LONDON (REUTERS).- A major exhibition in London brings together works by Italian painter Giovanni Antonio Canal, best known as Canaletto, and some of his biggest rivals who fought for artistic and commercial supremacy. Painting Venetian views was big business in the 18th century, as wealthy English aristocrats on their Grand Tour sought to take back with them a memento of the canal city. Canaletto adopted a more commercial approach to his art in the 1730s, largely thanks to his association with British patron and agent Joseph Smith who would promote the painter to patrons coming from abroad. One way the artist sought to earn more money was to paint smaller pictures which were more easily transported home. It was also in the 1730s that a significant rival to Canaletto emerged. Michele Marieschi focused on painting quickly and often brought in other artists to paint human elements of grand Venetian landscapes which he was less comfortable