# CSC420 Intro to Image Understanding Assignment 2 solved

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1. Interest Point Detection
(a) [2 points] Write two functions for computing the Harris corner metric using Harris (R) and
Brown (harmonic mean) methods. Display your results for the attached image building.jpg
showing your cornerness metric output. Compare the results corresponding to two different
methods. For Harris you can use the code provided in Tutorial C.
(b) [2 points] Write your own function to perform non-maximal suppression using your own
functions of choice. Use a circular element, and experiment with varying radii r as a parameter
for the output of harmonic mean method. Explain why/how the results change with r . MATLAB
users may want to use functions ordfilt2(), however it can be easily implemented.
(c) [2 points] Write code to search the image for scale-invariant interest point (i.e. blob) detection
using the Laplacian of Gaussian and checking a pixel’s local neighbourhood as in SIFT. You
must find extrema in both location and scale. Find the appropriate parameter settings, and display
your keypoints for synthetic.png using harmonic mean metric. Hint: Only investigate pixels with
LoG above or below a threshold.
(d) [1 point] Use open-source implementation of another local feature descriptor that is not covered
in the class, and show the output keypoints on synthetic.png and building.jpg. Describe the main
ideas of your algorithm of choice in a few sentences. You may want to look at Slide 7 in Lecture
8-A for a list of existing methods.
2. SIFT Matching (For this question you will use interest point detection for matching using SIFT. You
may use a SIFT implementation (e.g. http://www.vlfeat.org/), or another, but specify what you use)
(a) [0.5 points] Extract SIFT keypoints and features for book.jpg and findBook.jpg.
(b) [1.5 points] Write your own matching algorithm to establish feature correspondence between the
two images using the reliability ratio on Lecture 8. You can use any function for computing the
distance, but you must find the matches yourself. Plot the percentage of true matches as a
function of threshold. Also, after experimenting with different thresholds, report the best value.
(c) [2 points] Use the top k correspondences from part (b) to solve for the affine transformation
between the features in the two images via least squares using the Moore-Penrose pseudo inverse.
What is the minimum k required for solving the transformation? Demonstrate your results for
various k . Use only basic linear algebra libraries.
(d) [0.5 points] Visualize the affine transformation. Do this visualization by taking the four corners
of the reference image, transforming them via the computed affine transformation to the points in
the second image, and plotting those transformed points. Please also plot the edges between the
points to indicate the parallelogram. If you are unsure what the instruction is, please look at
Figure 12 of [Lowe, 2004].
(e) [1.5 points] Write code to perform matching that takes the colour in the images into account
during SIFT feature calculation and matching. Explain the rationale behind your approach. Use
colourTemplate.png and colourSearch.png, display your matches with the approach described
in part (d).
3. RANSAC
(a) [0.5 points] Assuming a fixed percentage of inliers p = 0.7 , plot the required number of
RANSAC iterations (S) to recover the correct model with a higher than 99% chance (P), as a
function of k (1:20), the minimum number of sample points used to form a hypothesis.
(b) [0.5 points] Assuming a fixed number of sample points required ( k = 5 ), plot S as a function of
the percentage of inliers p (0.1 : 0.5)
(c) [1 point] If k = 5 and the initial estimate on the percentage of inliers is p = 0.2 , what is the the
required number of iterations to recover the correct model with P≥ 0.99 chance? Assume that
you have implemented this and there are 1500 matches in total. In iteration #15, 450 points agree
with the current hypothesis (i.e. their error is within a preselected threshold), would the number of
required iterations chance? explain how and why.