Section 6.2. Simple Edge Detection: Conditionals with Two Options

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6.2. Simple Edge Detection: Conditionals with Two Options

What if we want to look for areas of high contrast between a pixel and the pixel below it? If the contrast is high, we can make the pixel black, and if the difference is low, we can make the pixel white. This is a simple form of edge detection. It results in a picture that looks like a pencil sketch.

What does high contrast mean? It means that the difference between the two colors is high. One way to calculate this is to average the red, green, and blue values in the top pixel and subtract this from the average of the red, green, and blue values in the bottom pixel. If the absolute value of the difference is greater than some amount, then there is high contrast. If there is high contrast set the top pixel color to black and if not set it to white.

6.2.1. Negation

You have seen a way to execute a statement or block of statements if some condition is true using an if. But how do you execute a statement or block of statements if a condition is false? One way is to use an if but negate the condition using the '!' (logical not) operator.

> !true false > !false true

So if we wanted to check whether a value was less than 20 and if it is print out "Yes, the value is less than 20" and if it isn't print out "No, this value is not less than 20" we could test for the value being less than 20 and also test for the value being not less than 20 using the '!' operator.

> int x = 30; > if (x < 20) System.out.println("Yes, the value is less than 20"); > if (! (x < 20)) System.out.println("No, not less than 20"); No, not less than 20

6.2.2. Testing for Both Conditions

Another way to test that a value isn't less than 20 is to test whether it is greater or equal to 20.

> int x = 30; > if (x < 20) System.out.println("Yes, the value is less than 20"); > if (x >= 20) System.out.println("No, not less than 20"); No, not less than 20

The problem with this approach is that we have to run both tests every time. But if the first test is true, there is no reason to run the second test (x can't be both less than 20 and greater or equal to 20). We would like a way to say that if the first test is true, execute one statement or block of statements, and if it is false, then execute a different statement or block of statements.

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6.2.3. Conditionals with Two Options

The way to do this is with if (expression) and else. As before, the if part will execute the statement or block of statements following the if when the expression is true. But, if we add an else this will also execute the statement or block of statements following the else when the expression following the if is false (Figure 6.6). And it will also mean that the else is skipped if the if test was true.

Figure 6.6. Flowchart of an if with an else.

To enter the following in DrJava's interaction pane, use SHIFT-ENTER after the if statement to let DrJava know that there will be a following else.

> int x = 30; > if (x < 20) System.out.println("Yes, the value is less than 20"); else System.out.println("No, this value is not less than 20"); No, this value is not less than 20

As you can see, the statement following the if didn't execute, but the statement following the else did. Try the same thing but this time give x a value less than 20.

> x = 15; > if (x < 20) System.out.println(       "Yes, the value is less than 20"); else System.out.println("No, this value is not less than 20"); Yes, the value is less than 20

Notice that this time the statement following the if did execute but the statement following the else did not.

6.2.4. Simple Edge Detection

To do simple edge detection, we will loop through the pixels with the outer loop incrementing y from 0 to less than picture height-1. Usually we have been looping while y is less than the height, so why stop at height-1? We are comparing the value at a y position with the pixel below it so the last y value that still has another row below it is at height-2.

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The inner loop will increment x from 0 to less than the picture width. The top pixel will be set to the pixel at the current x and y location. The bottom pixel will be set to the pixel at the current x but y+1. We will compare the absolute value of the difference between the intensities (average of color values), and if it is less than some passed limit, we will change the top pixel color to white, and otherwise we will set the top pixel color to black. The Pixel class has an object method getAverage() that returns the average of the three color values.

Program 37. Edge Detection (This item is displayed on pages 184 - 185 in the print version)

/**  * Method to do a simple edge detection by comparing the  * absolute value of the difference between the color  * intensities (average of the color values) between a  * pixel and the pixel below it. If the absolute value  * of the difference between the color intensities is  * less than a passed amount the top pixel color  * will be set to white.  Otherwise it is set to black.  * @param amount if the absolute value of the differences  * in the color average is less than this  * set the color to white, else black  */ public void edgeDetection(double amount) {   Pixel topPixel = null;   Pixel bottomPixel = null;   double topAverage = 0.0;   double bottomAverage = 0.0;   int endY = this.getHeight() - 1;   /* loop through y values from 0 to height - 1    * (since compare to below pixel) */   for (int y = 0; y < endY; y++) {    // loop through the x values from 0 to width    for (int x = 0; x < this.getWidth(); x++) {       // get the top and bottom pixels       topPixel = this.getPixel(x,y);       bottomPixel = this.getPixel(x,y+1);       // get the color averages for the two pixels       topAverage = topPixel.getAverage();       bottomAverage = bottomPixel.getAverage();       /* check if the absolute value of the difference         * is less than the amount */       if (Math.abs(topAverage - bottomAverage) < amount) {         topPixel.setColor(Color.WHITE); 
[Page 185]       // else set the color to black       } else {         topPixel.setColor(Color.BLACK);       }     }   } }

To execute this method and see the result (Figure 6.7) use:

> String fileName = FileChooser.getMediaPath("butterfly1.jpg"); > Picture p = new Picture(fileName); > p.explore(); > p.edgeDetection(10); > p.explore();

Figure 6.7. Original picture and after edge detection.

Making it Work Tip: Using Curly Braces

You may have noticed that the method edgeDetection shows the starting curly braces at the end of the line instead of on a new line as shown in the previous methods. Java doesn't care if the curly braces are at the end of a line or on a new line. Some programmers prefer one to another. It is often easier to see that you forgot a curly brace if it is by itself on a new line. However, the Java guidelines say to put the opening curly brace at the end of a line and the closing one on a new line.

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You may also notice that we are using curly braces after the if and else even though there is only one statement to be executed, so they aren't really needed. It is good practice to have them even if they aren't needed because the code is easier to read and change.