🚀 go-pugleaf

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