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Example Two-Dimensional Arrays

• Maps– That street is in D-5

• Battleship– Try I-5

• Hit or miss

• Chairs at a theater or game– Row C seat 20

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What is a two-dimensional array?

• The pixels in a picture are really stored in a two-dimensional array– Each pixel has a x value

(horizontal location)– Each pixel has a y value

(vertical location)– pictureObj.getPixel(x,y)

returns the pixel at that location

xy

Pixels

Pixel

• A pixel (abbr. for picture element) is the smallest unit of an image.

• Therefore, a 640x480 image is a matrix of 640 columns and 480 rows, each element of this matrix is called an image pixel.

RGB and Grayscale• In RGB format, each Pixel has 3 color components: Red,

Green, and Blue.

• A grayscale pictures has values from 0 to 255 that produce a black and white image in those ranges of gray. So it has one color ranging from 0 to 255 creating different scales of white to black (grayscale).

RGB Gray

GrayImage FreeResponse

Instance Variables to use: public static final int BLACK = 0;

public static final int WHITE = 255;

2D Array to use private int [][] pixelValues

private int pixelValues[][] =

{{255,184,178,84,135},

{84, 255, 255, 130, 84},

{78, 255, 0, 0, 78},

{84, 130, 255, 130, 84}};

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0 1 2 3 4 0 255 184 178 84 129 1 84 255 255 130 84 2 78 255 0 0 78 3 84 130 255 130 84

Grayscale picture with 4 rows and 5 columns

Methods to create

public int countWhitePixels() (return method)

{

This method will loop through the 2D array of pixels in row-major order (row first then column)

And return the number of pixels in the image that contain the value WHITE. Return an int

}

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0 1 2 3 4 0 255 184 178 84 129 1 84 255 255 130 84 2 78 255 0 0 78 3 84 130 255 130 84

It should return 5 since there are 5 pixel values with WHITE (255)

processImage()

public void processImage() (void method performs an action)

{

It will modify the image by chaing in the values in the instance variable pixelValues [][] by the following:

(1)Pixel value at position row col is decreased by the value at position [row+2] [col +2] if the row+2 and col + 2 position exist

(2)If the subtraction results in < 0 change the pixel value at the position to BLACK.

}Georgia Institute of Technology

• Work on Free Response today. (Let’s get finished)

• Work on multiple choice questions. These should be finished and turned in first thing Friday. I will grade these.

• If you need help come see me Thursday during IF once you have looked over them and tried them.

Georgia Institute of Technology