Threads and Multimedia

1
Threads and Multimedia

• Animation, Images, Sound

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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 lt 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

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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

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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

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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

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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

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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)

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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 urls content as Object
• openConnection()
• get a connection to this URL.
• openStream()
• returns InputStream used to read from the URL

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URL Class

• Methods, cont.
• 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

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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.

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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)

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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