Part VI

Part 6: Mirroring a Picture

Mirroring

Mirroring images is a simple and widely applicable tool. You've probably done it in your own designs. Car designers at GM Research Labs even use the idea of it -- they sculpt half the car out of clay, and then use a mirror to show what the whole car would look like. If they have a digital mirror, they can make the actual second half of the real thing totally symmetrical.

Today, you'll figure out how to write a method that flips pictures. But wait! We're not just flipping the whole thing backwards. That's boring. We're going to keep one half constant, and then make the other half a mirror of the first side. See Figure 6 for an example:

Figure 7 is the same process, but applied to an array of numbers.

Can you figure out the algorithm for this process? Test your algorithm on different sizes of two- dimensional arrays of integers. Will it work for 2D arrays with an odd number of columns? Will it work for 2D arrays with an even number of columns?

One algorithm is to loop through all the rows and half the columns. You need to get a pixel from the left side of the picture and a pixel from the right side of the picture, which is the same distance from the right end as the left pixel is from the left end. Set the color of the right pixel to the color of the left pixel. The column number at the right end is the number of columns, also known as the width, minus one. So assuming there are at least 3 pixels in a row, the first left pixel will be at row=0, col=0 and the first right pixel will be at row=0, col=width-1. The second left pixel will be at row=0, col=1 and the corresponding right pixel will be at row=0, col=width-1-1. The third left pixel will be at row=0, col=2 and its right pixel will be at row=0, col=width-1-2. Each time the left pixel is at (current row value, current column value), the corresponding right pixel is at (current row value, width - 1 - (current column value)).

The following method implements this algorithm. Note that, because the method is not looping through all the pixels, it cannot use a nested for-each loop.

Questions

1. Look at the mirrorVertical() method in the Picture class. Explain how it works. You can test it using the testMirrorVertical method in the PictureTester class.
2. Write a new method: mirrorVerticalRightToLeft(). This method will mirror in the opposite direction of the mirrorVertical method. (You can take advantage of the code in that method - don't re-invent the wheel.) Test your code by writing a static tester method in PictureTester and calling the method in that main method.. Copy/paste your method code (NOT the tester method).
3. Write the method mirrorHorizontal that mirrors the top of the picture down to the bottom (see Figure 8). Test in the PictureTester class and copy/paste your method code.
4. Write the method mirrorHorizontalBottomTop. Note: if you use your mirrorHorizontal method, you should be able to only change ONE LINE. Test in PictureTester as usual. Copy/paste the method code.
5. Write a new method: mirrorDiagonal. I strongly recommend working out your algorithm first. Figure 9 below is a demonstration of what the code should do. Copy/paste your method code.

---------Part V Part VII-------