Chuck Close is internationally-famous for his very large portraits. He paints from photographs, but divides his canvas up into tiny squares and fills each with little squiggles that are analogous to pixels. Viewers see the individual shapes up close, but as they back away, the image gels into a portrait. Chuck Close was famous before he was struck with a collapsed spinal artery. It left him a partial quadraplegic, but he still paints from his wheelchair using a motorized canvas. A major retrospective is now touring the country. Karen Michel reports.