# Midterm Exam Total Points: 100 PLEDGE: I have not given/received

```Visual Perception: Midterm Exam
Total Points: 100
PLEDGE: I have not given/received any assistance to/from anybody in this exam.
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1. Explain why windows in a house, when viewed from the inside, appear
transparent during the daylight hours, but act like mirrors at night. [4]
2. The sky on earth looks blue, but the same sky on moon looks black. Why? [4]
3. The color of the runway lights in airports is blue, and not red. Why? [4]
4. Explain how our perception of the Mach band would change if we had lateral
excitation instead of lateral inhibition. [4]
5. Cataracts are tissues that develop clouding the eye’s lenses. Cataracts mostly
happen in the old age. But there are also cased of juvenile cataracts that occur in
babies and often are existent from birth. In 1960s, when the art of cataract
removal was perfected, several old people were operated and cured. However,
operations performed on a bunch of young men who had juvenile cataracts, did
not cure them. Why? [3]
6.
The image on the left shows a
display made of two projectors P1
and P2. At any pixel x, the
contribution of the two projectors at
this pixel are expressed by P1(x)
and P2(x) respectively. To remove
the high brightness in the overlap
region, the intensity at any pixel x
in this region is blended using the
functions A1(x) and A2(x) from
projector P1 and P2 respectively
such that A1(x) + A2(x) = 1 and the
combined intensity at pixel x from
the two projectors is given by
A1(x)P1(x) + A2(x)P2(x).
Two types of function can be used.
(1) In the first, A1 and A2 are
assigned as follows.
A1(x) = d1/(d1+d2)
A2(x) = d2/(d1+d2)
(2) In the second, A1 and A2 are
assigned as follows
A1(x) = cos(d1/(d1+d2)*90)
A2(x) = cos(d2/(d1+d2)*90)
Which of these functions would
look perceptually better? Why? [5]
7.
In the image on the left, the gray
rectangle on the right looks lighter
than the rectangle on the left. But
they are actually the same gray. Can
you explain this by lateral
inhibition? [4]
8. We know that edge detection can be performed by the eye in different resolutions.
Do you think that the illumination and the reflectance edges will show up in
9. A person was presented an annular ring of luminance of 100cd/m2 with a circular
inset of luminance 50cd/m2. In an adjacent region, he was presented with a
circular inset of 150cd/m2 and asked to adjust the luminance of the annular ring
surrounding it so that it creates similar perception as that of the former pair. What
luminance do you expect him to adjust to? [4]
10. A sinusoidal grating has a mean of 100cd/m2 and contrast (defined by the range of
the luminance in the grating) of 50cd/m2. It is projected on a screen of width 20
inch with resolution of 50pixels/inch. The number of cycles of the grating made
over the whole screen is 10. What is the distance from the screen where the user
should be place so that this grating subtends 3cycles per degree of the eye? [5]
11. A two month old was being very jittery and crying. The room is lit by a whitish
tube light. In order to pacify him, the mother puts on a very bright yellowish table
lamp. The kid suddenly finds renewed interest in everything around him and stops
crying. What perceptual phenomenon can explain this behavior of the kid? [5]
12. You have your framed graduation certificate hanging on the wall. When you view
if from a distance of 12 or more feet, you cannot see the letters clearly. As you
approach the frame, slowly the letters become clearer and clearer. However, if
you are within an inch or two, you find that the letters have again become blurred.
Can you explain this with CSF? [4]
13. You are going on a train. The track is lined by electric poles placed regularly
along the track so that it is just a few feet away when you look out of the window.
When the train just starts you can make out these poles clearly as they keep
moving away from you. As the train increases speed, slowly the poles become
blurred as they move away from you. Can you explain this phenomenon with
CSF? [5]
14. In a movie auditorium, the projection system is used with mean luminance of 1222 foot Lamberts for refresh rates of 48 Hz. However, it is said that a refresh rate
has to be increased to 60 Hz for higher luminance projection system. Explain this
with CSF. [5]
15. Assume a gray scale projector which shows no spatial variation in luminance. The
contrast of the projector is defined by the ratio of the maximum and the minimum
luminance that can be projected by the projector. For an ideal projector, no light is
projected for black and hence infinite contrast can be achieved. However, in this
practical projector, black offset is present. What is the effect of this black offset
on the contrast of the projector? Based on CSF, which range of intensities (low
range, high range or mid range) will be more adversely affected perceptually by
the presence of this black offset and why? [2+4=6]
16. Let the maximum spatial frequency detected by a human at luminance L be 30
cycles/degree of the angle subtended on the human eye. Nyquist sampling
condition says that to generate a spatial pattern of a certain frequency f on a
display, at least 2f number of pixels are required. From this find out the minimum
display resolution needed for a person at a distance d, so that he can see all the
different spatial frequencies he is sensitive to. Plot the r with respect to d to find
how the minimum required resolution changes with the distance of the observer.
[4+4=8]
17. An audio signal of bandwidth 200 MHz, centered at 700 MHz is being
transmitted. There are several kinds of noise in the system. It is found that if the
noise has a bandwidth of 140 MHz and is also centered at 700 MHz, a noise of
large amplitude large amplitude can be tolerated without being audible. However,
as the frequency at which the noise is centered reduces or increases, the noise
becomes more and more audible. This phenomenon in audio is analogous to
which visual phenomenon? How does that explain these phenomena? If the
frequency at which the noise is centered reduces or increase, the noise can still be
made inaudible, but this time by reducing its amplitude significantly. What
phenomenon describes the noise being inaudible in this case? If both the noise
and the signal are centered at 300MHz, it is found that the bandwidth of the noise
needs to be reduced to 100 MHz to be inaudible. What does this tell you?
[3+2+2=7]
18. We have n devices. The gamut of the device i, 1≤ i ≤ n, is denoted by Gi. To
perform gamut matching, a common gamut Gc which is the intersection of all Gis
has to be found. And then every Gi should be transformed to the common gamut
Gc. In what kind of situations do you think this method of matching colors across
devices will be impossible? Even if it is possible, what kind of image degradation
would you expect from gamut matching? [5]
19. Let us consider a linear display with no spatial variation in color and no black
offset. The chromaticity coordinates of the red, green and blue channel of this
display are given by (0.5, 0.4), (0.2, 0.6) and (0.1, 0.2) respectively. The
maximum luminances of the red, green and blue channel are 100, 200 and
80cd/m2 respectively. Generate the matrix that converts the RGB coordinates for
this device to the XYZ coordinates. What is the XYZ coordinates of the color
generated by the RGB input (0.5, 0.75, 0.2) on this device? [5+2=7]
20. Let a projector coordinate system be defined by (s,t) with origin (0,0) at the center
of the projector. The projector shows a symmetric fall-off in the maximum
luminance for each channel from the center to the fringes. The fall off is inversely
proportional to the square of the distance of (s,t) from the center of the projector.
The black offset does not vary spatially and is defined by the XYZ vector (XB, YB,
ZB). Each channel has a quadratic input transfer function. If the color with
maximum luminance for red, green and blue channel at the center of the projector
is defined by the vectors (Xr, Yr, Zr) , (Xg, Yg, Zg) and (Xb, Yb, Zb), what would be
the equation defining XYZ coordinates of the color for any input (ir, ig, ib) at any
pixel (s,t) of the projector. [7]
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