Blick Comparator

The Blink Comparator is by today's standards an incredibly simple device. Using it, two images could be flipped one to the other and back to the first. The idea is that any differences between the two images would be easy to detect. Clyde Tombaugh used the one below to discover Pluto in 1930.

We will write a program in a few lines of Python to duplicate Tombaugh's discovering using the very same images.

After that you will rewrite the program to take another trip down memory lane to display this:

from ~1872.

opencv

We will be using the OpenCV package for Python. OpenCV is a super comprehensive suite of computer vision software routines used in thousands of applications in robotics, visual effects, image editing and self-driving cars to name a few.

The required packages should already be installed. Test this using interactive Python.

> python3
Python 3.6.9 (default, Nov  7 2019, 10:44:02) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cv2
>>> exit()
>

Remember, on your computer you may need to type python3 or just python and you will likely get a different Python version - which must begin with 3.

Step 1 - ensure the two pictures are in the right place

1. Create a folder called blink
2. Download p1.jpg and p2.jpg. They are in the zip file pluto.zip.
3. Put both in the folder named blink
4. Start a program called blink.py
5. Enter:

import cv2

images = []
images.append(cv2.imread('p1.jpg'))
images.append(cv2.imread('p2.jpg'))

6. Save and run this program. If there is no output, all is well. If any errors occur, all is not well.

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What's the point of coding so little and then testing?
Coding so little and testing is EXACTLY the point.

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Discussion

Our goal is to switch between two images, to ping-pong between them so to speak.

images is declared as an empty list.

cv2.imread() is given the name (or path) to the image to load. The return value is immediately appended to images. Note that this code does not perform well in the presence of errors. In fact, it performs no error checking at all.

After executing this code, images[0] and images[1] contain the image information for p1.jpg and p2.jpg

Step 2 - Enter remaining lines of code

7. Append the following to the code:

current_index = 0
key = ''
while key != 'q':
    cv2.imshow('Pluto', images[current_index])
    cv2.setWindowTitle('Pluto', str.format('Pluto Image: %d   -   \'q\' to quit' % current_index))
    key = chr(cv2.waitKey(0))
    current_index = 1 - current_index
cv2.destroyAllWindows()

8. Save and run this program. This program should open a window. If you see nothing and the program has not exited or generated an error, it is possible that the new window opened underneath other windows and is obscured.

9. With the new window selected hit any key other than q

10. Find Pluto

11. Hit q to exit

More Discussion

cv2.imshow()

This usage of the OpenCV function imshow() takes two parameters. The first serves as a window ID. Any OpenCV window relation function that supplies the same value will refer to the same window. The second parameter is what image to show within the window.

cv2.setWindowTitle()

Notice the first argument is the same as the call to imshow(). This function sets the contents of the window title.

cv2.waitKey()

This causes your program to wait indefinitely for keyboard input. The return value of this function is a numeric key code. The code can be turned into a character using chr().

current_index

This integer will ping-pong from 0 to 1 each time a key other than q is depressed. current_index starts out at 0. Thereafter current_index = 1 - current_index will flip it back and forth. When current_index is zero, images[0] is shown. When current_index is 1, images[1] is shown.

cv2.destroyAllWindows()

This function does what it says. Use it before exiting a program that uses OpenCV windows.

The "gist" of the main loop

The loop continues until the user hits the q key. On any other key, the displayed image alternates between picture 1 and picture 2 based upon current_index.

Your turn - making a flipbook from the Muybridge Horse photos

On June 19th, 1878 Eadweard Muybridge set up one dozen cameras with trip wires to settle a long standing question. Was there a time when all of a horse's hooves are off the ground at the same time.

Expanding on what you've learning above, you will create a flipbook that lets you run forwards or backwards over 12 images. If the user hits the a key, the current_index should decrement by 1. If the user hits the s key, the current_index should be incremented by one. As before, if the user hits the q key, the program should end.

Clearly current_index will grow beyond the number of images defined in the sequence and this would cause an index error. Similarly, if going backwards, current_index can become negative which will also end in tears.

You will need to keep current_index in the right range. Perhaps the % operator can be of assistance?

Due date

You have all of break and then one more week.

Work rules

Feel free to work together so long as your team each learn the details of the project and solution.