Android支持的颜色系统是RGB，Alpha最后的Alpha表示透明度，他的取值是0-255,。通过设置Alpha的值，我们可以改变图像的透明度。

下面举例说明，效果图如下：

public class MainActivity extends Activity implements OnSeekBarChangeListener{
	
	public static int alpha=100;
	private View myView;
	
    @Override
    public 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 stub
		alpha=progress;
		setTitle("Alpha:"+alpha);
		myView.invalidate();
	}
	
	private class MyView extends View{

		private Bitmap bitmap;
		public MyView(Context context) {
			super(context);
			// TODO Auto-generated constructor stub
			InputStream is=getResources().openRawResource(R.drawable.ic_launcher);
			bitmap=BitmapFactory.decodeStream(is);
			setBackgroundColor(color.white);
		}
		@Override
		protected void onDraw(Canvas canvas) {
			// TODO Auto-generated method stub
			Paint paint=new Paint();
			paint.setAlpha(alpha);
			//第二个参数表示源位图的复制区域，第三个参数表示绘制的目标区域
			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
		
	}
}