A quiz on the first laboratory: Getting started with OpenCV

Adrian F. Clark

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

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

   • 512 and 512.

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

2. What is the purpose of the doctest module?

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

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

   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.

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

   • 0, 10.
   • 0, 255.
   • 0, 15.

   • 10, 15.

   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.

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

   • cv2.imread

   • cv2.imshow

   • cv2.waitKey
   • cv2.display

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

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

   • So that you can see how to comment programs.
   • So you can record what you have done.
   • 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.

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

   • It outputs an error message and exits.
   • It throws an exception.
   • It crashes.

   • It returns None.

   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.

7. How are images represented in OpenCV?

   • in an opaque OpenCV-only format

   • as numpy arrays

   • as a list of lists
   • as a list of unsigned bytes

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

8. Where is the origin of an image in OpenCV?

   • the top left corner

   • the top right corner
   • the bottom left corner
   • the bottom right corner

   OpenCV images have the origin (row zero, column zero) at the top left corner of the image. This might seem obvious but is not always the case: for example if you program image operations using the 3D graphics package called OpenGL (note the similarity of the name to OpenCV), its image origin is at the bottom left corner of an image. Much confusion can result from this!

9. What is the standard deviation?

   • The average spread of values from the mean.

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

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

10. What is sys.argv[1]?

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

    • All the words on the command line.
    • An image.
    • The program name.

    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.