DPI settings for resolution in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does DPI stand for in matplotlib?

DPI stands for Dots Per Inch. It controls the resolution of the saved figure, affecting how sharp or detailed the image looks.

beginner

How does increasing DPI affect the saved image in matplotlib?

Increasing DPI makes the image sharper and more detailed by increasing the number of dots per inch, but it also increases the file size.

beginner

Which matplotlib function parameter controls the DPI of a saved figure?

The 'dpi' parameter in the savefig() function controls the resolution of the saved image.

beginner

True or False: DPI affects the size of the figure displayed on screen in matplotlib.

False. DPI mainly affects the resolution of saved images, not the size of the figure displayed on screen.

intermediate

How can you set the DPI for a figure when creating it in matplotlib?

You can set the DPI by passing the 'dpi' argument when creating a figure, like plt.figure(dpi=150), to control resolution from the start.

What does increasing DPI do to a matplotlib saved image?

A Decreases image sharpness

B Increases image sharpness and file size

C Changes the figure size on screen

D Removes colors from the image

Increasing DPI increases the number of dots per inch, making the image sharper and larger in file size.

Which parameter controls DPI when saving a figure in matplotlib?

A size in plot()

B linewidth in plot()

C color in figure()

D dpi in savefig()

The dpi parameter in savefig() sets the resolution of the saved image.

Does DPI affect the figure size shown on screen in matplotlib?

A No, it only affects saved image resolution

B No, it changes line thickness

C Yes, it changes colors

D Yes, it changes the size on screen

DPI mainly affects the resolution of saved images, not the on-screen figure size.

How do you set DPI when creating a new figure in matplotlib?

A plt.savefig(dpi=150)

B plt.plot(dpi=150)

C plt.figure(dpi=150)

D plt.show(dpi=150)

You set DPI at figure creation with plt.figure(dpi=150).

What happens if you save a matplotlib figure with a low DPI?

A Image looks blurry or pixelated

B Image file size is very large

C Image colors invert

D Figure size increases

Low DPI means fewer dots per inch, causing a blurry or pixelated saved image.

Explain what DPI means in matplotlib and how it affects your saved images.

Think about how many dots fit in one inch of your image.

Describe how to set DPI both when creating a figure and when saving it in matplotlib.

There are two places where DPI can be controlled.

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