# A short quiz on low-level vision

1. What does Otsu's method do?

• maximizes the within-class variance
• minimizes the background variance
• minimizes the within-class variance

• maximizes the background variance
2. Which of the following is not a valid equation for a straight line?

• the equation $x = m y - c$
• the equation $y = m + x - c$

• the equation $y = c + x / m$
• the equation $x cos A + y sin A = r$
3. What does one do with the Hough accumulator to find lines?

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

• Each 3 x 3 region in the accumulator is a line in the image
4. Which of the following is a sensible region descriptor?

• the colour of the region's background
• the value of the centre pixel of a region
• the circularity of a region

• the location of a region within an image
5. Why is a recursive region labelling algorithm poor in practice?

• it tends to overflow the computer's heap
• it is difficult to program
• it is too slow
• it tends to overflow the computer's stack

6. If a histogram has two peaks, where is the best place to put a threshold to separate foreground from background?

• half-way up the higher peak
• at the lower peak
• in the bottom of the dip between the two peaks

• half-way down the lower peak
• at the higher peak
7. What is the purpose of the Hough transform?

• To find edges in images
• To calculate an accumulator array
• To join together edges in images
• To find the parameters of lines, etc

8. In the parametric Hough transform, what is the accumulator array indexed by?

• end-points

9. What shape is the curve drawn in the parametric accumulator array from a single pixel in an image?

• a straight line
• an $x^2$ shape
• a $1/x$ shape
• a sinusoid

10. How many lines are drawn in the parametric accumulator array from a single pixel in an image?

• as many as there are angle entries in the accumulator array

• 1
• as many as there are radius entries in the accumulator array
• 360
11.