Image extent and aspect ratio in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the extent parameter control when displaying an image with matplotlib?

The extent parameter sets the bounding box in data coordinates for the image. It controls where the image is placed on the axes by specifying the left, right, bottom, and top limits.

beginner

What is the default behavior of the aspect parameter in matplotlib's imshow()?

By default, aspect='equal' which means each pixel is square and the image is scaled so that the x and y axes have the same length per unit.

intermediate

How does setting aspect='auto' affect an image in matplotlib?

Setting aspect='auto' allows the image to stretch to fill the axes area, ignoring the original pixel aspect ratio. This can distort the image but fits it to the plot size.

beginner

If you want an image to cover the x-axis from 0 to 10 and y-axis from -5 to 5, how would you set the extent parameter?

You would set extent=[0, 10, -5, 5] to place the image exactly within those data coordinate limits on the axes.

intermediate

Why is controlling the aspect ratio important when displaying images in data plots?

Controlling aspect ratio ensures the image is not stretched or squished, preserving the true shape and proportions. This is important for accurate visual interpretation.

What does the extent parameter in imshow() specify?

A The data coordinate limits where the image is placed

B The color map used for the image

C The interpolation method for the image

D The transparency level of the image

The extent parameter defines the bounding box in data coordinates for the image placement.

What happens if you set aspect='auto' in imshow()?

A The image keeps its original pixel aspect ratio

B The image stretches to fill the axes area

C The image is cropped to fit the axes

D The image is displayed in grayscale

aspect='auto' stretches the image to fill the axes, possibly distorting it.

Which aspect setting preserves the square shape of pixels in matplotlib?

A 'auto'

B 'square'

C 'equal'

D 'none'

aspect='equal' keeps pixels square and scales axes equally.

If you want an image to span from x=0 to x=5 and y=0 to y=10, what is the correct extent?

A [0, 5, 0, 10]

B [5, 0, 10, 0]

C [0, 10, 0, 5]

D [10, 0, 5, 0]

The extent is [left, right, bottom, top], so [0, 5, 0, 10] matches the axes limits.

Why might you want to change the aspect ratio when plotting an image?

A To change the image colors

B To fit the image to the plot area without distortion

C To add grid lines to the image

D To adjust the image size and preserve proportions

Changing aspect adjusts image size and preserves or changes proportions.

Explain how the extent parameter affects image placement in matplotlib plots.

Think about how you tell matplotlib where to put the image on the graph.

Describe the difference between aspect='equal' and aspect='auto' when displaying images.

Consider how the image looks when you want it to keep its shape versus fill the plot area.

Practice

1. What does the extent parameter control when displaying an image with matplotlib.pyplot.imshow()?

easy

A. The color map used for the image

B. The position and size of the image on the plot axes

C. The resolution of the image

D. The file format of the image

Solution

1. Step 1: Understand the role of extent

   The extent parameter defines the bounding box in data coordinates that the image will fill on the axes.

2. Step 2: Compare with other options

   Color map, resolution, and file format are unrelated to extent. They control different aspects of image display or file handling.

3. Final Answer:

   The position and size of the image on the plot axes -> Option B

4. Quick Check:

   Extent = position and size [OK]

Hint: Extent sets image box on axes, not colors or file type [OK]

Common Mistakes:

• Confusing extent with color map
• Thinking extent changes image resolution
• Assuming extent controls file format

2. Which of the following is the correct way to keep the image aspect ratio fixed when using imshow()?

easy

A. plt.imshow(img, aspect='equal')

B. plt.imshow(img, cmap='gray')

C. plt.imshow(img, extent=[0,1,0,1])

D. plt.imshow(img, aspect='auto')

Solution

1. Step 1: Identify aspect ratio options

   The aspect parameter controls image stretching. 'equal' keeps the aspect ratio fixed.

2. Step 2: Check other options

   'auto' allows stretching, extent sets position, and cmap sets colors, not aspect ratio.

3. Final Answer:

   plt.imshow(img, aspect='equal') -> Option A

4. Quick Check:

   Aspect='equal' fixes ratio [OK]

Hint: Use aspect='equal' to keep image shape correct [OK]

Common Mistakes:

• Using aspect='auto' which stretches image
• Confusing extent with aspect ratio
• Setting cmap instead of aspect

3. What will be the effect of this code snippet?

import matplotlib.pyplot as plt
import numpy as np
img = np.ones((10, 20))
plt.imshow(img, extent=[0, 5, 0, 10], aspect='auto')
plt.show()

medium

A. Image will be shown with default extent and fixed aspect ratio

B. Image will keep original shape and size ignoring extent

C. Code will raise an error due to wrong extent format

D. Image will stretch to fill x from 0 to 5 and y from 0 to 10, possibly distorted

Solution

1. Step 1: Analyze extent parameter

   The extent=[0,5,0,10] sets the image to cover x-axis 0 to 5 and y-axis 0 to 10 on the plot.

2. Step 2: Analyze aspect='auto'

   Aspect='auto' allows the image to stretch to fill the extent box, so the image shape may distort.

3. Final Answer:

   Image will stretch to fill x from 0 to 5 and y from 0 to 10, possibly distorted -> Option D

4. Quick Check:

   Extent sets size, aspect='auto' allows stretch [OK]

Hint: Extent sets size; aspect='auto' allows distortion [OK]

Common Mistakes:

• Assuming extent is ignored
• Expecting fixed aspect ratio with aspect='auto'
• Thinking code raises error

4. Identify the error in this code that tries to display an image with fixed aspect ratio:

import matplotlib.pyplot as plt
import numpy as np
img = np.random.rand(5,5)
plt.imshow(img, extent=[0,5,0], aspect='equal')
plt.show()

medium

A. The aspect='equal' is invalid and causes error

B. The image array shape is incompatible with imshow

C. The extent list has incorrect length; it should have 4 values

D. Missing plt.axis('equal') to fix aspect ratio

Solution

1. Step 1: Check extent parameter format

   Extent must be a list of 4 numbers: [xmin, xmax, ymin, ymax]. Here it has only 3 values, causing an error.

2. Step 2: Verify other parameters

   Aspect='equal' is valid. Image shape is fine. plt.axis('equal') is optional when aspect is set.

3. Final Answer:

   The extent list has incorrect length; it should have 4 values -> Option C

4. Quick Check:

   Extent needs 4 numbers [OK]

Hint: Extent must have 4 numbers: xmin, xmax, ymin, ymax [OK]

Common Mistakes:

• Using extent with less than 4 values
• Confusing aspect parameter validity
• Thinking plt.axis('equal') is required

5. You want to overlay a heatmap image on a scatter plot with x values from 0 to 10 and y values from 0 to 5. Which extent and aspect settings correctly align the image without distortion?

hard

A. extent=[0,10,0,5], aspect='equal'

B. extent=[0,5,0,10], aspect='auto'

C. extent=[0,10,0,5], aspect='auto'

D. extent=[0,5,0,10], aspect='equal'

Solution

1. Step 1: Match extent to data range

   The scatter plot x ranges 0-10 and y ranges 0-5, so extent must be [0,10,0,5] to align image correctly.

2. Step 2: Choose aspect to avoid distortion

   Aspect='equal' keeps the image shape correct, preventing distortion when overlaying.

3. Final Answer:

   extent=[0,10,0,5], aspect='equal' -> Option A

4. Quick Check:

   Extent matches data, aspect='equal' fixes shape [OK]

Hint: Match extent to data limits and use aspect='equal' [OK]

Common Mistakes:

• Swapping x and y in extent
• Using aspect='auto' causing distortion
• Ignoring data range when setting extent