Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!rutgers!mit-eddie!media-lab!turk@media-lab.media.mit.edu
From: turk@media-lab.media.mit.edu (Matthew Turk)
Newsgroups: comp.graphics
Subject: Re: Cleaning up frame-grabbed images: "Fourier plane"?
Message-ID: <3498@media-lab.MEDIA.MIT.EDU>
Date: 29 Sep 90 22:21:00 GMT
References: <1990Sep27.085647.13944@ste.dyn.bae.co.uk> <28315@pasteur.Berkeley.EDU>
Sender: turk@media-lab.MEDIA.MIT.EDU
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Organization: MIT Media Lab
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In-reply-to: ph@miro.Berkeley.EDU's message of 28 Sep 90 21:26:33 GMT


 > I don't know how the optical method you mentioned would work, except that
 > lenses effectively compute a Fourier transform.  I don't know how you
 > would do the division optically, however.  Maybe other netters can answer that.

Yep, you can do multiplications in the Fourier domain easily with
optics.  And the division by H(wx,wy) is implemented as a
multiplication by (1 / H(wx,wy)).

But note that you can do this digitally as well.  The impulse function
of the filter whose frequency response is (1/H(wx,wy)) is itself a
spatial filter h'(x,y), so once you find h'(x,y) you just convolute
the image with it.

In practice, though, this is kinda difficult, both because you
probably don't know H(wx,wy) very accurately and because designing FIR
filters (which is what we're talking about!) is not as straightforward
as it sounds.  Another good reference is Jae S. Lim's "Two-Dimensional
Signal and Image Processing" (Prentice-Hall, 1990).

But if I remember the original post properly, you're finding black
lines which spoil your digitized images.  I'd check your hardware,
because these aren't normally part of the video signal!  (Are you by
any chance mixing 50 and 60 hz components?)

	Matthew Turk
	MIT Media Lab			turk@media-lab.media.mit.edu
	20 Ames St., E15-391		uunet!mit-amt!turk
	Cambridge, MA  02139		(617)253-0381