A quiz on the first laboratory: Getting started with OpenCV

Adrian F. Clark

1. What values did your extrema routine return for testimage?

   • 0, 255.
   • 0, 15.

   • 10, 15.

   • 0, 10.

   Your extrema routine should return the smallest and largest values in the image it is given. For testimage, these are 10 and 15 --- in fact, the doctest for this routine gives them.

2. What is the standard deviation?

   • The median.
   • How different one image is from another.
   • The same as the mean.

   • The average spread of values from the mean.

   The standard deviation measures the average spread of value in an image.

3. What does cv2.imread do if it cannot read an image?

   • It crashes.
   • It throws an exception.

   • It returns None.

   • It outputs an error message and exits.

   OpenCV, or at least its Python wrapper, returns None when it cannot read an image. Good programs will catch this case and do something sensible...but not all programs are good.

4. How are the pixels of an image normally represented in OpenCV?

   • signed floats
   • signed bytes
   • unsigned ints

   • unsigned bytes

   Unless you take care to make them otherwise, the pixels of images are stored as unsigned bytes in OpenCV. It is possible to use the other representations listed in the answers but you have to go to some effort to do so when you create the image.

5. How many rows and columns are there in the image sx.jpg?

   • 512 and 512.

   • 1024 and 1024.
   • 640 and 480.
   • 480 and 640.

   This is something that was printed out when you ran hello.py on the image. You mean you didn't record the information...?

6. Why were you asked to store the routines you develop in sxcv.py?

   • So you can record what you have done.
   • So that you can see how to comment programs.
   • To give you practice at programming.

   • So that you can use them later in vision application programs.

   The underlying reason for getting you to store the routines you write (and hopefully test) in sxcv.py is so you can use them to extend the programs that appear in the lab experiments after the first progress test.

7. What underlying OpenCV routine does sxcv.display call to display an image?

   • cv2.display
   • cv2.imread

   • cv2.imshow

   • cv2.waitKey

   The underlying display routine in OpenCV is cv2.imshow, though the image is not posted onto the screen until cv2.waitKey is called.

8. What is the purpose of the doctest module?

   • To provide an overview of a complete module.
   • To document a method.
   • To provide an alternative way of commenting code.

   • To let you store unit tests in the top-of-routine documentation.

   The doctest module provides a way for a developer or tester to store a series of tests. These can be pulled out of the source code and executed automatically, checking that the routine produces when run exactly the documented output.

9. How are images represented in OpenCV?

   • in an opaque OpenCV-only format
   • as a list of lists
   • as a list of unsigned bytes

   • as numpy arrays

   The Python wrappers for OpenCV represent images as numpy arrays, 2D for monochrome ("black and white") images and 3D for colour ones.

10. What is sys.argv[1]?

    • The program name.

    • The first space-separated word on the command line.

    • An image.
    • All the words on the command line.

    sys.argv is a list of the space-separated words that were provided on the command line. The first element in it, element 0, is the program name and so element 1 is the first word that was given after the program name.