CPSC 425 Stereo, Motion and Optical Flow 20/21 (Term 1) Practice Questions
Multiple Part True/False Questions. For each question, indicate which of the statements, (A)–(D), are true and which are false? Note: Questions may have zero, one or multiple statements that are true.
Question 1. Consider conditions under which an epipolar constraint used in stereo matching holds between images from two cameras. Which of the following condi- tions are true? Which are false? Note: You can assume that the cameras perform standard perspective projection.
(A) The two cameras must have coplanar projection planes.
(B) The two cameras must face in the same direction (i.e., have parallel optical axes).
(C) The two images must be rectified.
(D) There are no restrictions on camera locations or orientations, an epipolar constraint always applies.
Question 2. Stereo matching can be performed by correlating windows of pixels between the two images. But, it is difficult to know what window size to use. The following statements identify problems when the selected window size is too large. Which are true? Which are false?
(A) There will be more false matches due to ambiguity and image noise.
(B) The exact location of correct matches will be known with less accuracy. (C) Places where depth is discontinuous will be poorly matched.
(D) The epipolar constraint is not as effective to limit the number of matches.
Question 3. The Lucas–Kanade method makes several assumptions about motion and optical flow. Which of the assumptions, (A)–(D), are true of Lucas–Kanade and which are false? Note: This is a question about the Lucas–Kanade method, not about assumptions that may or may not be true, in general, about the world.
(A) Corresponding points in a sequence of images of a moving object have ex- actly the same brightness values.
(B) Sampling in x, y and t is frequent enough that the partial derivative, Ix, Iy and It, are well-defined
(C) The motion, [u,v], is constant in the selected window about each image point, [x, y].
(D) The matrix
Ix2 IxIy
IxIy Iy2
has rank 2 in the selected window about each image point.
Short Answer Questions.
Question 4. The second edition of ’s textbook, Artificial Intelligence, published by Addison-Wesley, contains a discussion of stereo vision. Included is an extended example based on a stereo pair of images shown in the text as a figure. The figure caption reads, in part, “The two pictures are arranged so that you can see depth yourself with the aid of a stereoscopic viewer.” At the last minute, prior to printing, a graphic designer at Addison-Wesley made the artistic decision that the stereo pair looked better arranged above and below (i.e., top to bottom) rather than left to right. Accordingly, that is how the initial press run was printed – a left/right stereo pair printed with the left image above and the right image below.
Winston was not amused and insisted that Addison-Wesley reprint the entire book again, at its cost, with the figure in question corrected. Aside: This is a true story.
Briefly describe why Winston would insist that the figure be corrected.
Question 5. As we have seen, determining corresponding points in the left image and in the right image is the hardest part of stereo vision. A variety of things can go wrong in stereo matching. In a sentence or two for each, give a specific example of a scene where
(a) there are not enough locally distinct features that match (b) there are too many locally distinct features that match (c) locally distinct features match incorrectly
Hint: This is a question about the problem of stereo vision, not a question about the properties of any particular algorithm or technique used to do stereo matching.
Question 6. Lucas–Kanade estimate the 2-D motion, [u, v], at a given point, [x, y], in an image by computing the partial derivatives, Ix, Iy, It, in a window centered at the given [x, y]. Their method assumes all points in the window are “inliers” with respect to the estimation of a single motion, [u, v].
Suppose, instead, that there are multiple, distinct motions occuring within the window. Describe, in a few sentences, how you might use a Hough transform approach to detect and determine the multiple motions.