A short quiz on computational stereo

Adrian F. Clark

1. 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

   • The same pixel in the left and right images
   • Different pixels 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.

2. Why is a pin-hole camera rarely used in practice?

   • Only nearby objects are in focus

   • Exposure times are long

   • The pin-hole is always too large for an image to be made
   • Only distant objects are in focus

   A pin-hole lets very little light into the camera, so exposure times are long.

3. Which of the following is usually the most important source of error in computational stereo?

   • Failing to align the optical axes to be parallel

   • Locating the centre of the image
   • Measuring the baseline inaccurately
   • 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.

4. Which of the following is not an assumption we have made in considering computational stereo?

   • The object is stationary

   • The cameras are identical
   • The optical axes of the cameras are parallel
   • The images are captured by both cameras simultaneously

   The one thing we haven't assumed is that the object is stationary: this would make motion capture practically impossible!

5. As the baseline, the distance between the cameras, increases, what happens to the disparity?

   • The disparity decreases
   • The disparity stays the same

   • The disparity increases

   • The product of disparity and focal length stays the same

   The disparity must increase as the baseline increases. You can easily show this by pointing at something and moving your head from side to side.

6. How does disparity vary with distance?

   • Disparity is proportional to the square of distance

   • Disparity is inversely proportional to distance

   • Disparity is proportional to distance
   • Disparity is inversely proportional to the square of distance

   Disparity D and distance Z are related by Z = fB / D so Z is inversely proportional to D.

7. What is disparity?

   • The discrepancy in the apparent position of an object in the left and right images

   • The distance to an object in an image
   • The relative size of the same object when it is nearby and far away
   • The foreshortening of an object with distance

   Disparity, which is essentially the same as parallax, is the `shift' in appearance between the left and right eyes' views.

8. Which of the following equations determines the distance to an object $Z$ given its disparity $D$, baseline $B$ and focal length $f$?

   • $Z = D / fB$
   • $Z = B / fD$

   • $Z = fB / D$

   • $Z = fD / B$

   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!

9. In a camera, the incoming light is focussed onto:

   • the lens
   • the front focal plane

   • the back focal plane

   • the optical axis

   The place where a camera forms its image is known as the back focal plane.

10. The disparity of a nearby object is:

    • greater than that of a distant object

    • indeterminate
    • the same as that of a distant object
    • less 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.