COS 496 - Computer Vision

Spring 2003

Course home Outline and lecture notes Assignments


Assignment 1

Due Wednesday, Feb. 19


I. Getting familiar with Matlab

Matlab is available on several OIT machines (look here for details). You should be able run it remotely from any Unix workstation.

Read through any or all of the following for a basic introduction to Matlab

Work through the following tasks using an image of your choice. You do not need to submit any results, but make sure you are comfortable doing the following:

  1. Read an image into a variable.
    Hint #1: "help imread"
    Hint #2: Use single quotes around the filename.
    Hint #3: Ending a command with a semicolon supresses printing the result.

  2. Display the image.
    Hint: "help image"

  3. Adjust the axes so that the aspect ratio is 1:1 (i.e., "square pixels").
    Hint: "help axis"

  4. Convert the image to grayscale.
    Hint: "help rgb2gray"
    Note: if your version of Matlab doesn't have the rgb2gray function, download rgb2gray.m. Place this in your working directory, and it should be auto-loaded by Matlab.
  5. Convert the grayscale image to floating point.
    Hint: "help double"

  6. Display the floating-point image, using a grayscale colormap.
    Hint #1: "colormap(gray)"
    Hint #2: "help imagesc"

  7. Plot the intensities along one scanline of the image.
    Hint #1: Extracting a part of a matrix is done by
    	matrix2 = matrix1(row_min:row_max,col_min:col_max);
    
    Indices in Matlab are 1-based (not 0-based as in C). row_max or col_max may be "end" to indicate the last element.
    Just a ":" is equivalent to "1:end".
    Hint #2: "help plot"

  8. Store the width and height of the image in variables "width" and "height".
    Hint #1: "help size"
    Hint #2: Functions in Matlab may return multiple values. You can get at the values using the notation
    	[var1, var2] = func(x)
    
    Hint #3: In Matlab, the number of rows is the first dimension and the number of columns is the second.

  9. Write a pair of nested "for" loops to set a grid of every 10th pixel horizontally and every 20th pixel vertically to 0.
    Hint #1: "help for"
    Hint #2: "start:increment:stop"

  10. Create a function "maxrow" that takes a matrix and a row index and returns the brightest pixel in the given row. Store the function in a file called "maxrow.m" so that Matlab loads it automatically when you call the function.
    Hint #1: "help function"
    Hint #2: Matlab has many built-in functions that operate on entire vectors or matrices. There might be one to compute the maximum...

  11. Write the modified image back to a new file.
    Hint #1: "help uint8" to convert the image back to integer
    Hint #2: "help imwrite"

If you get stuck on any of these, feel free to ask for help, either by emailing smr@cs.princeton.edu or by asking a colleague.


II. Canny edge detector (65%)

Implement the Canny edge detection algorithm, as described in class and in Section 4.2 of Trucco and Verri. This consists of three phases:

Test your alogrithm on images of your choosing, experimenting with different values of the parameters sigma (the width of the Gaussian used for smoothing), T_h (the "high" threshold), and T_l (the "low" threshold). Also run your algorithm on the following images:


III. Corner detector (35%)

Implement the variance-based corner detection algorithm, as described in class and in Section 4.3 of Trucco and Verri. This consists of three phases:

Test your algorithm on the building image above, and on checker.jpg. (This is a picture of a target used for camera calibration. Note the barrel distortion in this lens.) Explore the effects of changing sigma (the width of the Gaussian), the size of the neighborhood, and the threshold.


IV. Extra credit

Experiment with heuristics for automatically determining the thresholds to be used for each image. Some of these are mentioned in Trucco and Verri, but feel free to be creative and come up with your own.


Submitting

This assignment is due Wednesday, February 19, 2002 at 11:59 PM Eastern Standard Time. Please see the general notes on submitting your assignments, as well as the late policy and the collaboration policy.

Please submit:

Note that programming in Matlab is not an excuse to write unreadable code. You are expect to use good programming style, including meaningful variable names, a comment or three describing what the code is doing, etc.


smr@cs.princeton.edu