CSC1401 Manipulating Pictures

1. CSC1401Manipulating Pictures

2. What we have done to date • We have modified pictures by writing on top of them • Using Turtles and using Graphics • Drawing lines, shapes and text, as well as inserting a picture into another picture

3. What would be nice to do • Changing colors on the pictures • Red-eye reduction • Blending pictures • Doing other fancy Adobe Photoshop-like transformations • But this would be almost impossible to do by simply drawing on top of the existing picture

4. The Goal • Develop an easier approach to modifying our pictures by changing the pictures themselves rather than writing on top of them

5. Digital Pictures • Represented by pixels • With a red, green, and blue value stored for each pixel • Stored in .jpg (JPEG) files • International standard • With lossy compression • Lossy means not all data is stored • But what is lost isn’t that important • Compression means made smaller • Other formats for storing digital pictures are GIF and BMP

6. Manipulating a Picture • To manipulate a picture we need to manipulate the pixels that make up the picture • Change the red, green, or blue values at the pixel • Pixel is a class created at Georgia Tech • Each pixel object has a red, green, and blue value

7. Pictures have lots of Pixels • How can we refer to each pixel? • pixel1, pixel2, pixel3, pixel4, pixel5, … • Do we really want to name each one? • On a 640 x 480 picture, there are 640 x 480 = 307,200 pixels

8. How do we change a Pixel? Pixel first; first = stevepicture.getPixel(10,120); // 10 refers to the row and 120 to the columnfirst.setColor(Color.black); /* The book writes the code as: stevepicture.getPixel(10,100).setColor(Color.black); */ Note that if you have already shown the picture, by invoking: stevepicture.show(); You can cause the picture to be updated by: stevepicture.repaint();

9. Pictures have lots of Pixels • How do we deal with lots of data of the same type? • Use an array

10. What is an Array? 0 1 2 3 4 5 • Storage for a sequence of items • Of the same type • You can access items by using an index • The index starts at 0 • The first item is at index 0 • The last item is at index (length – 1) • Arrays know their length (have a public length field) • arrayObj.length 3 7 9 2 1 5 0 1 2 3 8 3 2 6

11. But how does an array help us? • If we wish to change lots of pictures, we can use a loop!

12. Recall from Alice In this example, each time through the loop, the bunny hopped Note that the index variable changes from 0 to 1 to 2 to … each time through the loop. And we can take advantage of that when working with arrays!

13. In Java • Use a loop to access an array of pixels • The first time through the loop we access pixelArray[0] • The second time through the loop we access pixelArray[1] • …

14. What Data does a Picture Object Have? • A picture object has an array of pixel objects • That it read from the .JPG file • It knows the picture width pictureObj.getWidth() • It knows the picture height pictureObj.getHeight() • It knows how to return an array of pixels Pixel[] pixelArray = pictureObj.getPixels()

15. Pixel Objects • Each pixel has a red, green, and blue value • getRed(), getGreen(), getBlue() • setRed(v), setGreen(v), setBlue(v) • Each pixel knows the location it was in the picture object • getX(), getY() • You can also get and set the color at the pixel • getColor(), setColor(color)

16. Turning all of the pixels in our picture to red – using a loop • Notes: • We have created a counter that starts at 0, and goes up to the number of pixels in the picture • Each pixel has its color set to red! • How would this code have looked had it been written as a method inside of the Picture class?

17. A note on Color • You can either set the red, green and blue amounts individually, or all together using mypixel.setColor(someColor) • You can create a color object by giving the red, green, and blue values for it Color colorObj = new Color(255,10,125);

18. Predefined Colors • The Color class has defined class constants for many colors • Color.red, Color.green, Color.blue, Color.black, Color.white, Color.yellow, Color.gray, Color.orange, Color.pink, Color.cyan, Color.magenta • Or you can use all uppercase names • Color.RED, Color.BLUE, Color.BLACK, …

19. Getting and Setting Pixel Colors • To get a pixel’s color as a color object Color color1 = pixelObj.getColor(); int red = color1.getRed(); int green = color1.getGreen(); int blue = color1.getBlue(); • To set a pixel’s color using a new color object red = 20; green = 30; blue = 100; Color color2 = new Color(red,green,blue); pixelObj.setColor(color2);

20. Changing Pixel Colors • There are two ways to change the color of a pixel in a picture • Set the red, green, and blue values individually • pixelObj.setRed(value), • pixelObj.setGreen(value), • pixelObj.setBlue(value), • Or set the color • pixelObj.setColor(colorObj) • But, you won’t see any change in the picture • Until you ask it to repaint: pictureObj.repaint(); • Or you invoke the show method

21. Summary • Pictures have pixels • You can change the picture by changing the color of the pixels • Arrays let you store and retrieve values of the same type using an index • You can ask a picture for it’s width, height, and an array of pixels • You can get and set the color of a pixel

22. Assignment • Read Media Computation Chapter 4 – we’ll be covering while loops and for-each loops next week