3D Documentation of the Mural

Mural imaging at CCSF in December 2015. Photo: Marlin Lum

In 2011, at the invitation of City College of San Francisco's (CCSF) mural historian and physics lab manager Will Maynez, the CHI team walked into the lobby of the Diego Rivera Theater at CCSF to experience Diego Rivera’s Pan America Unity mural for the first time. Though the CHI team is based in San Francisco and familiar with many public art works, this mural was not known to them, and the team was wowed by the experience. The mural is impressive all by itself, and to experience it with a tour from Will Maynez is enlightening (read more on the RiveraMural.org website with lots of his mural history, or this New York Times article). From the day of that first visit, the team decided they wanted to digitally preserve the mural. It took some years for the stars to align and realize this possibility. More about the history of the project is found on the Cultural Heritage Imaging website.

To produce high-resolution shape and color of the mural’s surface, the CHI team employed the photogrammetry technique. Fundamentally, photogrammetry uses photographs to measure the imaging subject in three dimensions. Photogrammetry is used to build most of the world’s modern maps.

The CHI team captured enough resolution to show the fresco’s fine surface details both in the color information and in the surface shape geometry produced by the photogrammetry process. This resolution displays Rivera’s brush stroke impressions, the daily work area boundaries (giornata) and condition details such as cracks in the plaster. More about how CHI documented the mural’s color and shape using the 3D information is found on the 2D Color and Shape from 3D data page.

High quality photogrammetry requires a rules-based process. This includes well-placed, sharply focused photos, with even illumination of the subject. The photos must overlap by 66% to produce high precision geometry, and the use of a wide-angle lens for one pass over the subject also reduces errors in the calculations. More about the rules of photogrammetry is found on the Photogrammetry Background page.

For relatively flat imaging subjects, obtaining the photographic data set for a photogrammetry project is usually straightforward, even for a large canvas painting; this project was challenging due to its grand scale and the way the mural was installed in the Diego Rivera Theater at CCSF. The size of the mural at 6.7 meters X 22.4 meters (22’ X 74’) meant collecting a massive number of photos and managing all of that data through the processing pipeline. In addition, the mural was installed all the way to the ceiling and right up against the side walls of the theater lobby, and a railing protecting the mural also served to obstruct access and line-of-sight to the bottom of the mural. The 10 panels of the mural were stacked in 5 vertical panel pairs. Each pair was installed at an angle to it neighbor, making a curved shape –– not a flat surface as the mural was painted, and later installed at SFMOMA in June 2021. All of this made the photographic capture more complex.

When the mural butted into a perpendicular wall or ceiling, getting the necessary overlapping edge photos with a camera and lighting lashed to a scissor lift 6 meters off the ground required imaginative solutions. As the time-lapse videos show, CHI’s photo gear is reconfigured on the scissor-lift to capture each edge and the bottom of the mural.

The “Pan American Unity” 3D Documentation Project has Several Accomplishments:

  • The information provided by the high-precision 3D model demonstrates City College of San Francisco’s (CCSF’s) commitment to the mural’s long-term conservation and preservation.
  • The documentation also provides reliable 3D data for the architectural modeling of the mural’s new home on the CCSF campus, where it will reside following its time at SFMOMA in 2021-2023.
  • The 3D model and the special images it can produce supports the mural’s public access and enjoyment, use as educational content, and ongoing scholarship.
  • The 3D information supports conservation activities, and provides a baseline to monitor changes to the mural surface, which is particularly important given the extraordinary movement of the mural to and from SFMOMA. This imaging data has been used by conservators to determine if scratches and abrasions on the mural’s surface occurred before or during the move.
  • The 3D information in the form of Digital Elevation Maps (DEMs) demonstrates the hard to see, fundamentally sculptural character of frescos. The action of the artists brush in the wet plaster and the contours of the daily lay-up of the plaster work surface leave an information-rich shape record that is usually overpowered by the perception of color. By removing the painted color and replacing it with color keyed only to the depth of the surface relief contours, the fresco’s sculptural nature emerges.
  • Widely accessible information about the mural can spread the message of mutual cultural enrichment and respect that Diego Rivera directed to the Americas and the world.
  • Stanford Libraries commitment to the perpetual digital preservation of CHI’s Pan American Unity 3D documentation ensures the world’s ongoing digital access to Diego Rivera’s San Francisco masterpiece.
  • Finally, this 3D documentation can serve as a digital “lifeboat” for the mural in an uncertain world. San Francisco sits on the Pacific Ocean’s “Ring of Fire”: the potential for catastrophic earthquakes and the fire and destruction they bring is a fact of life.