_yahoo.com)
Date: 03/25/05
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Date: Thu, 24 Mar 2005 23:58:06 -0800
"lucy" <losemind@yahoo.com> wrote in message
news:d20avh$p2b$1@news.Stanford.EDU...
>I am always confused by the padding for image filtering.
>
> I have an 270x230 "image" and is to be filtered with a 250x250 "filter".
>
> I have made sure that the sum of all filter coefficients sum to be 1.
>
> In Matlab,
>
> sum(sum(filter)) =1.0000,
>
> then I do image filtering:
>
> result=imfilter(image, filter, 'same');
>
> so the result is the also a 270x230 image... since I've used 'same'
> option, ...
>
> I am not sure if the result is correct or not...
>
> in fact, I even don't understand when a 270x230 image convolved with a
> 250x250 filter, is the result meaningful at all?
>
> Anybody please give me some enlightment?
>
> And Matlab has so many options: such as "replicate", "symmetric", etc:
>
> X Input array values outside the bounds of the array
> are implicitly assumed to have the value X. When no
> boundary option is specified, IMFILTER uses X = 0.
>
> 'symmetric' Input array values outside the bounds of the array
> are computed by mirror-reflecting the array across
> the array border.
>
> 'replicate' Input array values outside the bounds of the array
> are assumed to equal the nearest array border
> value.
>
> 'circular' Input array values outside the bounds of the array
> are computed by implicitly assuming the input array
> is periodic.
>
> When do we choose which option?
>
>
>
Strange. I really don't understand this:
what happens if I have a 270x230 image convolved with a
600x600 filter using Matlab function?
result=imfilter(image, filter, 'same')?
what happens if I have a 270x230 image convolved with a 100 x 100 filter
using the same matlab function?
Are these results still correct?
I understand the small case: if an image 270x230 convolved with a 3x3
filter, I understand, the image will have false values on its 4 side 1pixel
width boundary. All other values should be correct value...
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