Vectorization

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• Questions?
• Vectorization
  • Section 5.4
    • Replace for loops driven by length or size of vector with functions or expressions that operate on each element internally
      • e.g.,
        

        for i = 1 : length(v)
            w( i ) = f( v(i) )
        end

      • vs
        

        w = f( v )

    • Matlab “vectorization” is not quite what everyone else means by “vectorization”
  • Example
    • Brightest color intensity in an image
    • Digital image is a 3-dimensional array of primary color intensities
      
    • Matlab has a number of commands for reading and displaying images
      

      >> mars = imread( 'Chromakey/MarsLookoutPoint.jpg' );
      >> size(mars)
      ans =
          387   516     3
      >> mars( 1, 1, 1 )
      ans =
          178
      >> mars( 1, 1, 2 )
      ans =
          135
      >> imshow( mars )

    • An unvectorized way to find the highest primary color intensity in an image
      

      function [ b ] = brightest( img )
      %BRIGHTEST Find the highest color intensity in an image
      b = 0;
      [maxR, maxC, maxP] = size( img );
      for r = 1 : maxR
          for c = 1 : maxC
              for p = 1 : maxP
                  if img(r,c,p) > b
                      b = img(r,c,p);
                  end
              end
          end
      end
      end

    • Figure out a vectorized equivalent and compare
      

      >> brightest(mars)
      ans =
          198
      >> max(mars)
      ans(:,:,1) =
          Columns 1 through 15
          178  179  178  179  180  180  180  178  179  177  178  178  178  178  184
          ...
      ans(:,:,2) =
          Columns 1 through 15
          137  137  136  135  136  138  137  137  135  135  136  136  136  134  134
          ...
      ans(:,:,3) =
          Columns 1 through 15
          96   95   94   96   97   96   95   96   96   95   96   96   96   95   95
      >> size(max(mars))
      ans =
          1   516     3
      >> max(max(mars))
      ans(:,:,1) =
          198
      ans(:,:,2) =
          138
      ans(:,:,3) =
          97
      >> max(max(max(mars)))
      ans =
          198
      >> tic
      >> toc
      Elapsed time is 4.999782 seconds.
      >> tic;brightest(mars);toc
      Elapsed time is 1.618765 seconds.
      >> tic;max(max(max(mars)));toc
      Elapsed time is 0.000567 seconds.

• Lab 7
  • Chroma key
    • i.e., is the me on Mars?
      
    • Or me and a blue sheet...
      
    • ...and a Nasa rover photo from Mars?
      
  • Handout
  • Move exam 2 to Friday April 17?
• Next
  • Image processing
  • Read
    • “Introduction to Digital Images” at http://cs.geneseo.edu/~baldwin/reference/images.html
    • Section 13.2

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