fspecial (Image Processing Toolbox)

Create predefined filters

Syntax

h = fspecial(type)
h = fspecial(type,parameters)

Description

h = fspecial(type) creates a two-dimensional filter h of the specified type. (fspecial returns h as a computational molecule, which is the appropriate form to use with filter2.) type is a string having one of these values:

'gaussian' for a Gaussian lowpass filter
'sobel' for a Sobel horizontal edge-emphasizing filter
'prewitt' for a Prewitt horizontal edge-emphasizing filter
'laplacian' for a filter approximating the two-dimensional Laplacian operator
'log' for a Laplacian of Gaussian filter
'average' for an averaging filter
'unsharp' for an unsharp contrast enhancement filter

Depending on type, fspecial may take additional parameters which you can supply. These parameters all have default values.

h = fspecial('gaussian',n,sigma) returns a rotationally symmetric Gaussian lowpass filter with standard deviation sigma (in pixels). n is a 1-by-2 vector specifying the number of rows and columns in h. (n can also be a scalar, in which case h is n-by-n.) If you do not specify the parameters, fspecial uses the default values of [3 3] for n and 0.5 for sigma.

h = fspecial('sobel') returns this 3-by-3 horizontal edge-finding and y-derivative approximation filter:

    [ 1  2  1
      0  0  0
     -1 -2 -1 ]

To find vertical edges, or for x-derivatives, use -h'.

h = fspecial('prewitt') returns this 3-by-3 horizontal edge-finding and y-derivative approximation filter:

    [ 1  1  1
      0  0  0
     -1 -1 -1 ]

To find vertical edges, or for x-derivatives, use -h'.

h = fspecial('laplacian',alpha) returns a 3-by-3 filter approximating the two-dimensional Laplacian operator. The parameter alpha controls the shape of the Laplacian and must be in the range 0 to 1.0. fspecial uses the default value of 0.2 if you do not specify alpha.

h = fspecial('log',n,sigma) returns a rotationally symmetric Laplacian of Gaussian filter with standard deviation sigma (in pixels). n is a 1-by-2 vector specifying the number of rows and columns in h. (n can also be a scalar, in which case h is n-by-n.) If you do not specify the parameters, fspecial uses the default values of [5 5] for n and 0.5 for sigma.

h = fspecial('average',n) returns an averaging filter. n is a 1-by-2 vector specifying the number of rows and columns in h. (n can also be a scalar, in which case h is n-by-n.) If you do not specify n, fspecial uses the default value of [3 3].

h = fspecial('unsharp',alpha) returns a 3-by-3 unsharp contrast enhancement filter. fspecial creates the unsharp filter from the negative of the Laplacian filter with parameter alpha. alpha controls the shape of the Laplacian and must be in the range 0 to 1.0. fspecial uses the default value of 0.2 if you do not specify alpha.

Example

I = imread('saturn.tif');
h = fspecial('unsharp',0.5);
I2 = filter2(h,I)/255;
imshow(I)
figure, imshow(I2)

Algorithms

fspecial creates Gaussian filters using:

fspecial creates Laplacian filters using:

fspecial creates Laplacian of Gaussian (LoG) filters using:

fspecial creates averaging filters using:

ones(n(1),n(2))/(n(1)*n(2))

fspecial creates unsharp filters using: