DPI settings for resolution in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Effect of DPI on saved image size

What will be the size in pixels of the saved image when running this code?

Matplotlib

import matplotlib.pyplot as plt
fig = plt.figure(figsize=(4, 3), dpi=100)
fig.savefig('test.png')
print(fig.get_size_inches() * fig.dpi)

A [100. 100.]

B [4. 3.]

C [400. 300.]

D [40. 30.]

💡 Hint

Multiply figure size in inches by dpi to get pixel dimensions.

The figure size is 4 inches by 3 inches. The dpi is 100 dots per inch. Multiplying 4*100 and 3*100 gives 400 by 300 pixels.

❓ data_output

intermediate

DPI effect on figure pixel count

Given a figure with size (5, 2) inches and dpi=200, what is the total number of pixels in the saved image?

Matplotlib

import matplotlib.pyplot as plt
fig = plt.figure(figsize=(5, 2), dpi=200)
width_px, height_px = fig.get_size_inches() * fig.dpi
print(int(width_px * height_px))

A 20000

B 400000

C 2000

D 10000

💡 Hint

Calculate width pixels * height pixels.

Width in pixels = 5 * 200 = 1000, height in pixels = 2 * 200 = 400. Total pixels = 1000 * 400 = 400,000.

❓ visualization

advanced

Visual difference with DPI settings

Which saved image will appear sharper when viewed at the same physical size on screen?

Matplotlib

import matplotlib.pyplot as plt
fig1 = plt.figure(figsize=(3, 3), dpi=50)
plt.plot([1, 2, 3], [1, 4, 9])
fig1.savefig('low_dpi.png')
fig2 = plt.figure(figsize=(3, 3), dpi=200)
plt.plot([1, 2, 3], [1, 4, 9])
fig2.savefig('high_dpi.png')

A The image saved with dpi=200 is sharper.

B Both images have the same sharpness.

C The image saved with dpi=50 is sharper.

D Sharpness depends only on figure size, not dpi.

💡 Hint

Higher dpi means more pixels per inch, so more detail.

Higher dpi means more pixels per inch, so the image has more detail and appears sharper when viewed at the same size.

🔧 Debug

advanced

Why does this saved figure look blurry?

This code saves a figure but the saved image looks blurry. What is the cause?

Matplotlib

import matplotlib.pyplot as plt
plt.figure(figsize=(6, 4))
plt.plot([0, 1], [0, 1])
plt.savefig('blurry.png', dpi=50)

A The savefig function needs a format argument to avoid blur.

B The figure size is too small, causing blur.

C The plot command is missing labels, causing blur.

D The dpi is too low, causing low resolution.

💡 Hint

DPI controls image resolution; low dpi means fewer pixels.

Low dpi means fewer pixels per inch, so the image looks blurry when viewed at normal size.

🧠 Conceptual

expert

Understanding DPI and figure size relationship

If you want to save a figure that is exactly 1200 pixels wide, which combination of figsize and dpi will achieve this?

A figsize=(6, 4), dpi=200

B figsize=(10, 4), dpi=100

C figsize=(4, 3), dpi=250

D figsize=(10, 3), dpi=100

💡 Hint

Width in pixels = width in inches * dpi.

To get 1200 pixels width, multiply width in inches by dpi. For option A: 6 * 200 = 1200 pixels.

Other options do not produce exactly 1200 pixels width.