How did a painter blow up a tiny photo into a wall-sized portrait?

By drawing a grid over both the photo and the canvas, and copying it one square at a time. That was the method the American painter Chuck Close built his whole career on: he would put a grid on a small photograph and a matching, much bigger grid on his canvas, then fill in each cell of the canvas to match the color of the corresponding cell in the photo. Up close, each square just looks like a rough smear or a ring of color. Step back, and the squares blend into a photo-realistic face taller than a person.

Mathematically, this square-by-square enlargement is an affine transformation: a way of mapping every point of one shape onto another while keeping straight lines straight and parallel lines parallel, and the ratios between parallel lengths unchanged. A uniform enlargement like this one is the simplest case: every distance grows by the same factor, so a small grid square and a huge grid square hold the same picture in the same proportions, however large the final canvas gets.

Close had an unusual personal reason to care so much about faces: he said he had prosopagnosia, or face blindness, and had trouble recognizing new faces. He said painting portraits made him better at recognizing and remembering faces, though the connection only occurred to him twenty years after he started. His first grid painting this way, Big Self Portrait in 1968, blew his own face up onto a canvas nearly nine feet tall, and took him more than four months to finish.

In 1988 a collapsed spinal artery left Close paralyzed from the neck down. He kept painting anyway, with a brush strapped to his wrist and grids built by an assistant, working in even lower-resolution squares. Seen up close, his later paintings look almost abstract, just blocks of color. Seen from across the room, the grid resolves back into a face, proof that the same simple trick, point for point, square for square, still worked.