Which artist or work changed the way you see?

Most artists can name one encounter that shifted something: a painting seen in person, a book, a teacher, a single image. To start the thread: Katsushika Hokusai's woodblock print "Under the Wave off Kanagawa", usually called The Great Wave, made around 1831. Prints like it reached Europe in large numbers in the second half of the 19th century and fed the wave of enthusiasm for Japanese art known as Japonisme. Painters including Claude Monet, Edgar Degas and Vincent van Gogh collected Japanese prints, and you can see their influence in flattened space, bold outlines and unusual cropping. A mass-produced print, made to be sold cheaply, helped change the course of European painting. Your turn: which artist or work changed the way you see, and what exactly did it change? Tell us where you first encountered it, too.