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Python for Everyone: The PreTeXt Interactive Edition

Section 18.10 Write Code Exercises

Note 18.10.1.

Remember that it can take a bit of time to process all the pixels in a picture! Check for errors below the code if it is taking a long time, but if you don't see any errors just wait.

Checkpoint 18.10.2.

Fix 4 syntax errors in the code below so that it correctly sets the red in all pixels to 0.
Solution.
Fix 4 syntax errors in the code below so that it correctly sets the red in all pixels to 0. 
# Import everything from the Image class
from image import *

# Add a closing parentheses
img = Image("gal2.jpg")

pixelList = img.getPixels()
for p in pixelList:

# Add 0 inside setRed() parentheses to set all red pixels to 0
  p.setRed(0)

  #Add a p inside updatePixel() parentheses
  img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.3.

The code below makes the image have a green-blue tint. Change 1 thing in order to make it have a red tint instead.
Solution.
The code below makes the image have a green-blue tint. Change 1 thing in order to make it have a red tint instead. 
from image import *

img = Image("kitten.jpg")

pixelList = img.getPixels()
for p in pixelList:

    # Red tint
    # Set red pixels to 255, not 0
    p.setRed(255)

    img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.4.

Fix the indention below to correctly set the red to the green, the green to the blue, and the blue to the red.
Solution.
Fix the indention below to correctly set the red to the green, the green to the blue, and the blue to the red. 
from image import *
img = Image("gal2.jpg")

pixels = img.getPixels()
for p in pixels:

    # Need to indent inside a for loop
    # Indent every line of code that modifies pixel values.
    r = p.getRed()
    g = p.getGreen()
    b = p.getBlue()

    p.setRed(g)
    p.setGreen(b)
    p.setBlue(r)

    img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.5.

Fix the 5 errors in the code, so that the red pixels get the value of the green, the green get the value of blue, and the blue get the value of the red. (The cat should look purple and gray)
Solution.
Fix the 5 errors in the code, so that the red pixels get the value of the green, the green get the value of blue, and the blue get the value of the red. (The cat should look purple and gray) 
from image import *

img = Image("kitten.jpg")

pixels = img.getPixels()

# pixels, not pixel, to match variable name
for p in pixels:

    # getRed() has a capital R
    r = p.getRed()
    b = p.getGreen()
    g = p.getBlue()

    p.setRed(g)
    p.setGreen(b)
    # The function is setBlue, instead of set.Blue
    p.setBlue(r)

    # updatePixel should have capitalized P
    img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
# img is lowercased
img.draw(win)

Checkpoint 18.10.6.

Fill in the missing code on lines 9, 12, and 18 below to set the red to half the original value in all pixels in the picture.
Solution.
Fill in the missing code on lines 9, 12, and 18 below to set the red to half the original value in all pixels in the picture. 
from image import *

img = Image("gal2.jpg")

pixels = img.getPixels();
# Add loop condition
for p in pixels:

    # Get current red value
    r = p.getRed()

    p.setRed(r * 0.5);

    # Update image with new pixels
    img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.7.

Complete the code in order to set the blue value to an eighth of the green value plus an eighth of the red value.
Solution.
Complete the code in order to set the blue value to an eighth of the green value plus an eighth of the red value. 
from image import *

img = Image("swan.jpg")

pixels = img.getPixels()

# Loop through each pixel in pixels
for p in pixels:

    # Add getBlue(), getGreen() and getRed()
    b = p.getBlue()
    g = p.getGreen()
    r = p.getRed()

    # Set blue to 1/8 of green and 1/8 of red
    p.setBlue((g / 8) + (r / 8))

    img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.8.

Fix the indention in the code below so that it correctly increases the red in each pixel in the picture by 1.5.
Solution.
Fix the indention in the code below so that it correctly increases the red in each pixel in the picture by 1.5. 
from image import *

img = Image("gal2.jpg")

pixels = img.getPixels();
for p in pixels:


    # Need to indent inside a for loop
    # Indent every line of code that modifies pixel values.
    r = p.getRed()
    p.setRed(r * 1.5);
    img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.9.

Fix the code (without adding anything new) to make the picture completely black.
Solution.
Fix the code (without adding anything) to make the picture completely black. 
from image import *

img = Image("motorcycle.jpg")

pixels = img.getPixels()
for p in pixels:

    # Removed getRed(), getGreen() and getBlue()
    #We are setting the pixel values to 0 regardless of current value

    p.setRed(0)
    p.setGreen(0)
    p.setBlue(0)

    img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.10.

Fix the code below to correctly set the green and blue values to 0.75 times their current values.
Solution.
Fix the code below to correctly set the green and blue values to 0.75 times their current values. 
from image import *
img = Image("gal2.jpg")

pixels = img.getPixels();
for p in pixels:

    # You need to first get current pixel values and theen modify to new values
    g = p.getGreen()
    b = p.getBlue()
    p.setGreen(g * 0.75)
    p.setBlue(b * 0.75)

    img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.11.

