DPI settings for resolution in Matplotlib

Introduction

DPI controls how clear and sharp your plot looks when saved or displayed. Higher DPI means better quality.

When saving a plot to share in a report or presentation.

When you want a high-quality image for printing.

When displaying plots on high-resolution screens.

When you need to reduce file size by lowering DPI for quick previews.

When adjusting plot clarity for different output devices.

Syntax

Matplotlib

plt.savefig('filename.png', dpi=VALUE)

# or when creating a figure
fig = plt.figure(dpi=VALUE)

dpi stands for dots per inch and controls image resolution.

Higher dpi means more pixels and sharper images but larger file size.

Examples

Saves the plot with low resolution (50 dpi).

Matplotlib

plt.savefig('plot_low.png', dpi=50)

Saves the plot with high resolution (300 dpi), good for printing.

Matplotlib

plt.savefig('plot_high.png', dpi=300)

Creates a figure with 200 dpi resolution for display.

Matplotlib

fig = plt.figure(dpi=200)
plt.plot([1, 2, 3], [4, 5, 6])
plt.show()

Sample Program

This code creates a simple line plot and saves it twice: once with 72 dpi and once with 300 dpi. The higher dpi image will be sharper and better for printing.

Matplotlib

import matplotlib.pyplot as plt

# Create a simple plot
plt.plot([1, 2, 3], [4, 5, 6])

# Save with low dpi
plt.savefig('plot_72dpi.png', dpi=72)

# Save with high dpi
plt.savefig('plot_300dpi.png', dpi=300)

print('Saved two plots with different DPI settings.')

Important Notes

Default dpi in matplotlib is usually 100.

Changing dpi affects only saved images or figure size, not the data itself.

Use dpi to balance image quality and file size.

Summary

DPI controls the sharpness of saved or displayed plots.

Higher DPI means better quality but larger files.

Use plt.savefig() with dpi to set resolution.

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