A quiz on the first laboratory: Getting started with OpenCV

Adrian F. Clark

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

   • It returns None.

   • It crashes.
   • It throws an exception.
   • 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.

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

   • signed bytes
   • signed floats
   • 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.

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

   • 480 and 640.
   • 640 and 480.
   • 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...?

4. What is the standard deviation?

   • The median.

   • The average spread of values from the mean.

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

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

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

   • 10, 15.

   • 0, 15.
   • 0, 255.
   • 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.

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

   • the top left corner

   • the bottom right corner
   • the top right corner
   • the bottom 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!

7. What is the purpose of the doctest module?

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

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

   • To document a method.

   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.

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

   • row, column, colour.

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

   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.

9. How are images represented in OpenCV?

   • in an opaque OpenCV-only format

   • as numpy arrays

   • as a list of unsigned bytes
   • 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.

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

    • cv2.imshow

    • cv2.imread
    • cv2.display
    • 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.