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Convolution and Edge Detection. 15-463: Computational Photography Alexei Efros, CMU, Fall 2005. Some slides from Steve Seitz. Fourier spectrum . Fun and games with spectra. Gaussian filtering. A Gaussian kernel gives less weight to pixels further from the center of the window

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convolution and edge detection
Convolution and Edge Detection

15-463: Computational Photography

Alexei Efros, CMU, Fall 2005

Some slides from Steve Seitz

gaussian filtering
Gaussian filtering
  • A Gaussian kernel gives less weight to pixels further from the center of the window
  • This kernel is an approximation of a Gaussian function:
  • Remember cross-correlation:
  • A convolution operation is a cross-correlation where the filter is flipped both horizontally and vertically before being applied to the image:
  • It is written:
  • Suppose H is a Gaussian or mean kernel. How does convolution differ from cross-correlation?
the convolution theorem
The greatest thing since sliced (banana) bread!

The Fourier transform of the convolution of two functions is the product of their Fourier transforms

The inverse Fourier transform of the product of two Fourier transforms is the convolution of the two inverse Fourier transforms

Convolution in spatial domain is equivalent to multiplication in frequency domain!

The Convolution Theorem
2d convolution theorem example
2D convolution theorem example








image gradient

The gradient direction is given by:

    • how does this relate to the direction of the edge?
  • The edge strength is given by the gradient magnitude
Image gradient
  • The gradient of an image:
  • The gradient points in the direction of most rapid change in intensity
effects of noise
Effects of noise
  • Consider a single row or column of the image
    • Plotting intensity as a function of position gives a signal

How to compute a derivative?

  • Where is the edge?
derivative theorem of convolution
Derivative theorem of convolution
  • This saves us one operation:
laplacian of gaussian
Laplacian of Gaussian
  • Consider

Laplacian of Gaussian


  • Where is the edge?
  • Zero-crossings of bottom graph
2d edge detection filters

Laplacian of Gaussian

  • is the Laplacian operator:
2D edge detection filters


derivative of Gaussian

matlab demo

g = fspecial('gaussian',15,2);



gclown = conv2(clown,g,'same');

imagesc(conv2(clown,[-1 1],'same'));

imagesc(conv2(gclown,[-1 1],'same'));

dx = conv2(g,[-1 1],'same');


lg = fspecial('log',15,2);

lclown = conv2(clown,lg,'same');


imagesc(clown + .2*lclown)

what does blurring take away1
What does blurring take away?

smoothed (5x5 Gaussian)

edge detection by subtraction
Edge detection by subtraction

Why does

this work?

smoothed – original

gaussian image filter
Gaussian - image filter



delta function

Laplacian of Gaussian