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From: tomg@hpcvlx.cv.hp.com (Thomas J. Gilg)
Newsgroups: comp.graphics
Subject: Re: Cleaning up frame-grabbed images: "Fourier plane"?
Message-ID: <101880025@hpcvlx.cv.hp.com>
Date: 28 Sep 90 18:02:27 GMT
References: <1990Sep27.085647.13944@ste.dyn.bae.co.uk>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 35


> My wife remembers from her physics undergraduate days that an image overlayed
> with a grating could be optically processed to remove the grating - light from
> the image was focused though a lens to come to a point at the fourier plane.
> Then, somehow, the image the other side was a magically cleaned up version of
> the original. (You can tell that I don't understand it very well :-).

There are many image processing/cleanup methods based on frequency domain
filters.  The usual computational sequence is:

    1. Convert the Spatial Domain Image (ie, your digitized image) to the
       Frequency Domain.  The Fast Fourier Transform (FFT) is popular here.

    2. Analyze the Frequency Domain representation of your image, and then
       apply some selected mask to it.

    3. Convert the Frequency Domain result back into the Spatial Domain.
       Magically, you'll have your cleaned image iff you did the right
       things in step #2.   BTW - the inverse FFT is popular here.

While the above is a standard sequence, and is usually more effective/faster
than pure Spatial Domain methods, it does take time.

Its my understanding that "optical" implementations of the above described
computational sequence are possible.  One lens setup deals with step #1,
a few optical filters modify the image for step #2, and another lens setup
converts the image back for step #3.  While this allows for real time image
processing, I'm not sure if its too handy for pre-digitized images.

Thomas Gilg
tomg@cv.hp.com

Great reference: "Digital Image Processing"
                 Rafael C. Gonzalez/Paul Wintz
                 Addison-Wesley Publishing Company