CS51 - Fall 2009 - Lecture 28

  • TicTacToe one last time
       - Given
          LEFT // x value of the left of the board
          TOP // y value of the top of the board
          CELL_SIZE // the size of the an individual cell in the board
          that the user clicked at x, y, which we know is in the board
       - How do we figure out what cell/board space the user clicked in?
          x = x - LEFT;
          y = y - TOP;
          int col = (int)(x/CELL_SIZE);
          int row = (int)(y/CELL_SIZE);

  • Lab attendance!

  • How is an image represented/stored in a computer?
       - a common approach is a matrix of Colors (often represented by RGB values)
       - How would we do this?
          - Color[][] image

  • show ImageEdit demo
       - we have two window areas
          - the one on the left is our image
          - the one on the right is our modified image
       - two variables
          - Color[][] source
          - Color[][] target
       - other private variables
          - private int height; // number of rows
          - private int width; // number of cols
       - grayscale
          - modifies one pixel at a time
          - What makes a color gray?
             - gray colors have identical RGB values
             - a measure of brightness
       - horizontal reversal
          - to do this we'll have two for loops and fill in "target"
             for( int row = 0; row < height; row++ ){
                for( int col = 0; col < width; col++ ){
          - let's look at a diagram - where does pixel row, col come from?
             - row stays the same
             - col is the distance from the col to the end of the image
          - target[row][col] = source[row][width-col-1]
       - vertical inversion
          - again, where does pixel row, col come from?
             - column stays the same
             - row is the distance from the row to the end of the image
             - target[row][col] = source[height - row - 1][col]
          - Is there a more efficient way of doing this?
             for( int row = 0; row < height; row++ ){
                target[row] = source[height-row-1]
       - rectangular averaging
          - target is the average of the pixels in the source in a rectangular range
             - private int range; // defines how far from the pixel we look in any direction
          - let's start by not worrying about the boundary cases
          for( int row = 0; row < height; row++ ){
             for( int col = 0; col < width; col++ ){
                int redSum = 0;
                int greenSum = 0;
                int blueSum = 0;
                int count = 0;

                for( int i = row-range; i < row+range+1; i++ ){
                   for( int j = col-range; j < col+range+1; j++ ){
                      redSum += source[i][j].getRed();
                      greenSum += source[i][j].getGreen();
                      blueSum += source[i][j].getBlue();
                      count++;
                   }
                }
             
             // average the values
             target[row][col] = new Color(redSum/count, greenSum/count, blueSum/count);
             }
          }
          - What do we need to add to account for the boundaries?
             - Some useful functions
                - Math.min
                - Math.max
             for( int i = Math.max(O,row-range); i < Math.min(width, row+range+1); i++ ){
                for( int j = Math.max(0,col-range); j < Math.min(hieght, col+range+1); j++ ){
  • extensible arrays
       - how would you do it?
       - java.util.ArrayList
          - get(index) // returns value at index
          - set(index, value) // sets value at index to be value
          - add(value) // adds value to the end of the array
    •