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Two-Dimensional Arrays and Nested Loops – mirroring

Two-Dimensional Arrays and Nested Loops – mirroring. Barb Ericson Georgia Institute of Technology. You’re going to have to think for this one!. Goals How to create an algorithm by solving several concrete examples And figuring out what is common in them

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Two-Dimensional Arrays and Nested Loops – mirroring

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  1. Two-Dimensional Arrays and Nested Loops – mirroring Barb Ericson Georgia Institute of Technology NestedLoops-Mod7-mirrors

  2. You’re going to have to think for this one! • Goals • How to create an algorithm by solving several concrete examples • And figuring out what is common in them • How to translate the algorithm into code • How to mirror a picture vertically and horizontally NestedLoops-Mod7-mirrors

  3. Vertical Mirroring • What if we want to pretend to place a mirror in the middle of the picture • We would see the left side of the picture mirrored on the right side NestedLoops-Mod7-mirrors

  4. Thinking Through Vertical Mirroring 0 1 2 3 4 original • If we just think of a number at each x and y location instead of a color • The mirroring would look like this: • Can you find the algorithm to do this? 0 1 2 mirrored NestedLoops-Mod7-mirrors

  5. What are the Vertical Mirror for these? Odd number of columns Even number of columns • Try the solve the problem for small samples • If you can’t solve it on a small sample • You can’t write a program to solve it 0 1 2 0 1 2 0 1 2 0 1 2 NestedLoops-Mod7-mirrors

  6. Which pixel goes where? • The far left goes to the far right • The first is copied to the last • Column width-1 is set to column 0 • Move inward until you get to the middle • Column width-2 is set to column 1 • The loop for each row starts at 0 and goes to the middle NestedLoops-Mod7-mirrors

  7. Is every row the same? • Yes • To mirror the whole picture how many rows should you go through? • All of them. • From y=0 to y= height-1 NestedLoops-Mod7-mirrors

  8. Mirror Vertical Algorithm to Code • A method inside the Picture class • public void mirrorVertical( ) { • } • We are going to need the midpoint int midpoint = this.getWidth() / 2; • Loop through the rows (y values) for (int y = 0; y < this.getHeight(); y++) { • Loop through x values up to the midpoint for (int x = 0; x < midpoint; x++) { // you write the code to get the left and right pixels and change the color of the right pixel… see the next slide for hints } NestedLoops-Mod7-mirrors

  9. Mirror Vertical Algorithm • Loop through all the rows (y starts at 0, while it is less than the picture height, increments by 1) • Loop with x starting at 0 and x less than the midpoint (mirror point) • Get the left pixel at x and y Pixel leftPixel = this.getPixel(x,y); • Get the right pixel at column width – 1 - x also in row y • Set the color for the right pixel to be the color of the left pixel NestedLoops-Mod7-mirrors

  10. Trying Mirror Vertical • Create a picture • Picture p1 = new Picture(“caterpillar.jpg”); • Invoke the method on the picture • p1.mirrorVertical(); • Show the picture • p1.show(); NestedLoops-Mod7-mirrors

  11. Mirror Horizontal • What about mirroring around a mirror held horizontally in the vertical center of the picture? • Like a reflection in a lake? NestedLoops-Mod7-mirrors

  12. Thinking Through Mirror Horizontal • Again think of a number at each x and y location • Instead of a color • And try it with a small sample • How can we write a nested for loop to do this? original 0 1 2 0 1 2 0 1 2 0 1 mirrored 2 NestedLoops-Mod7-mirrors

  13. What is the Horizontal Mirror for this? 0 1 2 3 4 • Try to solve the problem for several small samples problems • See if you can come up with the algorithm to solve it • Test it more small samples 0 1 2 NestedLoops-Mod7-mirrors

  14. Mirror Horizontal Algorithm Only go half way down • This is a method of the Picture class. • public void mirrorHorizontal(){ • } • Get the Picture height • int height = this.getHeight(); • Get the vertical midpoint • Picture height / 2 • Loop through all the x values (columns) • Loop from y=0 to y < vertical midpoint • Get the top pixel (source) • At x and y • Get the bottom pixel (destination) • Height - 1 - y • Set the bottom pixel’s color to the top pixel color NestedLoops-Mod7-mirrors

  15. Mirror Horizontal Exercise • Write the method to mirror the top half of the picture to the bottom half. • This is a motorcycle redMotorcycle.jpg • How about mirroring bottom to top? NestedLoops-Mod7-mirrors

  16. Exercise • Add mirrorVertical and mirrorHorizontal to your Picture class. • Test with the rubber duckie. public class TestMirrors{ public static void main(){ Picture duck1 = new Picture(“duckie.gif”); Picture duck2 = new Picture(“duckie.gif”); duck1.mirrorVertical(); duck1.explore(); duck2.mirrorHorizontal(); duck2.explore(); } } NestedLoops-Mod7-mirrors

  17. Challenge • Create a 4 way mirror creating a picture 4 times the size of the original Picture fourWayFlip = new Picture(source.getWidth()*2, source.getHeight()*2); Hint: copy the original picture to position (0,0) in the new picture. Flip the picture and then copy it into the new picture starting at the midway point. NestedLoops-Mod7-mirrors

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