An exhibition showing off colourful open-air artworks by major Impressionist painters such as Vincent van Gogh and Claude Monet is set to open its doors in Madrid on Tuesday. The "Impressionism and Open-Air Painting" show at the Thyssen-Bornemisza museum features 113 works and explores the evolution of outdoor painting -- an art-form perceived as having transformed the world of painting during the 19th century as Impressionists took it to its apex. Previously, artists had commonly produced landscape paintings indoors by using sketches.