Android 画布未设置到屏幕中心

我有两个画布和两个位图，当用户触摸并移动第一个位图时，一个位图覆盖另一个位图。它变为透明，显示第二个位图图像，直到听到所有东西都正常工作。但当我试图将两个位图移动到屏幕中央时，触摸无法正确检测，我必须在空白屏幕上触摸图像上方，使其可擦除

package com.example.canvastest;
import android.app.Activity;
import android.content.Context;
import android.content.res.Resources;
import android.graphics.AvoidXfermode.Mode;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.BitmapFactory;
import android.graphics.BlurMaskFilter;
import android.graphics.BlurMaskFilter.Blur;
import android.graphics.Canvas;
 import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.os.Bundle;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;

public class MainActivity extends Activity {
Canvas c2;
 @Override
 public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(new TouchView(this));


}



class TouchView extends View{
Bitmap bgr;
Bitmap overlayDefault;
Bitmap overlay;
Paint pTouch;
int X = -100;
int Y = -100;


public TouchView(Context context) {
    super(context);

    bgr = BitmapFactory.decodeResource(getResources(),R.drawable.sg);
   // overlayDefault = BitmapFactory.decodeResource(getResources(),R.drawable.g325);
    overlay = BitmapFactory.decodeResource(getResources(),R.drawable.g325).copy(Config.ARGB_8888, true);  
    c2 = new Canvas(overlay);

    pTouch = new Paint(Paint.ANTI_ALIAS_FLAG);         
    pTouch.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN)); 
    pTouch.setColor(Color.TRANSPARENT);
    pTouch.setMaskFilter(new BlurMaskFilter(10, Blur.NORMAL));


}

@Override
public boolean onTouchEvent(MotionEvent ev) {

    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;
}


@Override
public void onDraw(Canvas canvas){
    super.onDraw(canvas);


    //draw background
    canvas.drawBitmap(bgr, 0, 0, null);
    //copy the default overlay into temporary overlay and punch a hole in it                          
    //c2.drawBitmap(overlayDefault, 0, 0, null); //exclude this line to show all as you draw
    c2.drawCircle(X, Y, 40, pTouch);
    //draw the overlay over the background
    //canvas.drawBitmap(overlay, 0, 0, null);

    Paint new_paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    new_paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_ATOP));
    canvas.drawBitmap(overlay, 0,  0, new_paint);

}


}


}