Morphological operations in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of morphological operations in image processing?

Morphological operations are used to process images based on shapes. They help in removing noise, separating objects, and extracting useful structures by modifying the shapes in binary or grayscale images.

beginner

Explain the difference between dilation and erosion.

Dilation adds pixels to the boundaries of objects, making them larger. Erosion removes pixels from object boundaries, making them smaller. Both use a structuring element to decide how to modify the image.

beginner

What is a structuring element in morphological operations?

A structuring element is a small shape or mask used to probe and interact with the image. It defines the neighborhood for dilation, erosion, opening, and closing operations.

intermediate

Describe the opening operation and its effect on an image.

Opening is erosion followed by dilation. It removes small objects or noise while keeping the shape and size of larger objects intact.

intermediate

What does the closing operation do in morphological processing?

Closing is dilation followed by erosion. It fills small holes and gaps in objects, smoothing their boundaries without changing their overall size.

Which morphological operation would you use to remove small noise from a binary image?

A Closing

B Dilation

C Opening

D Erosion

Opening removes small objects or noise by eroding then dilating the image.

What is the effect of dilation on an object in an image?

A Expands the object

B Removes holes inside the object

C Shrinks the object

D Removes noise

Dilation expands the boundaries of objects by adding pixels.

In MATLAB, which function performs erosion on a binary image?

A imdilate

B imclose

C imopen

D imerode

The function 'imerode' performs erosion in MATLAB.

What does the structuring element define in morphological operations?

A The color of the image

B The shape and size of the neighborhood for processing

C The image resolution

D The file format

The structuring element defines the shape and size of the neighborhood used in morphological operations.

Which operation fills small holes inside objects in a binary image?

A Closing

B Erosion

C Opening

D Dilation

Closing fills small holes by dilating then eroding the image.

Explain how dilation and erosion work together in the opening operation and why this is useful.

Think about removing small dots but keeping big shapes intact.

Describe the role of the structuring element in morphological operations and how changing it affects the result.

Imagine using different shaped stamps to press on dough.

Practice

1. Which MATLAB function is used to create a structuring element for morphological operations?

easy

A. imerode

B. strel

C. imopen

D. imdilate

Solution

1. Step 1: Understand structuring element creation

   The structuring element defines the shape used in morphological operations.

2. Step 2: Identify the MATLAB function for structuring elements

   In MATLAB, strel creates structuring elements like disks or lines.

3. Final Answer:

   strel -> Option B

4. Quick Check:

   Structuring element = strel [OK]

Hint: Remember: strel makes shapes for morphology [OK]

Common Mistakes:

• Confusing strel with imopen or imerode
• Using imdilate to create structuring elements
• Thinking imopen creates structuring elements

2. Which of the following is the correct syntax to perform erosion on a binary image BW using a disk structuring element of radius 3?

easy

A. BW2 = imopen(BW, strel('disk', 3));

B. BW2 = imdilate(BW, strel('disk', 3));

C. BW2 = imerode(BW, strel('disk', 3));

D. BW2 = imclose(BW, strel('disk', 3));

Solution

1. Step 1: Identify the erosion function

   In MATLAB, imerode performs erosion on images.

2. Step 2: Check the syntax for erosion with a disk structuring element

   The correct syntax is imerode(image, strel('disk', radius)).

3. Final Answer:

   BW2 = imerode(BW, strel('disk', 3)); -> Option C

4. Quick Check:

   Erosion syntax = imerode(image, strel) [OK]

Hint: imerode + strel('disk', radius) erodes image [OK]

Common Mistakes:

• Using imdilate instead of imerode
• Wrong order of arguments
• Missing quotes around 'disk'

3. What will be the output size of the binary image BW2 after applying BW2 = imdilate(BW, strel('square', 3)); if BW is a 5x5 matrix?

medium

A. Cannot determine without image content

B. 7x7

C. 3x3

D. 5x5

Solution

1. Step 1: Understand dilation size effect

   Dilation expands shapes but does not change image size in MATLAB.

2. Step 2: Check MATLAB behavior on image size

   MATLAB morphological functions keep the output image size same as input.

3. Final Answer:

   5x5 -> Option D

4. Quick Check:

   Output size = input size for imdilate [OK]

Hint: Morphology keeps image size same in MATLAB [OK]

Common Mistakes:

• Assuming dilation increases image size
• Confusing structuring element size with output size
• Thinking output depends on image content

4. Consider the code snippet:
se = strel('line', 5, 45);
BW2 = imerode(BW, se);
What is the likely error if BW is a grayscale image instead of binary?

medium

A. No error; imerode works on grayscale images

B. Error because strel cannot create line elements

C. Error because imerode expects binary images only

D. Error due to missing image conversion to binary

Solution

1. Step 1: Check imerode input types

   MATLAB's imerode supports both binary and grayscale images.

2. Step 2: Verify structuring element creation

   strel('line', 5, 45) correctly creates a line structuring element.

3. Final Answer:

   No error; imerode works on grayscale images -> Option A

4. Quick Check:

   imerode supports grayscale and binary [OK]

Hint: imerode works on grayscale too, no conversion needed [OK]

Common Mistakes:

• Thinking imerode only works on binary
• Assuming strel('line') is invalid
• Forgetting imerode supports grayscale images

5. You want to remove small noise dots from a binary image BW without changing the main shapes. Which morphological operation and structuring element combination is best?

hard

A. imopen with strel('disk', 2)

B. imclose with strel('square', 5)

C. imerode with strel('line', 3, 0)

D. imdilate with strel('diamond', 1)

Solution

1. Step 1: Understand noise removal by morphology

   Opening removes small objects (noise) while preserving shape size.

2. Step 2: Choose suitable structuring element

   A disk with radius 2 is good for removing small dots without harming main shapes.

3. Final Answer:

   imopen with strel('disk', 2) -> Option A

4. Quick Check:

   Opening + disk removes noise best [OK]

Hint: Use imopen with small disk to clean noise [OK]

Common Mistakes:

• Using imclose which fills holes
• Using erosion alone which shrinks shapes
• Using dilation alone which enlarges noise