A quiz on the first laboratory: Getting started with OpenCV

Adrian F. Clark

1. What do the three subscripts of a colour image represent in OpenCV?

   • colour, row, column.
   • colour, column, row.

   • row, column, colour.

   • column, row, colour.

   Images are stored as numpy matrices, so their first two subscripts are row and column respectively. Images are stored so that the colour values of a single pixel are next to each other in memory, so the third subscript selects the colour.

2. What is the standard deviation?

   • The average spread of values from the mean.

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

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

3. How are images represented in OpenCV?

   • as a list of unsigned bytes

   • as numpy arrays

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

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

4. What is sys.argv[1]?

   • An image.
   • The program name.

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

   • 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.

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

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

   • the top left 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!

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

   • 480 and 640.

   • 512 and 512.

   • 640 and 480.
   • 1024 and 1024.

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

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

   • It outputs an error message and exits.

   • It returns None.

   • It crashes.
   • It throws an exception.

   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.

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

   • signed bytes
   • unsigned ints
   • signed floats

   • 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.

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

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

   • 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.

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

    • 0, 10.

    • 10, 15.

    • 0, 255.
    • 0, 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.