MONT 104S Perception and VR
College of the Holy Cross, Fall 2008
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Assignment 3Due: Wednesday, September 24, in class
Problem 1: Finding research articles on vision topics a) In class you learned how to use electronic databases to find research articles on a given topic. For this problem, each of you has been assigned a specific topic related to vision. Use the library databases to find at least 3 research articles relating to your assigned topic. The articles should be from good scientific sources, meaning either edited books or peer-reviewed journals. They should not be from popular science magazines or wikipedia. Cite each of the articles, giving the authors, date of publication, title of the article, journal name and volume and page numbers. A sample citation might look as follows: Duffy, C. J. & Wurtz, R. H. (1995). Response of Monkey MST neurons to optic flow stimuli with shifted centers of motion. Journal of Neuroscience, 15, 5192 - 5208. Here is a link to the list of assigned topics. b) Pick one of your articles and write a short paragraph describing what it is about. You do not need to read the full article. You should be able to get an idea of what it is about by reading the abstract. Do not copy from the abstract directly, but try to summarize it in your own words. What question did the authors set out to answer? What kind of experiments did they perform? What conclusions did they come to? Please submit a copy of the abstract for the paper in addition to your summary.
Problem 2: Judging where you are going In class we learned that, for a person moving in a straight line, the motion of points in the image forms a radial pattern. The center of this pattern indicates the person's direction of motion through the scene. In this problem, you will calculate the image velocities of points in the world for a person moving forward and a little to the right. You will then find the center of the radial pattern to determine that person's direction of motion. Consider a person moving forward and to the right toward a flat wall that is covered in dots, as shown in the picture below.
The observer is moving so that in 1 sec she moves forward by 100 cm, and to the right by 50 cm. This means that every point in the scene will change its position relative to the person. Specifically, each point's position will decrease in depth (Z value) by 100 cm and decrease in horizontal position (X value) by 50 cm. Below is a table of beginning and ending positions for 8 points for 1 second of motion by the observer.
a) For each point given above, calculate the image positions of the starting and ending positions of the point, using the equations for perspective projection as in assignment 1. Assume d = 1. Also assume that the image plane is in front of the center of projection as in assignment 1 (so there is no inversion of the image). b) On a piece of graph paper, plot the beginning and ending positions of each point. Draw an arrow from the beginning position to the ending position. Label each arrow to indicate which point it is associated with (e.g. P1). c) The arrows from part b should form a radial pattern. If you were to draw lines extending through each arrow, the lines would all intersect at a single point, known as the focus of expansion, which is the center of the radial pattern. This point corresponds to the observer's direction of motion through the scene. Compute the location of the focus of expansion by drawing lines through the horizontal and vertical arrows on your graph. The place where the lines intersect is the focus of expansion. What are the coordinates of this intersection point? Label it on your graph. Problem 3: Motion Parallax a) In your own words, explain what "motion parallax" is, and what information it can provide when analyzing the visual scene. b) Many neurons in the middle temporal area (MT) have an inhibitory surround. Describe how such cells might be used in determining the amount of motion parallax at a given location. (Hint: Think about how retinal ganglion cells detect changes in luminance. Now, think about how an MT cell could detect changes in speed.)
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Constance Royden--croyden@mathcs.holycross.edu
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