使位图的区域变为透明

Question

我在画布上有2个位图。打开顶部位图时，较低的位图应该可见并且顶部位图应该被擦除。 我从这个线程获得了帮助。 Make certain area of bitmap transparent on touch。我可以通过圆圈看到底部位图，但顶部位图不会在接触时擦除。如何擦除触摸的位图。我知道这个问题以前曾被问过，但我无法解决问题。

这是我的代码：

public class MainActivity extends Activity { 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 

    setContentView(new Panel(this)); 
} 

class Panel extends View { 
    Bitmap bmOverlay; 
    private Paint mPaint; 
    Bitmap bm2, bm1; 
    Bitmap bitmap; 
    Canvas pcanvas; 

    int x = 0; 
    int y = 0; 
    int r = 0; 

    public Panel(Context context) { 
     super(context); 
     setFocusable(true); 
     setBackgroundColor(Color.TRANSPARENT); 

     // setting paint 
     mPaint = new Paint(Paint.ANTI_ALIAS_FLAG); 
     mPaint.setColor(Color.TRANSPARENT); 
     mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT)); 
     mPaint.setAntiAlias(true); 

     bm1 = BitmapFactory.decodeResource(getResources(), R.drawable.aa); 
     bm2 = BitmapFactory.decodeResource(getResources(), R.drawable.aaa); 

     bmOverlay = Bitmap.createBitmap(bm1.getWidth(), bm1.getHeight(), 
       Bitmap.Config.ARGB_8888); 
     pcanvas = new Canvas(bmOverlay); 

    } 

    @Override 
    protected void onDraw(Canvas canvas) { 
     // draw a circle that is erasing bitmap 
     super.onDraw(canvas); 
     canvas.drawBitmap(bm2, 0, 0, null); 
     pcanvas.drawBitmap(bm1, 0, 0, null); 
     pcanvas.drawCircle(x, y, 40, mPaint); 
     canvas.drawBitmap(bmOverlay, 0, 0, null); 

    } 

    @Override 
    public boolean onTouchEvent(MotionEvent ev) { 
     // set parameter to draw circle on touch event 

     switch (ev.getAction()) { 

     case MotionEvent.ACTION_DOWN: { 

      x = (int) ev.getX(); 
      y = (int) ev.getY(); 
      invalidate(); 

      break; 
     } 

     case MotionEvent.ACTION_MOVE: { 

      x = (int) ev.getX(); 
      y = (int) ev.getY(); 
      invalidate(); 
      break; 

     } 

     case MotionEvent.ACTION_UP: 

      break; 

     } 
     return true; 
    } 



} 
} 

Answer 1

你需要作出两个变化让高层位抹掉。首先，使一个可变的位图，这样你可以改变的内容，因为它被删除：

Bitmap temp = BitmapFactory.decodeResource(getResources(), R.drawable.aa); 
bm1 = temp.copy(Bitmap.Config.ARGB_8888, true); // mutable = true 

要小心在这里内存不足的错误。

其次，在你的onDraw功能，通过写回BM1更新位图的内容：

@Override 
protected void onDraw(Canvas canvas) { 
    // draw a circle that is erasing bitmap 
    super.onDraw(canvas); 
    canvas.drawBitmap(bm2, 0, 0, null); 
    pcanvas.drawBitmap(bm1, 0, 0, null); 
    pcanvas.drawCircle(x, y, 40, mPaint); 
    canvas.drawBitmap(bmOverlay, 0, 0, null); 

    // erase the top bitmap: 
    Canvas bitmapCanvas = new Canvas(bm1); 
    bitmapCanvas.drawBitmap(bm2, 0, 0, null); 
    bitmapCanvas.drawBitmap(bmOverlay, 0, 0, null); 
} 

此外，停止一个圆将在左上角被删除，当你启动应用程序，创建一个布尔默认值为false，当你有有效的坐标时，将其设置在onTouchEvent中，并在调用drawCircle之前检查它。