Android支持的顏色系統是RGB，Alpha最后的Alpha表示透明度，他的取值是0-255,。通過設置Alpha的值，我們可以改變圖像的透明度。

下面舉例說明，效果圖如下：

public class MainActivity extends Activity implements OnSeekBarChangeListener{public static int alpha=100;private View myView;@Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);LinearLayout linearLayout=new LinearLayout(this);linearLayout.setOrientation(LinearLayout.VERTICAL);myView=new MyView(this);myView.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT, 260));SeekBar seekBar=new SeekBar(this);seekBar.setMax(255);//seekbar的初始值seekBar.setProgress(alpha);seekBar.setOnSeekBarChangeListener(this);linearLayout.addView(myView);linearLayout.addView(seekBar);linearLayout.setBackgroundColor(color.white);setContentView(linearLayout);setTitle("Alpha"+alpha);}public void onProgressChanged(SeekBar seekBar, int progress,boolean fromUser) {// TODO Auto-generated method stubalpha=progress;setTitle("Alpha:"+alpha);myView.invalidate();}private class MyView extends View{private Bitmap bitmap;public MyView(Context context) {super(context);// TODO Auto-generated constructor stubInputStream is=getResources().openRawResource(R.drawable.ic_launcher);bitmap=BitmapFactory.decodeStream(is);setBackgroundColor(color.white);}@Overrideprotected void onDraw(Canvas canvas) {// TODO Auto-generated method stubPaint paint=new Paint();paint.setAlpha(alpha);//第二個參數表示源位圖的復制區域，第三個參數表示繪制的目標區域canvas.drawBitmap(bitmap, new Rect(0,0,bitmap.getWidth(),bitmap.getHeight()),new Rect(10,10,310,235), paint);}}public void onStartTrackingTouch(SeekBar seekBar) {// TODO Auto-generated method stub}public void onStopTrackingTouch(SeekBar seekBar) {// TODO Auto-generated method stub} }

總結

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