BASIS FUNCTIONS:

The Fourier Transform ( in this case, the 2D Fourier Transform )
is the series expansion of an image function ( over the 2D space
domain ) in terms of “cosine” image (orthonormal) basis functions.

The definitons of the transform (to expansion coefficients) and
the inverse transform are given below:

	F(u,v) = SUM{ f(x,y)*exp(-j*2*pi*(u*x+v*y)/N) }
    and
	f(x,y) = SUM{ F(u,v)*exp(+j*2*pi*(u*x+v*y)/N) }

    where u = 0,1,2,...,N-1 and v = 0,1,2,...,N-1
	  x = 0,1,2,...,N-1 and y = 0,1,2,...,N-1
	  j = SQRT( -1 )
	  and SUM means double summation  over proper
	  x,y or u,v ranges

First we will investigate the “basis” functions for the Fourier
Transform (FT). The FT tries to represent all images as a summation
of cosine-like images. Therefore images that are pure cosines have
particularly simple FTs.

This shows 2 images with their Fourier Transforms directly underneath.
The images are a pure horizontal cosine of 8 cycles and a pure
vertical cosine of 32 cycles. Notice that the FT for each just has
a single component, represented by 2 bright spots symmetrically
placed about the center of the FT image. The center of the image is
the origin of the frequency coordinate system. The u-axis runs left
to right through the center and represents the horizontal component of
frequency. The v-axis runs bottom to top through the center and
represents the vertical component of frequency. In both cases there
is a dot at the center that represents the (0,0) frequency term or
average value of the image. Images usually have a large average
value (like 128) and lots of low frequency information so FT
images usually have a bright blob of components near the center.
Notice that high frequencies in the vertical direction will cause
bright dots away from the center in the vertical direction.
And that high frequencies in the horizontal direction will cause
bright dots away from the center in the horizontal direction.

Here are 2 images of more general Fourier components. They are images
of 2D cosines with both horizontal and vertical components. The one
on the left has 4 cycles horizontally and 16 cycles vertically. The
one on the right has 32 cycles horizontally and 2 cycles vertically.
(Note: You see a gray band when the function goes through gray = 128
which happens twice/cycle.)
You may begin to notice there is a lot of symmetry. For all REAL
(as opposed to IMAGINARY or COMPLEX) images, the FT is symmetrical
about the origin so the 1st and 3rd quadrants are the same and the
2nd and 4th quadrants are the same.
If the image is symmetrical about the x-axis (as the cosine images
are) 4-fold symmetry results.

MAGNITUDE VS. PHASE:

Recall that the definition of the Fourier Transform is:

	F(u,v) = SUM{ f(x,y)*exp(-j*2*pi*(u*x+v*y)/N) }
    and
	f(x,y) = SUM{ F(u,v)*exp(+j*2*pi*(u*x+v*y)/N) }

    where u = 0,1,2,...,N-1 and v = 0,1,2,...,N-1
	  x = 0,1,2,...,N-1 and y = 0,1,2,...,N-1
	  and SUM means double summation  over proper
	  x,y or u,v ranges

Note that f(x,y) is the image and is REAL, but F(u,v) (abbreviate as F)
is the FT and is, in general, COMPLEX. Generally, F is represented by
its MAGNITUDE and PHASE rather that its REAL and IMAGINARY parts, where:

	MAGNITUDE(F) = SQRT( REAL(F)^2+IMAGINARY(F)^2 )
	PHASE(F) = ATAN( IMAGINARY(F)/REAL(F) )

Briefly, the MAGNITUDE tells “how much” of a certain frequency component
is present and the PHASE tells “where” the frequency component is in
the image. To illustrate this consider the following.

Note that the FT images we look at are just the MAGNITUDE images.
The images displayed are horizontal cosines of 8 cycles, differing
only by the fact that one is shifted laterally from the other by
1/2 cycle (or by PI in phase). Note that both have the same FT
MAGNITUDE image. The PHASE images would be different, of course.
We generally do not display PHASE images because most people who see
them shortly thereafter succomb to hallucinogenics or end up in a
Tibetan monastery. Nevertheless, it is wise to remember that when
one looks at a common FT image and thinks about “high” frequency
power and “low” frequency power, this is only the MAGNITUDE part of
the FT.

By the way, you may have heard of the FFT and wondered if was different
from the FT. FFT stands for “Fast” Fourier Transform and is simply a
fast algorithm for computing the Fourier Transform.

