A short quiz on intermediate-level vision

Adrian F. Clark

1. How are BRIEF features matched?

   • angle between vectors
   • OR

   • exclusive OR

   • Euclidean distance

   BRIEF is intended to run in real time and uses strings of binary digits as the descriptor. The easiest way to measure dissimilarity of two bit-strings is to use exclusive-OR and then count the number of set bits.

2. Which of the following is not a valid equation for a straight line?

   • the equation $y^2 + x^2 = r^2$

   • the equation $x = m y - c$
   • the equation $x cos A + y sin A = r$
   • the equation $y = c + x / m$

   The cos-and-sin expression listed is the parametric form used by the Hough transform, while the second and third expressions can be manipulated into the form $y = m x + c$, which is the normal equation of a straight line. However, the other one cannot: it is the equation of a circle of radius r.

3. How should peaks chosen from the Hough accumulator array?

   • Largest first

   • Nearest to origin first
   • Smallest first
   • Smallest radius first

   The peak with the most lines in the accumulator contributing to it is the most significant line, and this is the pixel with the largest value.

4. Why does the Canny edge detector perform non-maximum suppression?

   • to stop the edges being maxima
   • to quantise the direction of the edge
   • to stop the edges being too bright

   • to make each edge only one pixel wide

   Non-maximum suppression is intended to make each edge segment only one pixel wide.

5. Which of the following schemes is unsuitable for matching arbitrary features in images?

   • matched filtering
   • image differencing

   • the Laplacian

   • cross-correlation

   The Laplacian emphasizes isolated points in images, and is not suited to matching any other feature in an image.

6. What is the purpose of the Hough transform?

   • To find edges in images

   • To find the parameters of lines, etc

   • To calculate an accumulator array
   • To join together edges in images

   The purpose of the Hough transform is to estimate the parameters that describe some image feature -- though we have looked only at straight lines in this module.

7. Which of the following schemes is unsuitable for matching corners in images?

   • FAST
   • Moravec

   • the Laplacian

   • Harris and Stephens

   The Laplacian emphasizes isolated points in images, and is not suited to matching any other feature in an image.

8. What information does the Canny edge detector find about each piece of edge?

   • the magnitude and direction

   • the direction
   • the magnitude
   • the magnitude, direction and width

   Canny finds the magnitude and direction of each line by forming differences along each row and column.

9. What does one do with the Hough accumulator to find lines?

   • Each line in the accumulator is a line in the image
   • Each 3 x 3 region in the accumulator is a line in the image

   • Each peak in the accumulator is a line in the image

   • Each corner in the accumulator is a line in the image

   As the code in the notes illustrates clearly at the end of Chapter 5, peaks in the Hough accumulator correspond to lines in the image. This is because more image pixels have `voted' for that set of parameters than any other set.

10. What is the profile of the mask used in the first stage of Canny?

    • Laplacian
    • 3 x 3 blur

    • Gaussian

    • Sobel

    The first stage of Canny involves convolving with a mask with a Gaussian profile.