Edge detection Flashcards
What is an intensity image?
A matrix whose values represent the intensity of light
What is an intensity gradient?
A function mapping coordinates to intensity.
How can you find the gradient of an intensity image in the x direction?
Use the mask [
[-1, 1],
[-1, 1]
]
How can you find the gradient of an intensity image in the y direction?
Use the mask [
[1, 1],
[-1, -1]
]
Edge direction
Perpendicular to the direction of maximum intensity change
What are the main steps in edge detection?
Smoothing
Enhancement
Thresholding
Localisation
What are the two ways edges are detected using derivatives?
Detecting the local maxima or minima of the first derivative
Detecting the zero-crossings of the second derivative
How does hysteresis thresholding work?
Uses a high and a low threshold. Points above the high threshold are definitely an edge. Points above the low threshold are an edge if they are connected to an existing edge.
Why are second order edge detectors better than first order?
First order give too many edge points, second order only give edge points that are the local maxima.
Describe the steps of canny edge detector
Smooth the image using a gaussian
Compute image gradient using the sobel operator
Compute laplacian along the gradient direction at each pixel
Find zero crossings in laplacian to find the edge location
What is a gaussian also known as?
Bell curve
The laplacian is
The second derivative of a signal
After taking a laplacian, what must you do to find edges?
Find the zero crossings
What are the advantages of the canny edge detector?
Produces smooth, single pixel thick edges.
Resistant to noise
What is the advantage of the sobel edge detector?
Quick to compute