ROTATION AND EDGE EFFECTS:

In general, rotation of the image results in equivalent rotation of its
FT. To see that this is true, we will take the FT of a simple cosine
and also the FT of a rotated version of the same function. The
results can be seen by:

At first, the results seem rather surprising. The horizontal cosine
has its normal, very simple FT. But the rotated cosine seems to
have an FT that is much more complicated, with strong diagonal
components, and also strong “plus sign” shaped horizontal and
vertical components. The question is, where did these horizontal
and vertical components come from? The answer is that the FT always
treats an image as if it were part of a periodically replicated
array of identical images extending horizontally and vertically to
infinity. And there are strong edge effects between the neighbors of
such a periodic array as can be seen by:

Thus, what we see as the FT in the “slant” image (lower right of the
image before last) is
actually the combination of the actual FT of the cosine function
and that caused by the edge effects of looking at a finite part of
the image. These edge effects can be significantly reduced by
“windowing” the image with a function that slowly tapers off to
a medium gray at the edge. The result can be seen by:

The windowed image is shown in the upper left. Its FT is shown in
the lower left. The non-windowed FT is shown in the upper right
and the actual, true FT of a cosine is shown in the lower right.
These images are all scaled differently and the comparison is only
qualitative, but it can be seen that the windowed image FT is much
closer to the true FT and eliminates many of the edge effects.

SOME IMAGE TRANSFORMS:

Now, with the above introduction, the best way to become familiar with
Fourier Transforms is to see lots of images and lots of their FTs.
First, an interesting pair of images, one sharp and clear, and the other
blurred and noisy.

There are 2 images, goofy and the degraded goofy, with FTs below each.
Notice that both suffer from edge effects as evidenced by the strong
vertical line through the center. The major effect to notice is that
in the transform of the degraded goofy the high frequencies in the
horizontal direction have been significantly attenuated. This is due
to the fact that the degraded image was formed by smoothing only in
the horizontal direction. Also, if you look carefully
you can see that the degraded goofy has a slightly larger background
noise level at high frequencies. This is difficult to see and perhaps
not even meaningful because the images are scaled differently, but if
really there, it is due to the random noise added to the degraded goofy.
Notice also that it is difficult to make much sense out of the low
frequency information. This is typical of real life images.

The next images show the effects of edges in images:

Notice the strong periodic component, especially
in the vertical direction for the bricks image. Horizontal components
appear closer together in the FT. In the blocks image, notice a
bright line going to high frequencies perpendicular to the strong
edges in the image. Anytime an image has a strong-contrast, sharp
edge the gray values must change very rapidly. It takes lots of high
frequency power to follow such an edge so there is usually such a line
in its magnitude spectrum.

Now lets look at a bunch of different shapes and their FTs.

Notice that the letters have quite different FTs, especially at the
lower frequencies. The FTs also tend to have bright lines that are
perpendicular to lines in the original letter. If the letter has
circular segments, then so does the FT.

Now lets look at some collections of similar objects:

Notice the concentric ring
structure in the FT of the white pellets image.
It is due to each individual
pellet. That is, if we took the FT of just one pellet, we would still
get this pattern. Remember, we are looking only at the magnitude
spectrum. The fact that there are many pellets and information about
exactly where each one is is contained mostly in the phase. The coffee
beans have less symmetry and are more
variably colored so they do not show the same ring structure. You
may be able to detect a faint “halo” in the coffee FT. What do you
think this is from?

Here are our first truly general images. Notice there is very little
structure. You can see a top left to bottom right slanting line in
the girl image FT. It is probably due to the edge between her hat
and her hair. There are also some small edge effects in both
images. The mandril image appears to have more high frequency
power, probably due to the hair.

The seafan image has a lot of little holes that are about the same
size and somewhat randomly oriented. The size of the holes is about
2 pixels wide so that corresponds to frequency components about 1/2
way out to the maximum. The strong horizontal components in the lake
image is probably due to the tree trunk edges.

Now, here is your first quiz. Consider an image that is all black
except for a single pixel wide stripe from the top left to the bottom
right. What is its FT? Also, consider an image that is totally
random. That is, every pixel is some random value, independent of all
other pixels. What is its FT?

Do you believe it? If not, you can check it yourself. By the way,
notice the single bright dot in the middle of the noise FT image.
Why is it there? Why does the noise FT look dark gray?

