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.

Practice

1. What does the dpi parameter control in matplotlib plots?

easy

A. The type of plot (line, bar, scatter)

B. The color scheme of the plot

C. The resolution or sharpness of the saved or displayed plot

D. The size of the plot in inches

Solution

1. Step 1: Understand the role of DPI in images

   DPI stands for dots per inch and controls how many pixels are used per inch in an image, affecting sharpness.

2. Step 2: Relate DPI to matplotlib plots

   In matplotlib, setting dpi changes the resolution of the saved or displayed plot, making it sharper or blurrier.

3. Final Answer:

   The resolution or sharpness of the saved or displayed plot -> Option C

4. Quick Check:

   DPI controls resolution = D [OK]

Hint: DPI means dots per inch, controlling image sharpness [OK]

Common Mistakes:

• Confusing DPI with plot size
• Thinking DPI changes plot colors
• Assuming DPI changes plot type

2. Which of the following is the correct way to save a matplotlib plot with 300 DPI resolution?

easy

A. plt.savefig('plot.png', dpi='300')

B. plt.save('plot.png', dpi=300)

C. plt.savefig('plot.png', resolution=300)

D. plt.savefig('plot.png', dpi=300)

Solution

1. Step 1: Recall the correct function to save plots

   The correct function to save a plot in matplotlib is plt.savefig().

2. Step 2: Check the parameter for resolution

   The parameter to set resolution is dpi, so the correct syntax is plt.savefig('filename', dpi=300).

3. Final Answer:

   plt.savefig('plot.png', dpi=300) -> Option D

4. Quick Check:

   Use plt.savefig with dpi=300 = A [OK]

Hint: Use plt.savefig(filename, dpi=number) to set resolution [OK]

Common Mistakes:

• Using plt.save instead of plt.savefig
• Using 'resolution' instead of 'dpi'
• Passing dpi as a string '300'

3. What will be the size in pixels of a saved plot with figsize=(4,3) inches and dpi=200?

medium

A. 4 x 3 pixels

B. 800 x 600 pixels

C. 200 x 150 pixels

D. 1000 x 750 pixels

Solution

1. Step 1: Calculate width in pixels

   Width in pixels = width in inches * dpi = 4 * 200 = 800 pixels.

2. Step 2: Calculate height in pixels

   Height in pixels = height in inches * dpi = 3 * 200 = 600 pixels.

3. Final Answer:

   800 x 600 pixels -> Option B

4. Quick Check:

   Pixels = inches * dpi = 800x600 [OK]

Hint: Multiply inches by dpi for pixel size [OK]

Common Mistakes:

• Confusing dpi with inches
• Multiplying dpi by 100 instead of inches
• Using dpi as pixel count directly

4. Identify the error in this code snippet that tries to save a plot with 150 DPI:

import matplotlib.pyplot as plt
plt.plot([1,2,3],[4,5,6])
plt.savefig('myplot.png', dpi='150')

medium

A. dpi value should be an integer, not a string

B. plt.plot syntax is incorrect

C. Filename must be .jpg not .png

D. Missing plt.show() before saving

Solution

1. Step 1: Check the dpi parameter type

   The dpi parameter expects an integer number, but here it is passed as a string '150'.

2. Step 2: Understand impact of wrong type

   Passing dpi as a string may cause a type error or unexpected behavior when saving the plot.

3. Final Answer:

   dpi value should be an integer, not a string -> Option A

4. Quick Check:

   dpi must be int, not string = A [OK]

Hint: dpi must be a number, not quoted text [OK]

Common Mistakes:

• Passing dpi as string instead of int
• Thinking plt.show() is needed before savefig
• Assuming file extension affects dpi

5. You want to save a plot with a fixed pixel size of 1200x900 pixels. Which combination of figsize and dpi will achieve this?

hard

A. figsize=(6,4.5) and dpi=200

B. figsize=(12,10) and dpi=100

C. figsize=(10,8) and dpi=120

D. figsize=(8,7) and dpi=150

Solution

1. Step 1: Understand pixel size formula

   Pixels = figsize (inches) * dpi. We want 1200 x 900 pixels.

2. Step 2: Check each option's pixel size

   A: 12*100=1200, 10*100=1000 (wrong)
   B: 6*200=1200, 4.5*200=900 (correct)
   C: 10*120=1200, 8*120=960 (wrong)
   D: 8*150=1200, 7*150=1050 (wrong).

3. Final Answer:

   figsize=(6,4.5) and dpi=200 -> Option A

4. Quick Check:

   Pixels = inches * dpi = 1200x900 [OK]

Hint: Pixels = figsize * dpi; pick balanced values [OK]

Common Mistakes:

• Choosing too large figsize with low dpi
• Ignoring pixel size formula
• Assuming dpi alone sets pixel size