Threads and Multimedia

1. Threads and Multimedia Animation, Images, Sound

2. Animation • Animation, displaying a sequence of frames to create the illusion of motion, is a typical application of threads • One thread is used to download the image(s) • Other thread(s) are used to paint the image(s) sequentially • See programs • ButtonToMove.java • MoveBall.java • AnimateBall.java

3. ButtonToMove • This program uses a button to allow the user to move a ball across the screen • Push button, move ball 9 pixels southeast • Solution to animate? • put movement into for loop (next slide)

4. Wrong Approach public void actionPerformed(ActionEvent ae) { for(int i = 0; i < 10; i++) { x += 9; y += 9; repaint(); // system collects them into // one call to paint() } } • Cannot put motion in a for loop in the actionPerformed method because the calls to repaint will be combined into one call to paint. • effect is to show only position at end of loop

5. MoveBall.java • Animates ball using a separate thread • The button pressed creates the thread to animate ball. • In an applet, the applet will implement runnable and the code for the run method will be used by the thread. • why does it have to implement runnable? • See actionPerformed in MoveBall.java • try pushing the Move button multiple times • Why does it act the way it does??

6. AnimateBall.java • Modified MoveBall so that the run method continually moves the ball until the web page is left. When the web page is re-entered, the thread will start again. • See start, stop, and run of AnimateBall.java

7. Line Animation • Uses a thread to move lines across an applet • The thread draws several lines repeatedly to create the illusion of the line moving. • When the lines reach the border of the applet, the motion reverses. • See LineAnimation.java

8. Text Animation • Demonstrates how a streaming banner can be shown in an applet. • Applet with string drawn inside • no init() • no nothing, except a graphics object used to draw a string • see what happens when you change its size in the html file • Animation.java

9. The Clock Frame • Demonstrates Frames, Event Handler, Threads, and Calendar class to build a clock with alarm. • Thread is used to run the clock • Can embed in another application • Application: ClockFrame.java • Note use of Toolkit

10. Images • Applets and applications can use images in the .gif or .jpg format • Applets usually cannot read images from the client computer even if the page came from the client. • images usually are in the base path of the web page or one of its subdirectories. • images use a URL to find them

11. Images in Applets Some Methods needed • getDocumentBase() • getCodeBase() • getImage(url, filename) • im.getWidth(this); • im.getHeight(this); • g.drawImage(im, x, y, this); • there are several versions of drawImage Example: ImageDemo.java

12. Images and URLs • getCodeBase() • location of applet code • getDocumentBase() • location of document in which applet is embedded • getImage(URL,Filename) • load named image from given URL • returns the image in a separate thread (Image is, of course, a class) • drawImage(image,x,y,which) • place the named image, with its upper left-hand corner at the given point. • which is the object to be notified as drawing progresses whether it is progressing • returns false if drawing is still occurring when it returns

13. Tracking Images MediaTracker • Keeps track of images added to its list Methods: • addImage(Image, id) • add an image for this media tracker to track • id is passed in and used to identify the image • removeImage(Image) • stop tracking the named image (why not its id?) • checkAll() • returns true if all tracked images are done loading • checkID(id) • returns true if all tracked images with this id are done loading (can be multiples)

14. Tracking Images More MediaTracker methods: • isErrorID(id) • check error status of images with given id; return true if error • isErrorAny() • check error status of all tracked images • waitForID(id,[ms]) • Starts loading all images tracked by this media tracker with the specified identifier. Can specify maximum time in milliseconds to wait for loading to complete • waitForAll(id,[ms]) • same as waitForID, but starts loading all images

15. The Class URL Used for accessing web sites Methods: • Constructors; URL(string url) • simplest; 5 other variations on constructor • getContent(); • retrieves url’s content as Object • openConnection(); • get a connection to this URL. • openStream(); • returns InputStream used to read from the URL

16. URL Class Methods, con’t.: • getHost() • return host as IPv6 address enclosed in square brackets • getFile() • get actual file this URL represents. Will not have web address if loacl • getProtocol() • returns http or https, as a string • getPort() • returns the port number allocated to this URL object • toExternalForm() • returns string form for this URL

17. Images in Applications • No applet limitations as to where the image can come from • must use different methods since application is not an applet • create a URL object for the file • load whatever the URL object refers to • we use getContent() to return an Object, since a URL can refer to any type of Object. If it is an image, getContent returns an ImageProducer object which can create an Image. • Alternative: Use the Toolkit version.

18. Image Demo Applications Examples: • ImageDemo2.java • Uses the URL and getContent() with ImageProducer • ImageDemo3.java • Use the Toolkit class • (actually, Image2Demo did, too; This example uses the toolkit to obtain the image)

19. Sequences of Images • Sequence of displayed images create animation. • Animation can flicker because of repaint. • Avoid by creating ImageIcon • Override the update method • Use double buffering • draw image twice • Examples: • ImageAnimation.java • AnimateImage_NoFlicker.java