🚀 go-pugleaf

In article <l8h39wm@rpi.edu>, raffuj@rebecca.its.rpi.edu (Jose Raffucci) writes:
|> 
|> I'm trying to add a low pass filter so I can check too see if it's 
|> doing what it's supposed to.  It's not.  I'm getting what appear to be 
|> the upper edges of some sinc waves, especially in the darker portions
|> of the pictures.  I've checked this out with another program I found
|> that fft's images, and the same thing happens.  

	That sounds exactly like it should. Since you are doing an *ideal*
low-pass filter (all 1's in the pass band, all zero's in the stop band), 
you will indeed get a diffraction pattern in your output image. You are 
effectively convolving the image with a sinc function, which causes the 
ringing.

	If you are trying to simulate lower-resolution optics, this is correct.

	You might try a smoother filter function that falls off more slowly
at higher frequencies. A gaussian is one idea.
|> 
|> Related question:  
|> 
|> 	What should I do with the data that dips below zero when I 
|> return from the frequency domain?  I Need to keep the intensity values
|> of the pixels, since they're linearly mapped to the signs' luminance.  
|> That pretty much eliminates moving everyting up, and making the lowest 
|> point the new zero.  any suggestions?
|> 
	Um, I'm not an optics type, but wouldn't luminance be the magnitude^2
of the output image? That would take care of any negative pixels.

|> Now, before anyone suggests I not do the fft and move over to a   
|> sliding kernel method, let me mention that if I could I would.  The 
|> problem is that I don't know how to change the filter functions to
|> their corresponding kernels.
|> 
	The FFT is just a convenient computational technique; if you do it right
you'll get the same results as the space-domain implementation. It sounds like
you are doing it right.

-- 
Dan                                             o
                                     ____  o o
Dan-Sullivan@orl.mmc.com            (_/\_)o
Martin Marietta Electronic Systems    ()=o
MP 1160, Orlando, FL USA
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 what you think it means."