private void setUpAlphaRect() {
if (!mShowAlphaPanel) return;
final RectF dRect = mDrawingRect;
float left = dRect.left + BORDER_WIDTH_PX;
float top = dRect.bottom - ALPHA_PANEL_HEIGHT + BORDER_WIDTH_PX;
float bottom = dRect.bottom - BORDER_WIDTH_PX;
float right = dRect.right - BORDER_WIDTH_PX;
mAlphaRect = new RectF(left, top, right, bottom);
mAlphaPattern = new AlphaPatternDrawable((int) (5 * mDensity));
mAlphaPattern.setBounds(
Math.round(mAlphaRect.left),
Math.round(mAlphaRect.top),
Math.round(mAlphaRect.right),
Math.round(mAlphaRect.bottom));
}