ABC D 
Figure.2 Original Image of straightedge to be measure 
2. FFT method to compute distance and 
the pattern position in the straightedge 
A very large class of important computational 
“Fourier transform methods’" or “spectral 
methods."' For some of these problems, the 
Fourier transform is simply an efficient 
computational tool for accomplishing certain 
common manipulations of data. In other cases, 
we have problems for which the Fourier 
transform (or the related "power spectrum'") is 
itself of intrinsic interest These two kinds of 
problems share a common methodology. 
Fourier methods are commonplace in research 
and we shall not treat them as specialized or 
arcane. A physical process can be described 
either in the time domain, by the values of 
some quantity h as a function of time t, e g. 
h(t), or else in the frequency domain, where 
the process is specified by giving its amplitude 
phase also) as a function of frequency /, that is 
//(/) as being two different representations of 
the same function. One goes back and forth 
between these two representations by means of 
the Fourier transform equations, 
w 
p ; - : *451* J 
.. . v&tiv-fj® Sii" ' 
yfe v *lPp%g 
r ***%№&&