How to get total area covered while drawing path on canvas android?

Im using below code to draw line on bitmap canvas while finger touch move... here i posted partial code and it is working fine..

As shown in below image, the black and white bitmap erased on touch drag.. I made canvas transparent so the parent layout background(color image) is getting visible.

I want to know , how much area is erased(like 50% or 60% of bitmap ).. is there any way to find that?

//Erasing paint

         mDrawPaint = new Paint();
    mDrawPaint.setAntiAlias(true); 
    mDrawPaint.setDither(true);  
    mDrawPaint.setStyle(Paint.Style.STROKE); 
    mDrawPaint.setStrokeJoin(Paint.Join.ROUND);
    mDrawPaint.setStrokeCap(Paint.Cap.ROUND);
    mDrawPaint.setStrokeWidth(50); 
    mDrawPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
    BlurMaskFilter mBlur = new BlurMaskFilter(10, BlurMaskFilter.Blur.NORMAL);
    mDrawPaint.setMaskFilter(mBlur);

private void doDraw(Canvas c) {

    c.drawBitmap(mBitmap, 0, 0,null );

}

private float mX, mY;
private static final float TOUCH_TOLERANCE = 1;

void touch_start(float x, float y) {
    mPath.reset();
    mPath.moveTo(x, y);
    mX = x;
    mY = y;
}
void touch_move(float x, float y) {
    float dx = Math.abs(x - mX);
    float dy = Math.abs(y - mY);
    if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
        mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
        mX = x;
        mY = y;
    }

     canvas.drawPath(mPath, mDrawPaint ); //Erasing Black and white image

}
void touch_up() {
    mPath.lineTo(mX, mY);
    // commit the path to our offscreen
    mCanvas.drawPath(mPath, mDrawPaint);
    // kill this so we don't double draw
    mPath.reset();
}