Manipulating Pictures, Arrays, and Loops Stepping through programs

1. Manipulating Pictures, Arrays, and LoopsStepping through programs Barb Ericson Georgia Institute of Technology May 2007 ManipulatingPictures-Mod6-part4

2. Learning Goals • How to trace code • How to fake a sunset ManipulatingPictures-Mod6-part4

3. Tracing Code • An important skill to develop is the ability to trace code • Also called walking through or stepping through your code • Look at each line and predict what will happen • Show the variables and their values ManipulatingPictures-Mod6-part4

4. Step Through decreaseRed() using a while loop • A picture object was created from your picture file and then was sent the message decreaseRed() • The picture object can be referred to by this because the decreaseRed () method is inside the Picture class • The array of pixel objects was returned from sending getPixels() to the picture object Pixel[] pixelArray = this.getPixels(); • Some variables were declared for later use in the loop Pixel pixelObj = null; int value = 0; // needed if we use a while loop instead of a for:each loop int index = 0; ManipulatingPictures-Mod6-part4

5. The loop in increaseRed() that allows you to choose where to start or stop public void increaseRed() { Pixel[] pixelArray = this.getPixels(); Pixel pixelObj = null; int value = 0; int index = 0;// set the starting position in the array while (index < pixelArray.length) // loop through all the pixels { pixelObj = pixelArray[index]; // get the current pixel value = pixelObj.getRed(); // get the value value = (int) (value * 1.3); // increase value by 30% pixelObj.setRed(value); // set the red value index++;// increment the index } } ManipulatingPictures-Mod6-part4

6. Step Through decreaseRed() - cont • The while loop tests if the index is less than the length of the array while (index < pixelArray.length) { • And if so it executes the statements in the body of the loop {} • It sets the variable pixelObj to the pixel at the current index in the array of pixels pixelObj = pixelArray[index]; • It gets the red value of that pixel value = pixelObj.getRed(); • it sets the value to the integer part of (red value/ 2) value = value / 2; • It sets the pixel’s red to the new value pixelObj.setRed(value); • It increments the index value index++; ManipulatingPictures-Mod6-part4

7. R=252, G=254, B=251, X=0, Y=0 R=253, G=255, B=254, X=1, Y=0 R=254, G=254, B=254, X=2, Y=0 Memory Map of decreaseRed() width=329 height=150 • What does memory look like the first time through the loop? • How about the 2nd time through? • How about the 3rd time through? • How about the last time through? Picture: Class getPixels() … this pixels … pixel Pixel: Class getRed() setRed()… value = 252 index = 0 ManipulatingPictures-Mod6-part4

8. To debug in BlueJ • 1stcompile your code • Click in the left margin to turn on the debugger at that point • When you run the program it will halt and allow you to single step through ManipulatingPictures-Mod6-part4

9. Press Step to look through the code as it runs • The variables will appear in the debugger window and change as you step through the code ManipulatingPictures-Mod6-part4

10. Faking a Sunset • If you want to make an outdoor scene look like it happened during sunset • You might want to increase the red • But you can’t increase past 255 • Another idea is to reduce the blue and green • To emphasize the red • Try to reduce the blue and green by 30% Only the top half of the picture is the sky ManipulatingPictures-Mod6-part4

11. Faking a Sunset Algorithm • Reduce the blue and green by 30% • Get the array of pixels from the picture • Set up an index to start at 0 • Loop while the index is less than the length of the array / 2 to get the top half of the picture • Get the pixel at the current index from the array of pixels • Set the blue value at the pixel to 0.7 times the original value • Set the green value at the pixel to 0.7 times the original value • Increment the index and go back to step 3 ManipulatingPictures-Mod6-part4

12. Faking a Sunset Method ManipulatingPictures-Mod6-part4

13. Testing makeSunset Picture pictureObj = new Picture(“beach.jpg”); pictureObj.explore(); pictureObj.makeSunset(); pictureObj.explore(); Challenge: Create a sunset method that will do this sunset effect on the top half of the picture and darken the bottom half of the picture. ManipulatingPictures-Mod6-part4

14. Summary • Tracing code is walking through code and saying what each line is doing • It can also help to show the values of the variables after each line executes • You can increase the red in a picture by multiplying the red value by a number > 1.0 • You can change more than one color in a pixel ManipulatingPictures-Mod6-part4