The code below sets all the pixels to half their original values with one for loop. Change the code so it uses 2 for loops that utilize the range function (1 for loop should be nested in the other).
Solution.
The code below sets all the pixels to half their original values with one for loop. Change the code so it uses 2 for loops that utilize the range function (1 for loop should be nested in the other). 
# STEP 1: USE THE IMAGE LIBRARY
from image import *

# STEP 2: PICK THE IMAGE
img = Image("baby.jpg")

# STEP 3: LOOP THROUGH THE PIXELS
pixels = img.getPixels()
for p in pixels:

    # STEP 4: GET THE DATA
    r = p.getRed()
    b = p.getBlue()
    g = p.getGreen()

    # STEP 5: MODIFY THE COLOR
    p.setRed(r/2)
    p.setGreen(g/2)
    p.setBlue(b/2)

    # STEP 6: UPDATE THE IMAGE
    img.updatePixel(p)

# STEP 7: SHOW THE RESULT
win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.12.

Change the following code to set the red to 0 for all pixels in the left half of the picture.
Solution.
Change the following code to set the red to 0 for all pixels in the left half of the picture. 
from image import *
img = Image("gal2.jpg")

# Since we need first half, only consider range for first half pixel values
# Range takes integer parameters, so need to convert float type to int
for x in range(int(img.getWidth()/2)):
    for y in range(img.getHeight()):

        p = img.getPixel(x, y)
        p.setRed(0)
        img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.13.

The code below makes the whole image have a blue-green tint. Change the code so that it makes an only blue tint in the bottom left corner.
Solution.
The code below makes the whole image have a blue-green tint. Change the code so that it makes an only blue tint in the bottom left corner. 
from image import *
img = Image("swan.jpg")

# Range should be the left 1/2 of the image
for x in range(int(img.getWidth()/2)):

    #Range should be bottom 1/2 of the image
    for y in range(int(img.getHeight()/2),int(img.getHeight())):

        p = img.getPixel(x, y)
        p.setRed(0)
        p.setGreen(0)
        img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.14.

Change the code below to set the red value in the pixels in the bottom half of the picture to 0.
Solution.
Change the code below to set the red value in the pixels in the bottom half of the picture to 0. 
from image import *
img = Image("gal2.jpg")

# Since we need first half, only consider range for first half pixel values
# Range takes integer parameters, so need to convert float type to int
for x in range(int(img.getWidth()/2), int(img.getWidth())):
    for y in range(int(img.getHeight()/2), int(img.getHeight())):

        p = img.getPixel(x, y)
        p.setRed(0)
        img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.15.

The code below makes the whole image seem red. Change it, so that only every 5 pixels get changed, so that it will look like a red grid.
Solution.
The code below makes the whole image seem red. Change it, so that only every 5 pixels get changed, so that it will look like a red grid. 
from image import *

# CREATE AN IMAGE FROM A FILE
img = Image("guy1.jpg")

# LOOP THROUGH THE PIXELS
for x in range(0, int(img.getWidth()), 5):
    for y in range(0, img.getHeight(), 5):
        # GET THE DATA
        p = img.getPixel(x, y)

        # SET THE PIXEL
        p.setGreen(0)
        p.setBlue(0)

        # UPDATE THE IMAGE
        img.updatePixel(p)
# SHOW THE RESULT
win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.16.

Change the following code into a procedure to keep only the green values in all pixels in a picture.
Solution.
Change the following code into a procedure to keep only the green values in all pixels in a picture. 
from image import *
img = Image("gal2.jpg")

pixels = img.getPixels();
for p in pixels:

    # Set Red and Blue values to 0
    p.setRed(0)
    p.setBlue(0)

    img.updatePixel(p)

win = ImageWin(img.getWidth(),img.getHeight())
img.draw(win)

Checkpoint 18.10.17.

Write code to create a subclass of the Image class with three different image effect functions and use each effect on a different part of the image. For example, you could turn the image into grayscale, keep only the green values, swap the red and green values, negate the image, etc. This is one possible solution. For example, the image shown below has one image effect on the first third of the image (negate), one on the second third (clear blue), and one on the final third (reduce red). It is using the image gal2.jpg
Solution.
Create a subclass of the Image class with three different image effect functions and use each effect on a different part of the image. For example, you could turn the image into grayscale, keep only the green values, swap the red and green values, negate the image, etc. This is one possible solution. For example, the image shown below has one image effect on the first third of the image (negate), one on the second third (clear blue), and one on the final third (reduce red). It is using the image gal2.jpg 
from image import *
class MyImage(Image):
    def __init__(self, file):
        super().__init__(file)

    def negate(self):
        for x in range(self.getWidth()):
            for y in range(int(self.getHeight() / 3)):
                p = self.getPixel(x,y)
                r = p.getRed()
                b = p.getBlue()
                g = p.getGreen()
                newPixel = Pixel(255 - r, 255 - b, 255 - g)

                self.setPixel(x, y, newPixel)

    def removeBlue(self):
        for x in range(self.getWidth()):
            for y in range(int(self.getWidth() / 3), int((self.getWidth() / 3)*2)):

                p = self.getPixel(x,y)
                p.setBlue(0)
                self.updatePixel(p)

    def reduceRed(self):
        for x in range(self.getWidth()):
            for y in range(int((self.getWidth() / 3)*2), self.getHeight()):
                p = self.getPixel(x,y)
                r = p.getRed()
                p.setRed(r/2)
                self.updatePixel(p)

def main():
    # CREATE AN IMAGE FROM A FILE
    img = MyImage("gal2.jpg")

    img.negate()
    img.removeBlue()
    img.reduceRed()

    # SHOW THE RESULT
    win = ImageWin(img.getWidth(),img.getHeight())
    img.draw(win)
main()
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