When Joan Leslie won a contract at Warner Bros. at age 15, she was instructed to report to the swimming pool at the Lakeside Country Club two blocks from the studio. Her duty: to pose for photographs in bathing suit and shorts.

This was her introduction to cheesecake, also known as leg art, a Hollywood ritual from the 1930s into the 1960s. Studios issued photos of curvy starlets – and stars – to the nation’s newspapers and magazines, and they were printed widely.

The photos were especially popular at holidays. “I can remember posing with other contract players bobbing for apples at Halloween and wearing hearts on my shoes, my stomach and my head for Valentine’s Day,” Leslie recalls.

The collapse of the studio system – and the women’s movement – took a big bite out of cheesecake. Actors, publicists and photographers were dropped from studio contract lists. A new generation of actresses considered posing in bathing suits demeaning, and feminists agreed.

Newspapers and magazines scorned such stunts as old-fashioned, corny and blatantly promotional. Men’s magazines preferred their centerfolds sans bathing suits. And the tabloids turned their attention to paparazzi photos of the stars.

The beginning of cheesecake can be traced to the silent era in which attractive young women in high-necked woolen bathing suits were photographed cavorting on California beaches. They were called the Mack Sennett Bathing Beauties, and they included future stars such as Carole Lombard and Phyllis Haver.

Marilyn Monroe famously poses over the updraft of a New York subway grate while in character for the filming of The

History does not record why the popular bathing suit photos acquired the name of the high-calorie dessert. Candid guess: because both were delectable.

An irrepressible redhead named Clara Bow converted cheesecake into an art form. She adored showing off as much of her voluptuous figure as the law would allow. In 1927 she starred in “It,” and ever afterward she was known as “The It Girl.” “It” was never defined, but whatever It was, Clara had It.

Despite the 1930s Depression – or perhaps because of it, since beleaguered citizens sought escape from their travails with cheap entertainment – Hollywood’s studios grew and flourished. All the majors embraced cheesecake as a means of publicizing their product and stars.

A remarkable 2003 book, “Sunkissed” by Joshua James Curtis, reproduced hundreds of photos showing actresses in bathing suits or other scanty attire.

Virtually everyone is pictured – even dramatic stars such as Katharine Hepburn, Olivia de Havilland, Barbara Stanwyck and Bette Davis.

Studios hired ace photographers such as George Hurrell and Clarence Sinclair Bull to produce glamour shots, not only at the beach or beside swimming pools but in arty poses at the photo galleries on the lots.

Fifty years later, Leslie – who starred in “High Sierra” with Humphrey Bogart, “Sergeant York” with Gary Cooper and “Yankee Doodle Dandy” with James Cagney – still recalls the photographers’ instructions: “Feet together, turn your hips sideways to the camera, the shoulders turned toward the camera, chest out, tummy in, wet your lips. And turn it on for me!”

World War II brought the heyday of cheesecake.

Homesick GIs yearned for photos of beauties in scanty attire to pin up beside their bunks, and the studios supplied millions of “pinups.”

The three most famous photos: Rita Hayworth kneeling on a bed in a negligee; Betty Grable in a bathing suit, smiling invitingly over her shoulder; Jane Russell nestling against a haystack, blouse cut low.

In the ’50s, cheesecake got a startling infusion from Marilyn Monroe’s electric presence. She was the subject of two historic photographs: the nude pose against red velvet and the shot of her skirt billowing up as she and Tom Ewell stood over a New York sidewalk vent for a scene in “The Seven Year Itch.”