A repaired fresco. [Photo by Wang Kaihao/China Daily]

He visits the site daily to make sure the restored frescoes resemble their original forms.

Speaking about the academy's work, Guo says: "When we repair frescoes, we stick to the principle of minimum intervention.

"If some parts of the frescoes look vague, we only prevent them from fading away and wouldn't do any redrawing."

But he says there are exceptions. "For example, when restoring a deity's image, we cannot leave it incomplete considering the pilgrims' emotions."

Tibetan painters are often invited to ensure accuracy, but people can tell which parts of the image have been restored.