private void PerformAutoCrop(BitMatrix matrix)
        {
            int left, right, top, bottom;

            GetLastEmptyLeftRow(matrix, out left);
            GetLastEmptyRightRow(matrix, out right);
            GetLastEmptyTopRow(matrix, out top);
            GetLastEmptyBottomRow(matrix, out bottom);

            matrix.Crop(
                left,
                top,
                matrix.Width - left - (matrix.Width - right),
                matrix.Height - top - (matrix.Height - bottom)
                );
        }