[Artwork] / [QisMLib] / [Programmer's Corner]

2x2 Subsampling of a monochrome bitmap image

Creating a suitable Recipe

• In 2x2 down-sampling, we map every 2x2 block of pixels from the input (two pixels on two rows) to a single pixel in the output

• The user-specified recipe controls which of the 16 possible 2x2 patterns from the input results in a black pixel in the output

• The recipe is an array of 16 bytes (unsigned char) where each byte can either be 0 (white/OFF pixel) or 1 (black/ON) pixel and the index/position of the byte in the array (0 .. 15) determines the corresponding 2x2 pattern from the input

• The default recipe uses the rule that the output pixel is black if the input pattern has more than 50% black pixels

  xxxxxxxxxx
  unsigned char default_recipe[] = { //{output}, {input-top-left}{top-right} {bottom-left}{bottom-right}
    0, //00 00
    0, //00 01
    0, //00 10
    0, //00 11
    0, //01 00
    0, //01 01
    0, //01 10
    1, //01 11 >50% black
    0, //10 00
    0, //10 01
    0, //10 10
    1, //10 11 >50% black
    0, //11 00
    1, //11 01 >50% black
    1, //11 10 >50% black
    1  //11 11 >50% black
  };
  

• An alternate recipe could allow black-white transitions from the input to set a pixel in the output as follows :-

  xxxxxxxxxx
  unsigned char alternate_recipe[] = { //{output}, {input-top-left}{top-right} {bottom-left}{bottom-right}
    0, //00 00
    0, //00 01
    0, //00 10
    1, //00 11 transition from white row (top) to black (bottom)
    0, //01 00 
    1, //01 01 transition from white column (left) to black (right)
    0, //01 10 
    1, //01 11 >50% black
    0, //10 00
    0, //10 01
    1, //10 10 transition from black column (left) to white (right)
    1, //10 11 >50% black
    1, //11 00 transition from black row (top) to white (bottom)
    1, //11 01 >50% black
    1, //11 10 >50% black
    1  //11 11 >50% black
  };