ITEC 120 2019fall asbrennem ibarland

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processing bitmaps (B&W)
2-D arrays of ints

This is a two-session lab; We will continue working on picture-processing methods next lab as well.

Pictures!

Download the provided Pict.java, and add it to your project. It has a helpful static method:

• Pict.displayPixels takes in any two-dimensional array of ints in [0,256), and creates a window showing that array as a picture.

  Task allocate-and-initialize the array below, and then call displayPixels on it).

  // int pixels as a literal array, then displayed: int[][] data1; data1 = { {30, 30, 30, 30, 30, 30, 30, 30, 30, 30}, {30, 30, 90, 200, 200, 200, 200, 200, 90, 30}, {30, 30, 200, 0, 0, 0, 0, 0, 200, 30}, {30, 30, 200, 0, 0, 0, 0, 0, 200, 30}, {30, 30, 200, 0, 255, 0, 255, 0, 200, 30}, {30, 30, 200, 0, 255, 0, 255, 0, 200, 30}, {30, 30, 200, 0, 0, 0, 0, 0, 200, 30}, {30, 30, 200, 0, 0, 0, 0, 0, 200, 30}, {30, 30, 200, 90, 0, 0, 0, 90, 200, 30}, {30, 30, 200, 90, 90, 0, 90, 90, 200, 30}, {30, 30, 200, 0, 90, 90, 90, 0, 200, 30}, {30, 30, 200, 0, 0, 0, 0, 0, 200, 30}, {30, 30, 90, 200, 200, 200, 200, 200, 90, 30}, {30, 30, 30, 30, 30, 30, 30, 30, 30, 30} }; Pict.displayPixels( data1 );

  It creates a gray-scale picture. That is, 0 corresponds to black, and 255 corresponds to white; 128 is a medium gray, and 30 is a dark gray.

• If you look at class Pict's documentation, you'll see there are methods that can take an existing file (.jpg etc.), and convert it to a 2-D array of pixels. After we review methods for modifying 2-D arrays, you'll be able to take an existing picture an make a photoshop filter for it!

Initializing 2-D arrays

Task, together:

1. create a variable to hold a two-dimentional array of ints.
2. Allocate space for a 400x500 array, and assign it to that variable.
3. Write a method fill_pattern then initializes location i,j is initialized to i % 256. Call it on your array above.
4. What do you think this will look like, as a picture? Display it with Pict.displayPixels, and find out.

Task: Now, tweak the above method to try different formulas. (For each one, you're encouraged to predict the result first.)

• (j) % 256,
• (i+j) % 256,
• Use the formula:

  (i*j)*256/(width*height)

  (how do we compute the array's height=#rows? Its width=#columns?)
• (i*j) % 256,
• Use the formula:

  if ((i*j)%59 == 0) set location i,j to 0 else set location i,j to 255.

• Use the formula:

  if ((i*j)%60 == 0) set location i,j to 0 else set location i,j to 255.

Methods on 2-D arrays

methods that return a value

In lecture, we already wrote a method to find the average value of a 2-D array:

    /** @return the sum of the numbers in row `r` of `nums`.
    static int addOneRow( int[][] nums, int r ) {
        int sumSoFar = 0;
        for (int c=0;  c < (nums[r]).length;  ++c) {
            sumSoFar += (nums[r])[c];
        }
        return sumSoFar;
    }
    
    
    /** @return the avg, cast to an int, of all the numbers in `nums`.
     *  @pre nums.length != 0 && ∃ i: nums[i].length != 0
     */
    static int avg( int[][] nums ) {
        int sumSoFar = 0;
        for (int r=0;  r < nums.length;  ++r) {
            sumSoFar += addOneRow(nums,r);
        }
        return sumSoFar / (nums.length * nums[0].length);
    }

Your Task Write a method which takes any 2-D array of ints, and finds the maximum value.
You can either use a helper method (findMaxInRow( int[][] nums, int rowNum)), or you can use a nested loop.

methods that modify array-contents

Your Task Write a method moveEachToward(int[][] nums, int target) which takes any 2-D array of ints, and replaces each number n with¹ (n+target)/2. That is, moveEachToward(0) ends up halving each item in the array, and moveEachToward(256) makes each item twice as close to 256.
You can either use a helper method (something like moveOneRowToward( int[][] nums, int target, int rowNum)), or you can use a nested loop.

Effects

Choose one effect from the following list, and implement it! To figure out what you should do, work a small test case by hand (say, a 3x4 array like

  int[][] data2 = { {  90,  90,  90,  90 }
                  , {  20,  90,  20,  90 }
                  , { 200, 200,  99, 200 }
                  }

• Make method invert, which complements each pixel: a black pixel (0) becomes white (255); a light-gray pixel (200) becomes dark-gray (55); etc..
• Enhance contrast: take pixels less than 128 and make them smaller (dividing them by 2, perhaps). Take pixels ≥128 and make them bigger (moving them halfway towards 256).
• Normalize brightness: First, compute the average brightess (value) of all the pixels. If that value is large, make all the pixels dimmer (move them halfway to 0); if that value is small, make all the pixels brighter (move them halfway to 256).
  (What effect would this filter have on your pictures, if you used it in Photoshop?)
• Make the right half of the image a mirror of the left half.

• Flip the picture left-to-right.
  To do this, you need to think: where should pixel 0,0 end up? What about pixel 2,0? or 2,3? See if you can figure out the general formula for pixel r,c. (Your formula should involve the array's width and height, in general.)

When you've finished one of the functions above, you can use Pict.fileToPixelsBW to convert an image to an array-of-pixels, display that image, then display the image after applying your effect!

There are also plenty of other photo effects you could do, instead of the above.

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