任何動畫的最基本的前提,是要在足夠快的時間內顯示和更換一張張的圖片,讓人的眼睛看到動的畫面效果。圖片必須按照順序畫出來。從一張圖片到下一張圖片之間的變化越小,效果會越好。
首先要做的,是使用你的圖片處理軟體(比如ps或者firework)建立一系列相同大小的圖片來組成動畫。每張圖片代表動畫一幀。你需要製作一定數量的禎--越多的幀會讓你的動畫看上去越平滑。製作好的圖片一定要儲存成PNG(Portable Network Graphics)格式,MIDP唯一支援的圖片格式。
有兩個辦法讓你剛做好的圖片在MIDlet上變成動畫。第一,把圖片都放到一個web服務器上,讓MIDlet下載他們,MIDP內建的HTTP支援。第二個辦法更簡單,把圖片用MIDlet打包成jar檔案。如果你使用的是J2ME開發工具,把PNG檔案放你的專案檔案裡面就可以了。
動畫的過程其實更像帳本記錄:顯示當前幀,然後適當地更換到下一幀。那麼使用一個類來完成這個工作應該是很恰當的,那好,我們就先定義一個AnimatedImage類:
import .*;
import i.*;
// 定義了一個動畫,該動畫其實只是一系列相同大小的圖片
// 輪流顯示,然後模擬出的動畫
public class AnimatedImage extends TimerTask {;
private Canvas canvas;
private Image[] images;
private int[][] clipList;
private int current;
private int x;
private int y;
private int w;
private int h;
// Construct an animation with no canvas.
public AnimatedImage( Image[] images ){;
this( null, images, null );
};
// Construct an animation with a null clip list.
public AnimatedImage( Canvas canvas, Image[] images )
{;
this( canvas, images, null );
};
// Construct an animation. The canvas can be null,
// but if not null then a repaint will be triggered
// on it each time the image changes due to a timer
// event. If a clip list is specified, the image is
// drawn multiple times, each time with a different
// clip rectangle, to simulate transparent parts.
public AnimatedImage( Canvas canvas, Image[] images, int[][] clipList ){;
as = canvas;
es = images;
List = clipList;
if( images != null && clipList != null ){;
if( th < th ){;
throw new IllegalArgumentException();
};
};
if( images != null && th > 0 ){;
w = images[0]idth();
h = images[0]eight();
};
};
// Move to the next frame, wrapping if necessary.
public void advance( boolean repaint ){;
if( current >= th ){;
current = 0;
};
if( repaint && canvas != null && own() ){;
int( x, y, w, h );
iceRepaints();
};
};
copy, otherwise
// set the clipping rectangle accordingly and
// draw the image multiple times.
public void draw( Graphics g ){;
if( w == 0 || h == 0 ) return;
int which = current;
if( clipList == null || clipList[which] == null ){;
Image( images[which], x, y,
| );
}; else {;
int cx = lipX();
int cy = lipY();
int cw = lipWidth();
int ch = lipHeight();
int[] list = clipList[which];
for( int i = 0; i 3 <= th; i =4 ){;
lip( x list[0], y list[1], list[2], list[3] );
Image( images[which], x, y,
| );
};
lip( cx, cy, cw, ch );
};
};
// Moves the animation′s top left corner.
public void move( int x, int y ){;
this.x = x;
this.y = y;
};
// Invoked by the timer. Advances to the next frame
軟體開發網
// and causes a repaint if a canvas is specified.
