Image Processing Toolbox User's Guide

Lookup Table Operations

Certain binary image operations can be implemented most easily through lookup tables. A lookup table is a column vector in which each element represents the value to return for one possible combination of pixels in a neighborhood.

You can use the makelut function to create lookup tables for various operations. makelut creates lookup tables for 2-by-2 and 3-by-3 neighborhoods. This figure illustrates these types of neighborhoods. Each neighborhood pixel is indicated by an x, and the center pixel is the one with a circle.

For a 2-by-2 neighborhood, there are 16 possible permutations of the pixels in the neighborhood. Therefore, the lookup table for this operation is a 16-element vector. For a 3-by-3 neighborhood, there are 512 permutations, so the lookup table is a 512-element vector.

Once you create a lookup table, you can use it to perform the desired operation by using the applylut function.

The example below illustrates using lookup table operations to modify an image containing text. You begin by writing a function that returns 1 if three or more pixels in the 3-by-3 neighborhood are 1; otherwise, it returns 0. You then call makelut, passing in this function as the first argument, and using the second argument to specify a 3-by-3 lookup table.

• f = inline('sum(x(:)) >= 3');
  lut = makelut(f,3);
  

lut is returned as a 512-element vector of 1's and 0's. Each value is the output from the function for one of the 512 possible permutations.

You then perform the operation using applylut.

• BW1 = imread('text.png'); 
  BW2 = applylut(BW1,lut);
  imshow(BW1)
  figure, imshow(BW2)
  
  
  

Image Before and After Applying Lookup Table Operation

For information about how applylut maps pixel combinations in the image to entries in the lookup table, see the reference page for applylut.

Note    You cannot use makelut and applylut for neighborhoods of sizes other than 2-by-2 or 3-by-3. These functions support only 2-by-2 and 3-by-3 neighborhoods, because lookup tables are not practical for neighborhoods larger than 3-by-3. For example, a lookup table for a 4-by-4 neighborhood would have 65,536 entries.

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