Overlaying data on images in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code overlaying points on an image?

Consider the following Python code using matplotlib to overlay red points on a grayscale image. What will be the color of the points shown on the plot?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

image = np.ones((5,5)) * 0.5  # gray image
points_x = [1, 3]
points_y = [2, 4]

plt.imshow(image, cmap='gray')
plt.scatter(points_x, points_y, color='red')
plt.axis('off')
plt.show()

A Red points on a gray background image

B Blue points on a gray background image

C Red points on a white background image

D No points visible, only gray image

💡 Hint

Check the color parameter in plt.scatter.

The color='red' argument in plt.scatter draws red points on top of the gray image.

❓ data_output

intermediate

How many points are plotted on the image?

Given this code snippet overlaying points on an image, how many points will appear on the plot?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

image = np.zeros((10,10))
points_x = [2, 5, 7]
points_y = [3, 5, 8]

plt.imshow(image, cmap='gray')
plt.scatter(points_x, points_y, color='yellow')
plt.show()

A 2 points

B 3 points

C 5 points

D 0 points

💡 Hint

Count the number of coordinates in points_x and points_y.

There are three x and y coordinates, so three points are plotted.

❓ visualization

advanced

Which option produces a heatmap overlay on the image?

Which code snippet correctly overlays a heatmap with transparency on a grayscale image?

A

plt.imshow(image, cmap='gray')
plt.scatter(data, color='hot', alpha=0.5)
plt.show()

B

plt.imshow(image, cmap='gray')
plt.scatter(data, cmap='hot', alpha=0.5)
plt.show()

C

plt.imshow(image, cmap='gray')
plt.imshow(data, cmap='hot')
plt.scatter(alpha=0.5)
plt.show()

D

plt.imshow(image, cmap='gray')
plt.imshow(data, cmap='hot', alpha=0.5)
plt.show()

💡 Hint

Heatmaps use imshow with a colormap and alpha for transparency.

Option D overlays the heatmap using imshow with alpha=0.5 for transparency on top of the grayscale image.

🔧 Debug

advanced

What error does this code raise when overlaying points on an image?

Examine this code snippet. What error will it raise when run?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

image = np.ones((5,5))
points_x = [1, 2, 3]
points_y = [1, 2]

plt.imshow(image, cmap='gray')
plt.scatter(points_x, points_y, color='blue')
plt.show()

A IndexError: list index out of range

B TypeError: color must be a string or sequence

C ValueError: x and y must be the same size

D No error, plot displays correctly

💡 Hint

Check if points_x and points_y have the same length.

The x and y coordinate lists have different lengths, causing a ValueError in plt.scatter.

🚀 Application

expert

What is the resulting shape of the overlayed image array after combining two images?

You have two numpy arrays representing images: img1 with shape (100, 100) and img2 with shape (100, 100, 3). You want to overlay img2 as a color layer on top of img1 grayscale image by stacking channels. What will be the shape of the resulting combined image array?

Matplotlib

import numpy as np

img1 = np.random.rand(100, 100)  # grayscale
img2 = np.random.rand(100, 100, 3)  # color

combined = np.dstack((img1, img2))
print(combined.shape)

A (100, 100, 4)

B (100, 100)

C (100, 100, 3)

D (100, 100, 2)

💡 Hint

Stacking a (100,100) array with a (100,100,3) array along the third axis adds channels.

np.dstack stacks arrays depth-wise (along the third axis). Adding a single channel to a 3-channel image results in 4 channels.