public void run(){;
if( w == 0 || h == 0 ) return;
advance( true );
};
};
你例項化一個AnimatedImage物件的時候你必須給AnimatedImage類的構造方法傳一個Image物件陣列,該陣列代表動畫的每一幀。使用的所有圖片必須具有相同的高度和寬度。
用teImage()方法從jar檔案裡面載入圖片:
private Image[] loadFrames( String name, int frames )
throws IOException {;
Image[] images = new Image[frames];
for( int i = 0; i < frames; i ){;
images = teImage( name i ".png" );
};
return images;
};
你也可以傳遞一個Canvas物件(可選),和一個剪輯列表(clip list)。如果你指定了一個canvas和使用一個timer來自動更換到動畫的下一幀,就如下面的例子程式碼中一樣,canvas在動畫向前滾動以後自動被重畫(repaint)。不過這樣的實現辦法是可選的,你可以這樣做,也可以讓程式選擇合適的時候重畫canvas。
因為MIDP 1.0不支援透明的圖片,AnimatedImage 類使用一個剪輯列表來模擬透明的效果,剪輯列表是圖片被剪成的方塊區域的系列。圖片被畫出來的時候分開幾次,每次畫一個剪輯列表裡面的剪輯區域。剪輯列表在幀的基礎上被定義好,所以你需要為圖片的每一幀建立一個數組。陣列的大小應該是4的倍數,因為每一個剪輯面積保持了四個數值:左座標,頂座標,寬度以及高度。座標的'原點是整個圖片的左上角。需要注意的是使用了剪輯列表會使動畫慢下來。如果圖片更加複雜的話,你應該使用向量圖片。
AnimatedImage類擴充套件了rTask,允許你設定一個timer。這裡有個例子說明如何使用timer做動畫:
Timer timer = new Timer();
AnimatedImage ai = ..... // get the image
dule( ai, 200, 200 );
每隔大約200毫秒,timer呼叫()方法一次,這個方法使得動畫翻滾到下一個幀。現在我們需要的是讓MIDlet來試試顯示動畫!我們定義一個簡單的Canvas類的子類,好讓我們把動畫“貼上上去”。
import .*;
import i.*;
// A canvas to which you can attach one or more
// animated images. When the canvas is painted,
it cycles through the animated images and asks
// them to paint their current image.
public class AnimatedCanvas extends Canvas {;
private Display display;
private Image offscreen;
private Vector images = new Vector();
public AnimatedCanvas( Display display ){;
lay = display;
// If the canvas is not double buffered by the
// system, do it ourselves...
if( !isDoubleBuffered() ){;
offscreen = teImage( getWidth(),
getHeight() );
};
};
// Add an animated image to the list.
public void add( AnimatedImage image ){;
lement( image );
};
// Paint the canvas by erasing the screen and then
// painting each animated image in turn. Double
// buffering is used to reduce flicker.
protected void paint( Graphics g ){;
Graphics saved = g;
if( offscreen != null ){;
g = raphics();
};
olor( 255, 255, 255 );
Rect( 0, 0, getWidth(), getHeight() );
int n = ();
for( int i = 0; i < n; i ){;
AnimatedImage img = (AnimatedImage)
entAt( i );
( g );
};
if( g != saved ){;
Image( offscreen, 0, 0,
| );
};
};
};
AnimatedCanvas 類的程式碼相當簡單,由一個動畫匯入方法和一個paint方法。canvas畫布每次被畫,背景都會被擦除然後迴圈每個匯入的AnimatedImage物件,直接畫到自己身上來(自己擴充套件了canvas類)。
import .*;
import .*;
import i.*;
import et.*;
// MIDlet that displays some simple animations.
// Displays a series of birds on the screen and
// animates them at different (random) rates.
public class AnimationTest extends MIDlet
implements CommandListener {;
private static final int BIRD_FRAMES = 7; 軟體開發網
private static final int NUM_BIRDS = 5;
private Display display;
private Timer timer = new Timer();
private AnimatedImage[] birds;
private Random random = new Random();
public static final Command exitCommand = new Command( "Exit",, 1 );
public AnimationTest(){; };
public void commandAction( Command c,Displayable d ){;
if( c == exitCommand ){;
exitMIDlet();
};
};
protected void destroyApp( boolean unconditional )
throws MIDletStateChangeException {;
exitMIDlet();
};
public void exitMIDlet(){;
el(); // turn it off...
notifyDestroyed();
};
// Generate a non-negative random number...
private int genRandom( int upper ){;
return( ( Int() ) % upper );
};