Here's a business-like way of looking at Italian Renaissance drawings: When Raphael or Leonardo picked up a piece of paper, chances are they were engaged in research, quality control or product development.

It's possible to watch them, and many other great artists, doing just those things at the British Museum's fabulous new exhibition, "Fra Angelico to Leonardo: Italian Renaissance Drawings," a collaboration between the BM and the Uffizi in Florence.

The formula is simple: 50 of the choicest works from each collection, just about the finest array of 15th- and early 16th-century Italian drawings in the world. It takes you inside the Renaissance workshop, and even into the heads of painters and sculptors who lived five centuries ago.

When Paolo Uccello (probably, the attribution isn't certain) drew a perspective exercise, "Study of a Chalice" (c. 1450-70), he broke down the object into more than 2,000 points of intersection — a dizzying exploration of the geometry of vision in a single cup.

This was an investigation as close to science as what we now call art. Leonardo was up to something similar, in a different context, when he scrutinized the "Abdomen and Left Leg of a Nude Man" (c. 1506-10), jotting down a note to himself "the fat is in the hollows, the lean in the relief."

ARTISTIC RIVALRY

That kind of inquiry — a revolutionary interest in how things really look — was only one application of drawing. Lines on paper were also a way of generating fresh ideas or, in 21st-century terms, brainstorming. Italian Renaissance artists were operating in a competitive market. There was a premium on originality, and a sheet of paper could be a virtual laboratory for visual innovation.

The paradigm-shifting genius of Leonardo is visible on one extraordinary sheet where he has overlaid so many ways of arranging his figures — "The Virgin and Child with St. Anne and the Child Baptist" (c. 1505-08) — that the center of the image has disappeared into a blur like a Rorschach blot or an over-exposed photograph.

It was the newly abundant supply of paper itself in the later 15th century, an unexpected result of the post-Gutenberg printing boom, that made such creative brainstorming possible. Previously, vellum (prepared animal skin) was the main medium for drawing. And that was too expensive for playing around.

The more artists drew, the more uses they found for drawing, such as refining details, which is what Antonio Pollaiuolo did in a study of St. John the Baptist. In the center is the saint, and all around, close-ups of crucially expressive parts of him: his hands, his feet.

"Fra Angelico to Leonardo: Italian Renaissance Drawings" opened April 22 and runs through July 25 at the British Museum, London. Information: www.britishmuseum.org.