DPI settings for resolution in Matplotlib - Mini Project: Build & Apply

DPI Settings for Resolution in Matplotlib

📖 Scenario: You want to create a simple plot and control its resolution by setting the DPI (dots per inch). This is useful when you save images for presentations or reports to make sure they look sharp.

🎯 Goal: Learn how to create a plot with Matplotlib and set the DPI to control the image resolution.

📋 What You'll Learn

Create a simple line plot using Matplotlib

Set a variable for DPI value

Use the DPI setting when creating the figure

Display the plot

💡 Why This Matters

🌍 Real World

Setting DPI is important when creating images for reports, presentations, or publications to ensure they look clear and professional.

💼 Career

Data scientists and analysts often need to create high-quality visualizations for sharing insights with others.

1

Create a simple line plot data

Create two lists called x and y with these exact values: x = [1, 2, 3, 4, 5] and y = [2, 3, 5, 7, 11].

Matplotlib

# Create lists x and y with the given values
# Your code here

Hint

Use square brackets to create lists and separate numbers with commas.

2

Set the DPI value

Create a variable called dpi_value and set it to 150.

Matplotlib

x = [1, 2, 3, 4, 5]
y = [2, 3, 5, 7, 11]
# Create dpi_value variable and set it to 150
# Your code here

Hint

Use a simple assignment to create the variable.

3

Create the plot with DPI setting

Import matplotlib.pyplot as plt. Then create a figure using plt.figure() with the dpi parameter set to dpi_value. Plot x and y using plt.plot().

Matplotlib

x = [1, 2, 3, 4, 5]
y = [2, 3, 5, 7, 11]
dpi_value = 150
# Import matplotlib.pyplot as plt
# Create figure with dpi=dpi_value
# Plot x and y
# Your code here

Hint

Use plt.figure(dpi=dpi_value) to set the resolution before plotting.

4

Display the plot

Use plt.show() to display the plot with the DPI setting.

Matplotlib

import matplotlib.pyplot as plt

x = [1, 2, 3, 4, 5]
y = [2, 3, 5, 7, 11]
dpi_value = 150

plt.figure(dpi=dpi_value)
plt.plot(x, y)
# Display the plot
# Your code here

Hint

Call plt.show() to see the plot window.

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