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EECS 556 – Image Processing– W 09
2D SPACE SIGNALS/SYSTEMS
2D discrete‐space signals and systems
n,m are integers
We are going to study properties of these signals and their transformations
LSI (linear shift invariant) systems
An LSI system is completely specified by its impulse response.
sifting property of the delta function
superposition
Discrete convolution
2D Discrete‐Signal Fourier SeriesIf g[n,m] is periodic with period (N,M):
with
2D discrete‐space Fourier transform
2D discrete‐space Fourier transform (DSFT) of a 2D discrete‐space signal g[n,m] :
Inverse 2D DSFT:
Valid if g[n,m] has: finite energy or absolutely summable
Convergence
If g absolutely summable:
then
If g is square summable(energy signal):
DSFT is periodic2D DSFT is periodic with period (2pi, 2pi):
Properties
Multiplication: [what’s FT in continous space? ]
(2pi ) periodic convolution
Notice
•Transpose
•Reflection property
•Rotational symmetry propertyIf g[n,m] has 2‐ 4‐ or 8‐fold rotational symmetry, then so does its 2D DSFT.
Properties
PropertiesShift:
Convolution:
Delta function:
Example: DSFT of moving average filter
],[],[91],)[(
,lnkmhlkfnmhf
lk−−=∗ ∑
111
111
111
h
Complex exponential
‐
repeated periodically with horizontal period 2 pi and vertical period 2 pi
Special case:
Periodic signalsIf g[n,m] is periodic with period (N,M):
Sampling revisitedRelationship between spectra of original and sampled signals
This expressions holds regardless of whether the CS signal ga(x, y) is band‐limited!
ga (x,y) gd [n,m]
Magnitude and Phase
2D DSFT is generally complex
• Encode image using magnitude and phase [image encoding]• How to retrieve phase from magnitude [image restoration]• Reconstruct signal from just magnitude or phase
impossible unless….
☺
inverse FT:
inverse FT:
inverse FT:
Why this is so bad?
Inverse transform of
Autocorrelation of g!
Later in this course we will explore iterative techniques for extracting original image from phase of image spectrum
LSI systems in Frequency‐domain
• A LSI system is characterized completely by its frequency response
Good news:•BIBO stable iff its impulse response h[n,m] is absolutely summable•DSFT always exists for absolutely summable signals•H(wx, wy) always exists for stable LSI systems
Intro to Filter Design
Crucial in pre‐processing tasks such as•Noise removal (low pass filtering)•Enhancing (high pass filtering)
How to design a low‐pass filter in 2D?Let’s give a look at this 1D filter:
h[n] = [1/4 1/2 1/4]
Frequency response of this filter?
Designing a 1D low pass filter
h[n] = [1/4 1/2 1/4]
Designing a 1D low pass filter
h[n] = [1/4 1/2 1/4]
Since:
pi‐pi
unit delay or shift associated with this filter!
It does actas a low pass filter!
Designing a 2D low pass filter
• In 2D: h[n] = [1/4 1/2 1/4]
• How about this?
Designing a 2D low pass filter
Designing a 2D low pass filter
• H is real (due to Hermitian symmetry of h[n,m])
~ constantno shift!
‐ pi ‐ pi
Designing a 2D low pass filter
It doesn’t kill frequencies at pi or ‐pi
Designing a 2D low pass filter
Not a good 2D low pass filter:We would like a lowpass design that is zero along the edge of the ± pi ,± pi box
],[],[91],)[(
,lnkmhlkfnmhf
lk−−=∗ ∑
111
111
111
h
Designing a 2D low pass filter
Not that good after all….
Designing a 2D low pass filter
Let’s try his:
Notice: this is separable
Designing a 2D low pass filter
Let’s try his:
Notice: this is separable
Let’s look see at 45o slice:
This filter design is not circularly symmetric!
Rotation invarianceCS: rotation invariant if rotating any input image causes the output image to rotate by the same amount
What should we expect after filtering with
Designing a 2D low pass filter
Let’s try again!This time we start from the frequency domain
In 1D we liked
2D filter with the following circularly symmetric frequency response
Designing a 2D low pass filter
•This is not FIR but IIR
•There is no analytical solution for the inverse 2D DSFT of this spectrum
•Trick: use matlab to get h[n,m]
‐ design H in the frequency domain‐ use ifft2.m to estimate h (numerical solution)
Designing a 2D low pass filterIf we don’t care about having a FIR filter, then let’s look at the ideal low pass filter:
(2pi, 2pi)
Let’s compute h[n,m]…
We can use the sampling theorem: h[n,m] is sampled from some ha(x, y)
We follow 3 steps:
1. Let’s find the spectrum of ha
(2pi, 2pi)
2. Compute inverse FT of Ha
3. Compute inverse FT of Ha
Polar coordinates