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Lecture 2: Edge detection

CS5670: Computer Vision

From Sandlot Science

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Announcements

  • Project 1 (Hybrid Images) is now on the course webpage (see Projects link)
    • Due Friday, Feb 7, by 8pm on Github Classroom
    • Artifact due Monday, Feb 10, by 8pm on CMSX
    • Project to be done individually
    • Skeleton code available soon on Github Classroom – instructions for setting up Python environment on the project webpage
  • Course webpage: https://www.cs.cornell.edu/courses/cs5670/2025sp/
    • Has lectures, projects, office hours, etc
  • In-class Quiz first 10 minutes of class this Thursday

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Project 1: Hybrid Images

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Project 1 Demo

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Reading

  • Szeliski 7.2

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Edge detection

TexPoint fonts used in EMF.

Read the TexPoint manual before you delete this box.: AAA

  • Convert a 2D image into a set of curves
    • Extracts salient features of the scene
    • More compact than pixels

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Origin of edges

  • Edges are caused by a variety of factors

depth discontinuity

surface color discontinuity

illumination discontinuity

surface normal discontinuity

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Images as functions…

  • Edges look like steep cliffs

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Characterizing edges

  • An edge is a place of rapid change in the image intensity function

image

intensity function�(along horizontal scanline)

first derivative

edges correspond to�extrema of derivative

Source: L. Lazebnik

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Image derivatives

  • How can we differentiate a digital image F[x,y]?
    • Option 1: reconstruct a continuous image, f, then compute the derivative
    • Option 2: take discrete derivative (finite difference)

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How would you implement this as a linear filter?

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Source: S. Seitz

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Image gradient

  • The gradient of an image:

The gradient points in the direction of most rapid increase in intensity���

The edge strength is given by the gradient magnitude:

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The gradient direction is given by:

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    • how does this relate to the direction of the edge?�

Source: Steve Seitz

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Image gradient

Source: L. Lazebnik

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Effects of noise

Where is the edge?

Source: S. Seitz

Noisy input image

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Solution: smooth first

f

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f * h

Source: S. Seitz

To find edges, look for peaks in

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Associative property of convolution

  • Differentiation is convolution, and convolution is associative:

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  • This saves us one operation:

f

Source: S. Seitz

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The 1D Gaussian and its derivatives

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2D edge detection filters

Gaussian

derivative of Gaussian (x)

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Derivative of Gaussian filter

x-direction

y-direction

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The Sobel operator

  • Common approximation of derivative of Gaussian

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  • The standard definition of the Sobel operator omits the 1/8 term
      • doesn’t make a difference for edge detection
      • the 1/8 term is needed to get the right gradient magnitude

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Sobel operator: example

Source: Wikipedia

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Which method is the best way to compute the gradient of an image?

ⓘ Start presenting to display the poll results on this slide.

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Example

original image

Demo: http://bigwww.epfl.ch/demo/ip/demos/edgeDetector/

Image credit: Joseph Redmon

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smoothed gradient magnitude

Finding edges

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thresholding

Finding edges

where is the edge?

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Get Orientation at Each Pixel

  • Get orientation (below, threshold at minimum gradient magnitude)

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θ = atan2(gy, gx)

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Gradient orientation angle

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Non-maximum supression

  • Check if pixel is local maximum along gradient direction
    • requires interpolating pixels p and r

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Before Non-max Suppression

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After Non-max Suppression

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Thresholding edges

  • Still some noise
  • Only want strong edges
  • 2 thresholds, 3 cases
    • R > T: strong edge
    • R < T but R > t: weak edge
    • R < t: no edge
  • Why two thresholds?

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Connecting edges

  • Strong edges are edges!
  • Weak edges are edges �iff they connect to strong
  • Look in some neighborhood�(usually 8 closest)

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Canny edge detector

  1. Filter image with derivative of Gaussian

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  • Find magnitude and orientation of gradient

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  • Non-maximum suppression

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  • Linking and thresholding (hysteresis):
    • Define two thresholds: low and high
    • Use the high threshold to start edge curves and the low threshold to continue them

Source: D. Lowe, L. Fei-Fei, J. Redmon

MATLAB: edge(image,‘canny’)

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Canny edge detector

  • Our first computer vision pipeline!
  • Still a widely used edge detector in computer vision

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  • Depends on several parameters:

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J. Canny, A Computational Approach To Edge Detection, IEEE Trans. Pattern Analysis and Machine Intelligence, 8:679-714, 1986.

: width of the Gaussian blur

high threshold

low threshold

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Canny edge detector

Canny with

Canny with

original

  • The choice of depends on desired behavior
    • large detects “large-scale” edges
    • small detects fine edges

Source: S. Seitz

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Scale space [Witkin 83]

  • Properties of scale space (w/ Gaussian smoothing)
    • edge position may shift with increasing scale (σ)
    • two edges may merge with increasing scale
    • an edge may not split into two with increasing scale

larger

Gaussian filtered signal

first derivative peaks

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Demo

http://bigwww.epfl.ch/demo/ip/demos/edgeDetector/

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Questions?