code: private void drawPreviousShadow(Canvas canvas) {
  		final float degreesFlipped = getDegreesFlipped();
  		if (degreesFlipped > 90) {
  			final int alpha = (int) (((degreesFlipped - 90) / 90f) * MAX_SHADOW_ALPHA);
  			mShadowPaint.setAlpha(alpha);
  			canvas.drawPaint(mShadowPaint);
  		}
  	}
docs: draw top/left half shadow
  
  @param canvas