A conservator works on a section of Paul Philippoteaux's famous “Battle of Gettysburg” cyclorama painting, a 360-degree canvas that gives viewers the feeling of being placed in the middle of Pickett's Charge, the climactic clash on the final day of battle, at the new Museum and Visitor Center at the Gettysburg National Military Park in Gettysburg.

GETTYSBURG — Standing in the middle of the gruesome battle scene during Pickett’s Charge, surrounded by the horrific sight of 12,000 infantrymen fighting, bloodied, mangled and dying, some of the most hardened soldiers were brought to tears.

But that was two decades after the decisive 1863 battle. The soldiers described in newspaper accounts as being overcome with emotion were Civil War veterans looking at a massive in-the-round painting, known as a cyclorama, a grand illusion recreating the pivotal battle with a level of realism not seen before.

“They were the IMAX of their day,” said Katie Lawhon, spokeswoman for Gettysburg National Military Park. The National Park Service has owned the cyclorama since 1942.

Visitors coming for the 150th anniversary of the Battle of Gettysburg will experience the four-story cyclorama, one of only three known to survive in the U.S., fully restored in a new visitor center with a dramatic light and sound show.

As much novelty as fine art, cycloramas commonly depicted military scenes and were displayed in purpose-made buildings.

Hung inside a rotunda, a properly installed cyclorama creates an immersive 360-degree experience thanks to its colossal size and meticulous details. A diorama in front of the canvas adds to the effect by extending the scene in 3-D.

French artist Paul Philippoteaux and a team of 20 painters created four identical Gettysburg cycloramas in all, based on his sketches and photos of the landscape, historical accounts and interviews with Pickett’s Charge survivors. They were so popular that a number of Philippoteaux knockoffs started making the state fair rounds.

“When Paul Philippoteaux is finished with the paintings and lays everyone off, those master drawings get out there and other competing companies acquired the artist and drawing,” said Sue Boardman, program manager with the Gettysburg Foundation and a cyclorama expert.

Philippoteaux signed his work by adding his likeness, leaning against a tree in a Union uniform, as soldiers around him charge the front line. The work was otherwise politically neutral: Whether Union or Confederate, all Philippoteaux’s soldiers fight heroically and die with honor.

“He would tinker with it if someone who was there told him a detail that wasn’t right,” Lawhon said. “He continued to make changes after it was completed.”

Motion pictures killed the cyclorama craze, and they began to quickly vanish by the late 1800s.

Gettysburg’s cyclorama was originally displayed in Boston, then spent a decade languishing boxed in an open shed that a Boston newspaper colorfully mourned as a “mausoleum of greatness.” It was purchased by a retail mogul, who chopped it up for display, and eventually arrived in its current hometown exactly a century ago.

Time was also not any kinder in the decades that followed. By the 1990s, it “was so aesthetically compromised that ... the illusion it was once capable of creating could only be read in century-old accounts,” Boardman said.

A five-year, $15 million upgrade completed in 2008 repaired deterioration from heat, moisture, pollution, neglect and ill-advised repair attempts.