Case Studies

Automatic colorization of classic statues?

Automatic colorization of black and white photographs has recently been enabled by advances in machine learning (see [Zhang et al 2016] for the methods we used for the following results). Basically deep neural networks are shown millions of black and white photographs and their color versions to learn a mapping from black and white to color. After successful training, it is possible to colorize black and white photos which the machine learning algorithm has never seen before. Online services like ALGORITHMIA enable everyone to test and use this technique by simply uploading their images.

Successful colorization via ALGORITHMIA of our family’s dog Ozzy

One focus for DUST AND DATA is the Glyptothek of the Academy of Fine Arts, Vienna, which is a collection of plaster casts dating back to the late 17th century. Its main task was to serve as study material for Academy students, but it also became publicly accessible as a museum. The collection contains copies of a canon of world renown sculptures, ranging from plaster casts of Egyptian originals to copies of Greek and Roman, medieval, renaissance and historism statues. Thanks to German archaeologist Vinzenz Brinkmann, it is now an established fact that classic statues from e.g. Greek antiquity were originally painted in bright colors.

This color restoration shows what a statue of a Trojan archer from the Temple of Aphaia, Aegina would have originally looked like (CC BY-SA 2.5, from Wikipedia)

As for DUST AND DATA, all we are given are black and white photos of plaster casts from the Glyptothek. These are digital photo copies of analog plaster copies of statues. Is this sufficient to obtain any kind of meaningful result when trying to automatically colorize classic statues?

Plaster cast of dying warrior from the Temple of Aphaia, Aegina (Photo: Gemäldegalerie der Akademie der bildenden Künste Wien)
Automatic colorization via ALGORITHMIA

The automatic colorization of our dying warrior did not quite succeed, but is has many interesting features nevertheless. E.g. the algorithm did correctly “understand” that the statue is held by a real person, hence the colorization to skin tones of the person’s arm and dark color of the trousers. As for the statue, it is rendered in a light brown color, probably imitating the color of statues it has seen during training of the machine learning system. But what about the arm bottom left in the picture? It has almost a lifelike skin tone. And even more astonishing, the red bloodlike colorization of the amputed arm stump!

The Glyptothek also has a copy of Michelangelo’s David, or at least its head. The colorization above does provide a skinlike pink tone for the face and even blond hair.

Applying the colorization to the full David statue gives a lifelike pinkish skin tone, at least much more so than for the dying warrior above. The fact that Michelangelo’s David is such a realistic sculpture probably made it possible to trick the algorithm into treating the photo of David as the photo of a real naked person.

So can we use machine learning to automatically colorize photos of plaster casts from the Glyptothek? This would require training an algorithm with thousands of color restorations of antique statues (see photo of the Trojan archer above) to have any chance of success. But application of state-of-the-art colorization algorithms already now provides interesting results by exposing some of the biases and failures of their machine learning machinery.