Digital Images

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• Misc
  • REU Talk
    • Megan Brunner
    • Measuring intercultural competence in middle school students
    • Thursday, April 14, 3:30, South 328
    • Extra credit for write-ups
• Questions?
• Images
  • Sections 11.1, 11.2, 11.5
    • 3D arrays
      • x, y, and z dimensions, 3rd dimension is “layer” dimension
      • Apartment building example
      • Creating layers in for loop
    • Images
      • Pixel = picture element = region of uniform color
      • RGB image stored in m-by-n-by-3 array
      • Color determined by red, blue, green intensity
    • Operations on images
      • imread reads image from file (e.g., JPEG)
      • image creates (and displays) image from array
      • imshow displays image
      • imfinfo returns structure that describes an image file
      • imwrite writes image to file
  • Chroma Key Lab
    • Handout
    • About chroma (i.e., color) key compositing (i.e., combining images)
  • Examples
    

    >> % Make a black color swatch
    >> swatch = zeros( 100, 100, 3 );
    >> swatch = uint8( swatch );
    >> % Display it
    >> imshow( swatch )
    >> % Make it white
    >> swatch( :, :, : ) = 255;
    >> imshow( swatch )
    >> % Make it gray
    >> swatch( :, :, : ) = 155;
    >> imshow( swatch )
    >> % Make it red-gray (dull pink)
    >> swatch( :, :, 1 ) = 255;
    >> imshow( swatch )
    >> % Save the swatch as a JPEG file
    >> imwrite( swatch, 'Swatch.jpg' );
    >> % Read in another image and replace part of it by a piece of the pink color swatch
    >> baldwinParlor = imread( 'BaldwinParlor.jpg' );
    >> imshow( baldwinParlor )
    >> size( baldwinParlor )
    ans =
       387   516     3
    >> size( swatch )
    ans =
       100   100     3
    >> baldwinParlor( 100:110, 100:110, : ) = swatch( 1:11, 1:11, : );
    >> imshow( baldwinParlor )
    >> % Check the numeric values of the colors in a section of the image
    >> baldwinParlor( 10:15, 30:35, : )
    ans(:,:,1) =
      206  206  206  206  206  205
      207  207  207  206  206  205
      209  209  209  208  208  208
      207  207  207  208  208  208
      205  205  205  205  205  205
      206  206  206  205  205  206
    ans(:,:,2) =
      207  207  207  207  207  206
      207  207  207  206  206  205
      209  209  209  208  208  208
      207  207  207  208  208  208
      205  205  205  205  205  205
      206  206  206  205  205  206
    ans(:,:,3) =
      202  202  202  202  202  201
      205  205  205  204  204  203
      207  207  207  206  206  206
      205  205  205  206  206  206
      203  203  203  203  203  203
      204  204  204  203  203  204

• Next
  • Chroma key algorithm
    • High-level idea:
      

      chromakey( subject, background, ... )
          look for subject pixels with RGB in certain range
          replace found pixels in subject with same pixels f/ background

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