A quiz on the stomata-counting experiment

Adrian F. Clark

1. Why is the variable 'me' given as an argument to the call that creates a slider?

   • it contains the command line
   • it contains the image name
   • the program name is needed to create a slider

   • there may be more than one window

   There may be more than one window in the graphical user interface, so you have to specify in which window you want the slider to appear. The choice of window name is arbitrary but here it is the program name.

2. How are the stomata positions found?

   • the centre of the bounding box of the contour

   • the centre of the minimum enclosing circle
   • using the ratio of the first two moments
   • the centre of the minimum enclosing ellipse

   To be honest, all of these answers could work but the code uses the middle of the bounding box

3. Which colour space is most effective at segmenting stomata?

   • monochrome

   • RGB

   • HSV
   • HLS

   My experience is that, unusually, RGB space is the most effective at segmenting the stomata.

4. What does a space character do in the graphical user interface of count-stomata.py?

   • it quits the program
   • it tells the program to count the number of stomata

   • it moves on to the next image

   • it controls the segmentation process

   The space bar tells count-stomata.py to move on to the next image given on the command line.

5. What are stomata?

   • lumps caused by a virus
   • microscopic wasp larvae
   • spores of a fungal disease

   • microscopic pores on leaves through which transpiration takes place

   Stomata are the parts of a leaf through which gases pass, forming an essential part of plant growth. As a rule of thumb, the more stomata and the larger they are, the healthier and quicker-growing the plant is.

6. If you are using count-stomata.py on a system without a right mouse button, such as a Macintosh, what does the program let you do instead?

   • hold down the ALT key
   • hold down the windows key
   • hold down the control key

   • hold down the shift or control key

   You are able to hold down either the shift or control keys; the relevant code is in the callback that handles events.

7. Which Python package is used to handle the command line?

   • argparse

   • taskfile
   • os
   • sys

   The standard Python package for handling the parsing of the command line is argparse, and it is available on all platforms. Yes, even Windows.

8. Which RGB slider has the most effect on the detection of stomata?

   • green

   • red

   • hue
   • blue

   In my experience, setting the red and green sliders to encompass most values allows the blue slider to control the effectiveness of segmentation.

9. How is the slider position communicated to the segmentation code in count-stomata.py?

   • using a global variable
   • using a local variable

   • using a callback

   • there is no need to do anything

   a callback is defined when the slider is created and this sets a global variable with the value from the slider, and that is passed into cv2.inRange.

10. What is the main purpose of this experiment?

    • to let you understand that OpenCV can be used to build graphical user interfaces

    • to practise writing calls to cv2.inRange
    • to see how contours centres are found
    • to count the number of stomata

    The main purpose of the experiment is to help you understand how OpenCV can be used to build useful GUIs. Although the main aim of computer vision is to take the human out of the task, it is sometimes (as here) necessary to involve them. A GUI is the currently conventional way to do it.