The sign "Please do not touch the art" might be a common instruction for many museum visitors, but not for those at the Museo del Prado in Spain. Based in Madrid, the museum invites visually-impaired visitors to enjoy classical paintings by touching their texture and relief at a special exhibition "Touching the Prado." Open earlier this year and running until June 28, the project showcases six 3-D works from different pictorial genres created through a special technique called "Didu," which is developed by Spanish design agency Estudios Durero. As stated on its website, the technique "[turns] a digital image into an image that can be touched and which makes sense not only to the eyes but also to the touch." The display also includes informative materials like braille and audio guides. According to the museum, blind visitors are able to "obtain a heightened degree of artistic-aesthetic-creative enjoyment in order to explain, discuss and analyze these works in the Prado." Exhibition commissioner, Fernando Perez told Reuters about the technique, "It is different inks that are applied in different layers. When you put them under chemical processes like ultra violet light or other techniques, there is a reaction and they increase their volume. They can work with different textures, flat and corroded. And in that way you decide what to highlight." In collaboration with professionals in the visual impairment sector, a group of artists were asked to re-create the museum's popular paintings through this technique. The exhibition collection includes Diego Rodriguez's "Vulcan's Forge," El Greco's "The Noblemen with his Hand on his Chest," Leonardo da Vinci's "La Gioconda" (Mona Lisa), Francisco de Goya's "The Parasol," Correggio's "Noli me tangere" and Juan van der Hamen's "Still Life with Artichokes, Flower and Glass Vessels." Touching the Prado is not the world's first 3-D art exhibition for visually-impaired people. In previous periods, similar events were held in New York, London and Florence. The project creators will reportedly expand it to other European countries after Spain.