Image colormaps in Matplotlib - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to display an image with the 'viridis' colormap.

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

image = np.random.rand(10,10)
plt.imshow(image, cmap=[1])
plt.show()

Drag options to blanks, or click blank then click option'

A'cool'

B'viridis'

C'hot'

D'gray'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the colormap name in quotes.

Using a colormap name that does not exist.

The 'viridis' colormap is a popular default colormap in matplotlib for displaying images.

2fill in blank

medium

Complete the code to display the image with the 'hot' colormap.

Matplotlib

plt.imshow(image, cmap=[1])
plt.colorbar()
plt.show()

Drag options to blanks, or click blank then click option'

A'hot'

B'spring'

C'cool'

D'winter'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a colormap that does not represent heat.

Not adding quotes around the colormap name.

The 'hot' colormap shows colors from black to red to yellow to white, useful for heat maps.

3fill in blank

hard

Fix the error in the code to correctly apply the 'cividis' colormap.

Matplotlib

plt.imshow(image, cmap=[1])
plt.show()

Drag options to blanks, or click blank then click option'

Acividis

Bcividis()

C'cividis'

D['cividis']

Attempts:

3 left

💡 Hint

Common Mistakes

Using the colormap name without quotes.

Trying to call the colormap as a function.

The colormap name must be a string in quotes, not a variable or function call.

4fill in blank

hard

Fill both blanks to create a grayscale image with a colorbar.

Matplotlib

plt.imshow(image, cmap=[1])
plt.[2]()
plt.show()

Drag options to blanks, or click blank then click option'

A'gray'

B'viridis'

Ccolorbar

Dshow

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong colormap name for grayscale.

Forgetting to add the colorbar function.

Use 'gray' for grayscale colormap and 'colorbar' to add the color scale bar.

5fill in blank

hard

Fill all three blanks to create a heatmap with 'hot' colormap, add a colorbar, and show the plot.

Matplotlib

plt.imshow(image, cmap=[1])
plt.[2]()
plt.[3]()

Drag options to blanks, or click blank then click option'

A'hot'

Bcolorbar

Cshow

D'cool'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong colormap name.

Forgetting to add colorbar or show the plot.

Use 'hot' colormap, then add colorbar and finally show the plot.

Practice

1. What does the cmap parameter do in plt.imshow() when displaying an image?

easy

A. It changes how numbers map to colors in the image.

B. It changes the image size.

C. It adds a title to the image.

D. It saves the image to a file.

Solution

1. Step 1: Understand the role of cmap in plt.imshow()

   The cmap parameter controls the colormap, which maps numeric values to colors in the image.

2. Step 2: Identify what cmap affects visually

   Changing cmap changes the colors shown, helping interpret data better.

3. Final Answer:

   It changes how numbers map to colors in the image. -> Option A

4. Quick Check:

   Colormap = color mapping [OK]

Hint: Remember: cmap controls colors, not size or titles [OK]

Common Mistakes:

• Thinking cmap changes image size
• Confusing cmap with adding titles
• Assuming cmap saves the image

2. Which of the following is the correct way to apply the 'viridis' colormap to an image using matplotlib?

easy

A. plt.imshow(image, color='viridis')

B. plt.imshow(image, cmap='viridis')

C. plt.imshow(image, cmap=viridis)

D. plt.imshow(image, colormap='viridis')

Solution

1. Step 1: Recall the correct parameter name for colormap

   The parameter to set colormap in plt.imshow() is cmap, and it expects a string name.

2. Step 2: Check the syntax for passing the colormap

   Passing cmap='viridis' is correct. Using color or colormap is incorrect, and omitting quotes causes an error.

3. Final Answer:

   plt.imshow(image, cmap='viridis') -> Option B

4. Quick Check:

   Use cmap='name' syntax [OK]

Hint: Use cmap='colormap_name' exactly [OK]

Common Mistakes:

• Using color instead of cmap
• Forgetting quotes around colormap name
• Using colormap instead of cmap

3. What will the following code display?

import matplotlib.pyplot as plt
import numpy as np
image = np.array([[0, 1], [2, 3]])
plt.imshow(image, cmap='gray')
plt.colorbar()
plt.show()

medium

A. A 2x2 image with shades of gray from black (0) to white (3) and a colorbar showing values 0 to 3.

B. A 2x2 image with random colors and no colorbar.

C. A 2x2 image with rainbow colors and a colorbar showing values 0 to 1.

D. An error because 'gray' is not a valid colormap.

Solution

1. Step 1: Understand the image data and colormap

   The image is a 2x2 array with values 0 to 3. The 'gray' colormap maps low values to black and high values to white.

2. Step 2: Analyze the colorbar and display

   The colorbar shows the numeric range from 0 to 3, matching the image values. The image colors range from black (0) to white (3).

3. Final Answer:

   A 2x2 image with shades of gray from black (0) to white (3) and a colorbar showing values 0 to 3. -> Option A

4. Quick Check:

   Gray cmap maps 0-3 to black-white [OK]

Hint: Gray cmap means black to white shades [OK]

Common Mistakes:

• Thinking 'gray' is invalid
• Ignoring the colorbar range
• Assuming rainbow colors with 'gray'

4. The code below is intended to show an image with the 'hot' colormap and a colorbar, but it raises an error. What is the problem?

import matplotlib.pyplot as plt
import numpy as np
image = np.random.rand(3,3)
plt.imshow(image, cmap=hot)
plt.colorbar()
plt.show()

medium

A. plt.colorbar() must be called before plt.imshow().

B. The image array shape is invalid for imshow.

C. The colormap name 'hot' should be a string: cmap='hot'.

D. np.random.rand() cannot be used for images.

Solution

1. Step 1: Check the cmap parameter usage

   The colormap name must be a string. Here, hot is used without quotes, causing a NameError.

2. Step 2: Verify other code parts

   The image shape is valid, and colorbar usage is correct. np.random.rand() is valid for image data.

3. Final Answer:

   The colormap name 'hot' should be a string: cmap='hot'. -> Option C

4. Quick Check:

   Colormap names need quotes [OK]

Hint: Always put colormap names in quotes [OK]

Common Mistakes:

• Forgetting quotes around colormap name
• Thinking image shape is wrong
• Misordering colorbar and imshow

5. You have a 5x5 numpy array with values from 0 to 24. You want to display it using plt.imshow() with the 'coolwarm' colormap but only show colors for values between 5 and 20. Which code snippet correctly applies this?

hard

A. plt.imshow(data, cmap='coolwarm'); plt.colorbar(vmin=5, vmax=20)

B. plt.imshow(data, cmap='coolwarm', min=5, max=20); plt.colorbar()

C. plt.imshow(data, cmap='coolwarm', clip=(5,20)); plt.colorbar()

D. plt.imshow(data, cmap='coolwarm', vmin=5, vmax=20); plt.colorbar()

Solution

1. Step 1: Understand how to limit color range in imshow

   Use vmin and vmax parameters to set the data range for colormap scaling.

2. Step 2: Check each option for correct syntax

   plt.imshow(data, cmap='coolwarm', vmin=5, vmax=20); plt.colorbar() uses vmin=5 and vmax=20, which is correct. Other options use invalid parameters or place them incorrectly.

3. Final Answer:

   plt.imshow(data, cmap='coolwarm', vmin=5, vmax=20); plt.colorbar() -> Option D

4. Quick Check:

   Use vmin and vmax to clip colors [OK]

Hint: Use vmin and vmax to set color limits [OK]

Common Mistakes:

• Using min/max instead of vmin/vmax
• Trying to clip with non-existent parameters
• Passing vmin/vmax to colorbar instead of imshow