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lab14a
2-D arrays
processing pictures I

We will:

Review lect13c—2-D arrays, and example code.
write some methods for 2-D arrays;
try out some methods which convert between int[][] and black-and-white pictures.

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

Methods on 2-D arrays

Group Task Write a method which takes any 2-D array of ints, and calculates the average value (that is, it adds them all up, and divides by the length×width).
To add up all the values in the array, you can either use a helper method ( something like addOneRow( int[][] nums, int rowNum)), or you can use a nested loop.

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.

Your Task Write a method moveEachToward(int[][] nums, int target) which takes any 2-D array of ints, and replaces each number n with ¹ (n+target)/2. That is, moveEachHalfwayTo(0) ends up halving each item in the array, and moveEachHalfwayTo(256) makes each item “twice as close” to 256.
You can either use a helper method (something like moveEachInRowToward( int[][] nums, int target, int rowNum)), or you can use a nested loop.

Pictures!

We have provided a class (documentation and source code). with two helpful static methods:

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

        // int pixels as a literal array, then displayed:
        int[][] 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.

Pict.fileToPixelsBW takes the name of a jpeg or gif file (either a filename, or a URL), and returns a 2-D array of ints in [0,256).
Here are some pictures you might want to convert to an array-of-ints, and then modify:

See further documentation on this class. You can also look at and run the main method of that class. You can put your methods in a separate class; just be sure to call the provided (static?!) methods by using their class-name: Pict.methodName.

Task, together: Write a method which creates a 500x500 array, then initializes location i,j is initialized to i % 256. What do you think this will look like, as a picture? Display it, and find out. example code.

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)
(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.

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, the 3x4 array data1 shown above).

Make method invert, which complements each pixel: a black pixel (0) becomes white (255); a light-gray pixel (200) becomes dark-gray (55); etc..
Normalize brightness: 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 large, 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?)
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).
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.)

Recall how our selectionSort code from lecture moved items around inside the array (using one variable tmp when swapping).
Make the right half of the image a mirror of the left half.

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

¹ Hey, for once integer division is what we want, since we're storing the result back into an int. Note that any number in [0,256), when average with 256, is still in [0,256). That's because (255+256)/2 is 255, by integer division. This will be important below. ↩

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©2010, Ian Barland, Radford University
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lab14a 2-D arraysprocessing pictures I

Methods on 2-D arrays

Pictures!

Effects

lab14a
2-D arrays
processing pictures I