What is the real frequency of image "Lena"?

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2 total messages Started by Chen Jie Mon, 21 Dec 1992 06:24

#847

Author: Chen Jie
Date: Mon, 21 Dec 1992 06:24

32 lines
1896 bytes

          Merry Christmas to everybody,
I would like to ask a question about standard image "Lena" with size of
512x512x8bit.
For this digitalized image, we can use 2D-FFT (with input data length 512)
to estimate the spatial frequency property of H(wx,wy),where wx and wy
are spatial frequences in X-direction and Y-direction,respectively.
And generally, wx and wy are normalized to [-pi,pi] as following figure.
                      wy|
                        Pi
                        |
                        |
           --  -Pi------o-------Pi--->wx
                        |
                        |
                       -Pi

My question is that,
for wx and wy, what are the real frequencies (in Hz) at Pi ?
The answer may be related to the sampling frequency f_s for 512x512 images,
What is the f_s?  and for 256x256 images, does the f_s become half of 512's?

Thanks in advance.
-Jie Chen
--
____________________________________________________________________________              
               Jie Chen 
	       ITL ( Information Transmission Laboratory )
               Depart. of Electronics
               Univ.   of Electro-Communications 
               Chofu city, Tokyo 182, Japan
               E-mail: chen@mahler.ee.uec.ac.jp
-----------------------------------------------------------------------------

Re: What is the real frequency of image "Lena"?

#770

Author: Jeffrey Mulligan
Date: Tue, 22 Dec 1992 03:06

33 lines
1603 bytes

chen@haydn.ee.uec.ac.jp (Chen Jie) writes:

>          Merry Christmas to everybody,
>I would like to ask a question about standard image "Lena" with size of
>512x512x8bit.

Actually, this question is not about lena...

>For this digitalized image, we can use 2D-FFT (with input data length 512)
>to estimate the spatial frequency property of H(wx,wy),where wx and wy
>are spatial frequences in X-direction and Y-direction,respectively.
>And generally, wx and wy are normalized to [-pi,pi] as following figure.
>My question is that,
>for wx and wy, what are the real frequencies (in Hz) at Pi ?

The units of frequency are not Hz unless the original signals come
from the time domain.  The output of the DFT is often expressed
in cycles/sample, in which case the range is +- 0.5.
(0.5 c/s is also known as the Nyquist frequency).
For images viewed by humans frequency is also sometimes
expressed as cycles/picture width or cycles/degree of visual angle.

-- 

	Jeff Mulligan (jbm@vision.arc.nasa.gov)
	NASA/Ames Research Ctr., MS 262-2, Moffett Field CA, 94035-1000
	(415) 604-3745

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