A short quiz on computational stereo

Adrian F. Clark

Why is a pin-hole camera rarely used in practice?
- Only nearby objects are in focus
- Exposure times are long
- Only distant objects are in focus
- The pin-hole is always too large for an image to be made
A pin-hole lets very little light into the camera, so exposure times are long.
In order to use computational stereo, we must find:
- the location of different features in the left and right images
- the location of the same feature in the left and right images
- different pixels in the left and right images
- the same pixel in the left and right images
We must find the same feature in the left and right images in order to estimate the distance to it.
A particular measurement of the height of a building is repeated many times. The mean and standard deviation of the measurements are calculated and found to be (50.0 +/- 0.5) m. Assuming the uncertainties are Gaussian-distributed, which of the following statements is true?
- With 95% confidence, you can say that the height of the building is in the range 49.75--50.25 m
- With 95% confidence, you can say that the height of the building is in the range 49.0--51.0 m
- With 95% confidence, you can say that the height of the building is in the range 48.5--51.5 m
- With 95% confidence, you can say that the height of the building is in the range 49.5--50.5 m
We obtain 95% confidence with 2 standard deviations (SDs) from the mean. As the SD is 0.5 m, 2 x SD = 1 m, and that corresponds to 49--51 m.
What is disparity?
- The relative size of the same object when it is nearby and far away
- The distance to an object in an image
- The foreshortening of an object with distance
- The discrepancy in the apparent position of an object in the left and right images
Disparity, which is essentially the same as parallax, is the `shift' in appearance between the left and right eyes' views.
Which of the following is usually the most important source of error in computational stereo?
- locating the centre of the image
- measuring the baseline inaccurately
- failing to align the optical axes to be parallel
- having dissimilar cameras
The only item listed which is measured, which means its error can be estimated and accommodated in calculations of distance, is the baseline.
The disparity of a nearby object is:
- less than that of a distant object
- the same as that of a distant object
- indeterminate
- greater than that of a distant object
Disparity increases as an object approaches the cameras. Disparity D and distance Z are related by Z = fB / D so as Z increases then D must decrease.
When dividing two quantities that have associated errors, which of the following procedures is correct?
- We subtract the absolute errors
- We add the fractional errors
- We add the absolute errors
- We subtract the fractional errors
When we multiply or divide quantities, we must add their fractional errors.
How does disparity vary with distance?
- disparity is inversely proportional to the square of distance
- disparity is proportional to distance
- disparity is proportional to the square of distance
- disparity is inversely proportional to distance
Disparity D and distance Z are related by Z = fB / D so Z is inversely proportional to D.
Which of the following equations determines the distance to an object $Z$ given its disparity $D$, baseline $B$ and focal length $f$?
- $Z = fB / D$
- $Z = D / fB$
- $Z = fD / B$
- $Z = B / fD$
This is an equation that you need to be very familiar with; if you got this wrong, make sure you learn the equation and are able to derive it!
If the length of the sides of a brick is (20.00 +/- 0.02) cm, the length of two bricks placed side by side is
- The result is (40.02 +/- 0.04) cm
- The result is (40.04 +/- 0.02) cm
- The result is (40.00 +/- 0.02) cm
- The result is (40.00 +/- 0.04) cm
When we add quantities, we must add their errors.