Albion College CS 171 Fall 2021

Lab 11: Optical Illusions

In this lab you will create some simple geometric optical illusions. In the process, you will familiarize yourself with the graphics and image display capabilities of python's opencv package, and gain experience using single and nested loops.

Optical Illusions

Checkerboard. A normal black and white checkerboard. For a more interesting look at what can be done with a checkerboard, see Craig Kaplan's Swirled Series project --- I contributed two of the segments.

Hermann Grid Illusion. Dark spots appear at the intersections of white lines.

Scintillating Grid Illusion. Black dots flash in the center of white circles.

Cafe Wall Illusion. Parallel rows appear sloped.

Installing Python packages

Install the opencv and numpy packages using pip.


python -m pip install --user opencv-python
python -m pip install --user numpy

Task

Complete the methods in Lab11.py, as shown below.

# CS 171 - Lab 11 # Your name(s) here # use # python -m pip install --user opencv-python # python -m pip install --user numpy # to install these python packages import numpy as np import cv2 as cv # Define image dimensions width = 512 height = 512 def example(): # create a blank image img = np.zeros((width,height,3), np.uint8) # draw a white (255,255,255) rectangle with corners at (100,200) and (300,400) # the -1 indicates it is filled cv.rectangle(img, (100,200), (400,500), (255,255,255), -1) # draw a filled circle at (250, 350) with a radius of 125 # the -1 indicates it is filled # lineType=cv.LINE_AA indicates anti-aliasing (less jagged boundary) cv.circle(img, (250, 350), 125, (0,0,0), -1, lineType=cv.LINE_AA) # draw a blue circle at (250, 350) with a radius of 125 # and a line thickness of 10 # note colors are specfied as a (blue, green, red) tuple cv.circle(img, (250, 350), 50, (255,0,0), 10, lineType=cv.LINE_AA) # draw a gray diagonal line # since this is drawn last, it is on top of everything else cv.line(img, (0,0), (width,height), (128,128,128), 10, lineType=cv.LINE_AA) return img def checkerboard(nx, ny): img = np.zeros((width,height,3), np.uint8) dx = width//nx dy = height//nx for i in range(nx): for j in range(ny): x = i*dx y = j*dy if (i+j)%2 == 0: cv.rectangle(img, (x,y), (x+dx,y+dy), (0,0,0), -1) else: cv.rectangle(img, (x,y), (x+dx,y+dy), (255,255,255), -1) return img # https://michaelbach.de/ot/lum-herGrid/index.html # Hint: use a loop to draw the vertical lines # and a loop for the horizontal lines def HermannGrid(nx, ny): img = np.zeros((width,height,3), np.uint8) return img # https://michaelbach.de/ot/lum-scGrid/index.html # Hint: Start with your Hermann Grid code # use a doubly nested loop to draw the circles def ScintillatingGrid(nx, ny): img = np.zeros((width,height,3), np.uint8) return img # https://en.wikipedia.org/wiki/Caf%C3%A9_wall_illusion # Hint: Start with your checkerboard code # change the offsets based on the row mod 4 def CafeWall(nx, ny): img = np.zeros((width,height,3), np.uint8) return img def main(): img = example() output = "output.png" while True: cv.imshow('CS 171 Lab', img) key = chr(cv.waitKey(0)) print(key) if key == 'q': break elif key == 'e': img = example() elif key == 's': cv.imwrite(output, img) elif key == '1': img = checkerboard(8, 8) output = "checkerboard.png" elif key == '2': img = HermannGrid(8, 8) output = "HermannGrid.png" elif key == '3': img = ScintillatingGrid(8, 8) output = "ScintillatingGrid.png" elif key == '4': img = CafeWall(12,12) output = "CafeWall.png" elif key == '5': img = bulgingCheckerboard(13,13) output = "output.png" cv.destroyAllWindows() # close all windows main()

When you are finished, email your lab files to your lab partner(s) and instructor.