the point of passing through a filter in image processing

Hi,

I have a basic question on image processing, after FT, I got the casted
image on different colors, or different freq. Next is to pass through a
filter to get the freq that I want. I am wondering, I can just pick up
any freq I want, and the band of freq I want. Why do I have to
construct a filter? Is it really needed? Or how a filter is being used
in image processing? 


thanks
PZ

PZ wrote:
> Hi,
> 
> I have a basic question on image processing, after FT, I got the casted
> image on different colors, or different freq. Next is to pass through a
> filter to get the freq that I want. I am wondering, I can just pick up
> any freq I want, and the band of freq I want. Why do I have to
> construct a filter? Is it really needed? Or how a filter is being used
> in image processing? 

Are you equating color to frequency? if so, you don't understand what FT 
does. Is there a textbook you can consult?

Jerry
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