SOME FILTERS:

Now we start to illustrate the use of some filters on the girl image.
The first is a lowpass filter. The upper left is the original
image. The lower left is produced by:

	fft2d 128 < girlimage > girlfft
	mag2d 128 < girlfft > girlmag

The lower right is then produced by:

	fftfilt 128 low ideal 50 < girlfft > lpgirlfft
	mag2d 128 < lpgirlfft > lpgirlmag

Finally, the upper right is produced by:

	ifft2d 128 < lpgirlfft > lpgirl

To see the results:

The left side of the image we have seen before. In the lower right,
notice how sharply the high frequencies are cut off by the “ideal”
lowpass filter. Notice also that not very much power is being thrown
away beyond the circle that is cut off. In the upper right, the
reconstructed image is obviously blurrier due to the loss of high
frequencies. Overall contrast is still pretty good due to that fact
that not too much power was thrown away. Notice also that there are
obvious “ringing” artifacts in the reconstructed image. This is
due to the very sharp cutoff of the “ideal” filter. A Butterworth
or Exponential filter with reasonably low order would not cause these.

Now we will do a highpass filter. The following image is produced in
the same way as the previous one except:

	fftfilt 128 high butter 50 < girlfft > hpgirlfft

In other words, a butterworth filter of 1st order is used.

Notice in the lower right that this filter does not cut off sharply
at the 50% point as the lowpass did. However, the center bright
spot, which accounts for most of the power in the image, is clearly
gone. The image in the upper right, which looks totally black, in
fact is not totally black. If you use the colormap capability of
“dym” to stretch the gray values from 0-20 out over the entire
range, you can see that this highpass filter has preserved the
image information where there are very rapid changes in gray level.
Such a process is frequently what is desired in an edge detector.
However, it is not an improvement in the image. There are 2 problems.
First, it is too dark. This can be fixed by rescaling or re-contrast-
stretching the image after filtering. This is commonly done and is
easy. Second, and harder, is the fact that too much of the low
frequency tonal information is gone.

Image sharpening requires a “sharpening” filter or high frequency
emphasis filter. This kind of filter preserves some of the low
frequency information but relatively boosts the higher frequencies.
To do such a thing, we will construct our own filter which will be
piecewise-linear. The filter will be circularly symmetrical and will
have coefficients as follows:

	  0 	0.5
	 96	4.0
	127 	4.0

In other words, Fourier coefficients of frequency-distance 0 from the
origin will be multiplied by 0.5. As you go away from the origin or
zero frequency, out to frequency-distance 96, the multiplier will be
interploated between 0.5 and 4.0. From then outward, the multiplier
will be 4.0. So higher frequency coefficients are multiplied by
values greater than 1.0 and lower frequency coefficients are
multiplied by values less thatn 1.0. The overall net effect on the
image power is that it is unchanged. The above values are in a file
called “filter_coeffs”. To apply the filter, the following steps are
carried out:

	filttabler < filter_coeffs > filter_file
	fftfilt 128 file filterfile < girlfft > mfgirlfft

The rest of the image is constructed as before. To see the result:

Notice the relative brightness at high frequencies in the lower
right image. Which upper image is sharper? Which upper image looks
better? Portraits are one of the few contradictions to the general
principal that sharper is better.

Filtering can also be used to reduce noise. It is particularly
effective when the noise is confined to just a few frequencies:

The image on the upper left is goofy with a superimposed cosine
added to it, representing noise. In the lower left, notice the
strong cosine “dots” just to the left and right of the origin.
In the lower right, these “dots” have been removed ( I actually did
it with the “trace” capability in dym ). The resulting magnitude
file is then used with the “filter” command to filter the Fourier
coefficients. The file of coefficients is then inverse FT’d to get
the upper right image. The cosine “noise” is gone.

Life is not always this easy as is shown in the next example:

In this case, a grid has been placed over goofy. The lower left
shows the resulting FT. Notice that the grid is quite sharp so it
has lots of high frequencies so its impact on the frequency domain
is very spread out. Dym was again used to “paint” out the grid
frequencies as much as possible. The right half of the lower right
image is not painted because it is the symmetric reflection of the
left half and is not used by the filter.

YOUR ASSIGNMENT: (SHOULD YOU CHOOSE TO ACCEPT IT)

(1) Pick an image.
(2) FFT it and find the magnitude spectrum.
	see man for fft2d and mag2d
(3) Do something to the spectrum or the fft.
	ex:	filter
		fftfilt
		something like:
			cm
			double
			multiply by alternating +1,-1
			take phase only
			take magnitude only
(4) Reconstruct an image by inverse fft.
	see man for ifft2d
(5) Put the results together like the above images using "group"
	see man for group
(6) Explain your results (1-2 pages).
    More credit will be given to the imagination of what you do than
    to the correctness of your explanation.

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