code: public void init(int index, Bitmap bitmap, int horizontalOffset, int verticalOffset) {
          this.index = index;
  
          this.groupBitmapHalfHeight = bitmap.getHeight() / 2;
          this.groupBitmapHalfWidth = bitmap.getWidth() / 2;
          this.groupHOffset = horizontalOffset;
          this.groupVOffset = verticalOffset;
      }
docs: Set group marker parameter. To know index and calculate position on spiral.
  
  @param index            the index of this child marker.
  @param bitmap           the bitmap of the group marker.
  @param horizontalOffset the horizontal offset of the group marker.
  @param verticalOffset   the vertical offset of the group marker.