Da Vinci's 'Mona Lisa' part of first 3D artwork?

Leonardo Da Vinci's famous 'Mona Lisa' painting may be part of the world's oldest 3D artwork, scientists say.
In 2012, scientists discovered that a seemingly insignificant "knock-off" of the 'Mona Lisa' in the Museo del Prado in Madrid was in actuality very close to the original hanging in the Louvre Museum in Paris, revealing the same subject with the same mountain landscape background.
That painting may have been painted by Da Vinci or possibly one of his students, researchers believe.
"When I first perceived the two paintings side by side, it was very obvious for me that there is a very small but evident difference in perspectives," study researcher Claus-Christian Carbon of the University of Bamberg in Germany told Live Science.

The real 'Mona Lisa', or 'La Gioconda', and the Prado cousin were painted from slightly different perspectives.
Carbon and Vera Hesslinger of Germany's University of Mainz figured out this perspective shift by looking at so-called trajectories, or the paths from a distinctive point on the source, such as the tip of Mona Lisa's nose, to a target, or the observer's (or painter's) eyes.

The scientists also asked people to estimate the perspective of the 'Mona Lisa' sitter, something Carbon called a psychological assessment of the perspective.
"This is particularly clear if you observe the chair on which La Gioconda sits: In the Prado version, you can still see the end of the end corner of the chair at the background of the painting, which you cannot see in the Louvre version, because the painter of the Prado version looked at the' Mona Lisa' more from the left than the painter of the Louvre version," Carbon said.

The researchers then recalculated the position the painters took relative to each other and to the 'Mona Lisa' sitter in Da Vinci's studio.
They found that the horizontal difference between the two paintings was about 2.7 inches, which is close to the average distance between a person's two eyes.
Researchers believe the two paintings form a stereoscopic pair, meaning when viewed together create an impression of depth, a 3D image of the 'Mona Lisa'.
The researchers are not sure if Da Vinci, and possibly one of his students, intentionally created this stereoscopic pair.
However, Carbon pointed out that Da Vinci "intensively worked on the 3D issue."

In addition, in inventory lists there were hints of the existence of two 'Mona Lisa' paintings on his property at the same time, and that he owned coloured spectacles, Carbon said.
This evidence "might indicate that he did not only [think] about the 3D issue theoretically but in a very practical sense in terms of experiments," Carbon added.