Hemami 220 Lec HVS_freq 1  9/26/06
Using the Frequency Response of the Human
Visual System to Analyze Applying DT
Filtering to Digital Images
•
Outline
•
Understanding of “frequency” in space
•
Human contrast sensitivity function
•
Analysis of Image Filtering
ECE 220
Fall 2006
Prof. Hemami
Hemami 220 Lec HVS_freq 2  9/26/06
Applying DT System Analysis to Images
•
We know how to compute the frequency response
of a DT LTI filter given the impulse response
.
•
We can describe the behavior of a filter (lowpass,
highpass, bandpass) by looking at the frequency
response.
•
For image processing when the receiver is a
human, we need to understand how to relate
frequencies
to
what we see
— that is, to spatial
frequencies in the image.
hn
()
ω
ˆ
Hemami 220 Lec HVS_freq 3  9/26/06
Spatial Frequency
•
A frequency in time is a function of a single
variable, e.g.
•
A frequency in space is a function of two variables,
x
and
y
in the horizontal and vertical directions.
2
π
17
t
cos
Hemami 220 Lec HVS_freq 4  9/26/06
Units of Spatial Frequency
•
The spatial frequency unit is
cycles/degree
.
•
cycles
refers to the number of cycles of a sine (or
cosine)
•
degree
measures the size of the field of view
occupied by the stimulus
180
o
field of view
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Defining Spatial Frequency
10 cycles
60
o
The spatial frequency
of the stimuli is
10 cycles/60
o
=
0.167 cycles/degree
Hemami 220 Lec HVS_freq 6  9/26/06
Spatial Frequency is NOT Constant for an
Image
10 cycles
45
o
Same stimuli, different
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 Fall '05
 JOHNSON
 Frequency, Lowpass filter, Spatial frequency, digital Images, 1dimension, Lec HVS_freq, Prof. Hemami

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