# Digital Image Processing Questions and Answers – Gaussain Lowpass and Sharpening Frequency Domain Filters

This set of Digital Image Processing Multiple Choice Questions & Answers (MCQs) focuses on “Gaussain Lowpass and Sharpening Frequency Domain Filters”.

1. If the Gaussian filter is expressed as H(u, v) = e(-D2 (u,v)/2D 02),where D(u, v) is the distance from point(u, v), D0 is the distance defining cutoff frequency, then for what value of D(u, v) the filter is down to 0.607 of its maximum value?
a) D(u, v) = D0
b) D(u, v) = D02
c) D(u, v) = D03
d) D(u, v) = 0

Explanation: For the given Gaussian filter of 2-D image, the value D(u, v) = D0 gives the filter a down to 0.607 of its maximum value.

2. State the statement as true or false. “The GLPF did produce as much smoothing as the BLPF of order 2 for the same value of cutoff frequency”.
a) True
b) False

Explanation: For the same value of cutoff frequency, the GLPF did not produce as much smoothing as the BLPF of order 2, because the profile of GLPF is not as tight as BLPF of order 2.

3. In general, which of the following assures of no ringing in the output?
a) Gaussian Lowpass Filter
b) Ideal Lowpass Filter
c) Butterworth Lowpass Filter
d) All of the mentioned

Explanation: Using Gaussian Lowpass Filter no ringing is assured, but Ideal Lowpass Filter and Butterworth Lowpass Filter of order 2and more produces significant ringing.

4. The lowpass filtering process can be applied in which of the following area(s)?
a) The field of machine perception, with application of character recognition
b) In field of printing and publishing industry
c) In field of processing satellite and aerial images
d) All of the mentioned

Explanation: In case of broken characters recognition system, LPF is used. LPF is used as preprocessing system in printing and publishing industry, and in case of remote sensed images LPF is used to blur out as much detail as possible leaving the large feature recognizable.

5. The edges and other abrupt changes in gray-level of an image are associated with_________
a) High frequency components
b) Low frequency components
c) Edges with high frequency and other abrupt changes in gray-level with low frequency components
d) Edges with low frequency and other abrupt changes in gray-level with high frequency components

Explanation: High frequency components are related with the edges and other abrupt changes in gray-level of an image.
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6. A type of Image is called as VHRR image. What is the definition of VHRR image?
a) Very High Range Resolution image
b) Very High Resolution Range image
c) Very High Resolution Radiometer image
d) Very High Range Radiometer Image

Explanation: A VHRR image is a Very High Resolution Radiometer Image.

7. The Image sharpening in frequency domain can be achieved by which of the following method(s)?
a) Attenuating the high frequency components
b) Attenuating the low-frequency components
c) All of the mentioned
d) None of the mentioned

Explanation: The Image sharpening in frequency domain is achieved by attenuating the low-frequency components without disturbing the high-frequency components.

8. The function of filters in Image sharpening in frequency domain is to perform reverse operation of which of the following Lowpass filter?
a) Gaussian Lowpass filter
b) Butterworth Lowpass filter
c) Ideal Lowpass filter
d) None of the Mentioned

Explanation: The function of filters in Image sharpening in frequency domain is to perform precisely reverse operation of Ideal Lowpass filter.
The transfer function of Highpass filter is obtained by relation: Hhp(u, v) = 1 – Hlp(u, v), where Hlp(u, v) is transfer function of corresponding lowpass filter.

9. If D0 is the cutoff distance measured from origin of frequency rectangle and D(u, v) is the distance from point(u, v). Then what value does an Ideal Highpass filter will give if D(u, v) ≤ D0 andifD(u, v) >D0?
a) 0 and 1 respectively
b) 1 and 0 respectively
c) 1 in both case
d) 0 in both case

Explanation: Unlike Ideal lowpass filter, an Ideal highpass filter attenuates the low-frequency components and so gives 0 for D(u, v) ≤ D0 and 1 for D(u, v) >D0.

10. What is the relation of the frequencies to a circle of radius D0, where D0 is the cutoff distance measured from origin of frequency rectangle, for an Ideal Highpass filter?
a) IHPF sets all frequencies inside circle to zero
b) IHPF allows all frequencies, without attenuating, outside the circle
c) All of the mentioned
d) None of the mentioned

Explanation: An Ideal high pass filter gives 0 for D(u, v) ≤ D0 and 1 for D(u, v) >D0.

11. Which of the following is the transfer function of the Butterworth Highpass Filter, of order n, D0 is the cutoff distance measured from origin of frequency rectangle and D(u, v) is the distance from point(u, v)?
a) b) c) d) none of the mentioned

14. For a given image having smaller objects, which of the following filter(s), having D0 as the cutoff distance measured from origin of frequency rectangle, would you prefer for a comparably smoother result?
a) IHLF with D0 15
b) BHPF with D0 15 and order 2
c) GHPF with D0 15 and order 2
d) All of the mentioned

Explanation: For the same format as for BHPF, GHPF gives a result comparably smoother than BHPF. However, BHPF performance for filtering smaller object is comparable with IHPF.

15. Which of the following statement(s) is true for the given fact that “Applying Highpass filters has an effect on the background of the output image”?
a) The average background intensity increases to near white
b) The average background intensity reduces to near black
c) The average background intensity changes to a value average of black and white
d) All of the mentioned

Explanation: The Highpass filter eliminates the zero frequency components of the Fourier transformed image HPFs are applied on. So, the average background intensity reduces to near black.

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