create masked overlay the user can clear with touch

create masked overlay the user can clear with touch

I'm beaking my head over something that sounds realy simple but apparently
is not. I want to create some grey with alpha 50% overlay over some
framelayout with drawable background.
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/wrapper"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/ic_launcher" >
<com.example.DrawView
android:id="@+id/draw_view"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:visibility="visible" />
</FrameLayout>
I want the user to be able to "clear" the over lay by touch, what I mean
is that the pixels that he touches - their color will turn from grey to
transparent. This is my solution:
@Override
protected void onDraw(Canvas canvas) {
mBitmap = getDrawingCache();
mCanvas = new Canvas(mBitmap);
for (Line line : lines) {
mCanvas.drawCircle(line.startX, line.startY, 10, mPaint);
mCanvas.drawLine(line.startX, line.startY, line.stopX, mPaint);
}
if (currentLine != null)
mCanvas.drawLine(currentLine.startX, currentLine.startY,
currentLine.stopX, currentLine.stopY, mPaint);
// mCanvas.drawBitmap(mBitmap, new Matrix(), null);
// if (currentLine != null) {
// for (int i = 0; i < 30; i++) {
// for (int j = 0; j < 30; j++) {
// mBitmap.setPixel((int) Math.min(currentLine.startX
+ i, mBitmap.getWidth()),
// (int) Math.min(currentLine.startY + j,
mBitmap.getHeight()), Color.TRANSPARENT);
// }
//
// }
//
// }
}
And from the constructor of the View I'm calling:
private void init(int width, int height) {
setBackgroundColor(R.color.mobli_dark_40);
setDrawingCacheEnabled(true);
mPaint = new Paint();
mPaint.setAntiAlias(true);
mPaint.setDither(true);
mPaint.setStyle(Paint.Style.STROKE);
mPaint.setStrokeWidth(40);
mPaint.setAlpha(100);
mPaint.setColor(Color.TRANSPARENT);
mBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
for (int i = 0; i < width; i++) {
for (int j = 0; j < height; j++) {
mBitmap.setPixel(i, j, R.color.mobli_dark_40);
}
}
mCanvas = new Canvas(mBitmap);
mPath = new Path();
mBitmapPaint = new Paint(Paint.DITHER_FLAG);
setOnTouchListener(this);
}
Now, no matter what I'm doing - the mCanvas.drawCircle and
mCanvas.drawLine DOES NOTHING when I'm trying to create transparent
circle/line in order to remove the pixels' color. Only when I'm setting
the pixels' one-by-one color to transparent(like in the comment in
OnDraw(..) the their color is realy being changed to transparent.
Thank you for the help!