What is 3D Photogrammetry?

Scale bars for photogrammetry

Learn more about photogrammetry, the powerful technique grounded in measurement science.

San Francisco Chronicle Article about the Mural Imaging

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NBC Video Coverage of the Mural Imaging

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Diego Rivera Mural Project, City College of San Francisco

The Diego Rivera Mural Project, sponsored by City College of San Francisco, has as its mission the return of the mural to the position of public importance and influence envisioned by its creator.

Imaging Services and Consulting by CHI

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Capturing High Resolution 3D of a Diego Rivera Mural

Applying 3D Photogrammetry to the 1940 Pan American Unity Fresco Mural at
City College of San Francisco

CHI's Carla Schroer imaging the Diego Rivera Mural, Pan American Unity, at City College of San Francisco, December 2015

Carla Schroer of CHI shooting the mural at City College of San Francisco, December 2015

“My mural will picture the fusion between the great past of the Latin American lands, as it is deeply rooted in the soil, and the highly mechanical developments of the United States.” — Diego Rivera

In 1940, Mexican artist Diego Rivera (1886-1957) painted a huge mural (22 feet x 74 feet), an inspiring vision of the unity of art, religion, history, politics, and technology in the Americas. Originally titled The Marriage of the Artistic Expression of the North and of the South on this Continent, it is commonly known as Pan American Unity. Rivera created the work during the 1940 season of the Golden Gate International Exposition (GGIE) at Treasure Island on the Bay in San Francisco, California. The mural was the centerpiece of a program called “Art In Action” that featured many artists creating their works while the public watched.

Rivera’s imagery in the mural is wide-ranging and detailed, joining visual themes of North American artistry and technology, reflecting Rivera's fervent belief that a multicultural artistic expression would be emblematic of a unified cultural entity in the Americas, regardless of individual locations of origin. In late 1940 the Mexican artist saw the expediency of projecting a united front as the seemingly unstoppable Nazis turned their eyes to the Americas.

Origins of the Rivera Mural Project

Carla Schroer adjusting for color in the mural shoot

Collecting color information in the mural shoot

After the exposition, the original plan for the mural was to install it in a new library at City College of San Francisco. But the library was never built: World War II intervened, and building materials for civilian use were scarce. The mural was put into storage until 1961 when it was moved to the lobby of the Diego Rivera Theatre on campus.

The mural’s ten large panels make up a work that is 22 feet high and 74 feet long. It is Rivera’s largest contiguous work of art. Given its size, the mural’s current location at City College is not optimal for viewing the entire work as Rivera meant it to be seen. William Maynez, the mural’s historian at City College, says the space where the mural is housed now is just too small. “You should be able to get as far away from a work of art as it is wide,” he says. “This is 74 feet wide, and you can only get back 14 feet.”

A group from the private sector known as Friends of the Diego Rivera Murals is committed to underwriting restoration of the mural to its rightful prominence. The goals of the City College of San Francisco Diego Rivera Mural Project are to conserve and preserve the mural, expand the viewing audience, enhance understanding of the mural, and develop educational programs about it.

The plan of record is to move the mural to a new building on campus where it can be located for enhanced public viewing. Relocating such a large, complex and precious work must be undertaken with great care to avoid damage to it, and Mexican conservators well versed in this task will continue to advise. After a one–day assessment in 1999, conservator Francesca Pique of the Getty Conservation Institute reported that the mural was in very good condition, but that a more thorough assessment had to be undertaken before any move.

CHI's Role in the Rivera Mural Project

Maynez contacted Cultural Heritage Imaging (CHI) in 2011 and brought them in to do an initial viewing of the work. Finally, in 2015, with funding from the Friends group transferred to City College, he engaged CHI to bring their imaging techniques and expertise to the mural. The goals were to create an important historic document of the mural’s current state, to determine its condition for safe transport, and to support education and research. CHI has an unblemished object safety record, and the imaging techniques CHI employs are both non-contact and non-destructive.

CHI’s aim from the start was to create a collection of data-rich 3D images using photogrammetry, setting a data baseline against which future images can be compared to better understand and address any changes in the mural's condition. Convinced of the cultural importance of the project, CHI donated significant time for planning, data collection, and image processing.

Watch the CHI Team in Action: Imaging the Mural

This video is a time-lapse document taken during the four days the CHI imaging team spent on site at City College of San Francisco, applying 3D photogrammetry to capture the fresco mural.

Capturing High Resolution 3D of a Diego Rivera Mural from Cultural Heritage Imaging on Vimeo.

CHI's 3D Photogrammetry of the Mural

In December 2015, the CHI team spent four days on site at City College, digitally capturing the mural in over 1500 images. The team rented a 20-foot “person-lifter” (also known as a “scissor jack”; see photo) to aid in capturing the farthest reaches of the mural’s height. They also tested lighting equipment to come up with the most even illumination and consistent color management, and to eliminate “motion blur” that might be caused by the swaying scissor jack. And finally, the team used a 50-megapixel, high-resolution camera with a wide angle (24mm) lens followed by a second pass using a 50mm lens. This combination of equipment, distance from the mural, and two passes with two lenses of different focal lengths provided the optimum 3D geometric data, resolution detail, and color fidelity.

A crucial element of a successful photogrammetric process is obtaining a “good” photographic sequence, based on a few simple rules. The basics are explained on the CHI photogrammetry page in the “How To Capture” section. The CHI photogrammetry training class goes much deeper, exploring the reasons behind these rules and showing how to make informed choices in the face of challenging subjects.

The photogrammetric image capture and image processing techniques the CHI team used is based on the work of Neffra Matthews and Tommy Noble, developed at the US Bureau of Land Management (BLM). By carefully planning and executing the capture of overlapping photos that are crisply focused and properly exposed, these images can be fed into the photogrammetry software (CHI currently uses Agisoft PhotoScan Pro), which can create a very detailed accurate 3D surface with color.

Video of the Plaster Surface of the Mural

Video showing results from detailed 3D imaging collected using photogrammetry to document the surface shape and color; from Cultural Heritage Imaging on Vimeo.

While a fresco mural may appear to have a two-dimensional surface, the plaster actually has a lot of texture that follows the design of the mural and was created by the application of the paint in the wet plaster.

Project Outcomes

The image data collection and the initial image processing are complete. Because of the size of the project and amount of data collected, additional processing is still underway. The collected and processed data will produce a high-resolution, distortion-corrected 2D image with approximately 200 ppi (pixels per inch). In addition, the CHI team will build several resolutions of the 3D surface topology with submillimeter precision. All of the original photos will be archived, allowing others to use the data in novel ways now and in the future. The completed 3D representation and 2D image will support precise analysis and a lasting record of the mural in its current state.

Impact of 3D Photogrammetry on Mural Conservation and Research

Acknowledgments

Friends of the Diego Rivera Murals, a San Francisco Bay Area group of people devoted to protecting the mural and restoring it to prominence, provided major funding for the imaging project.

Nylda Gemple and the late Herb Gemple convened the group in 2006.

Will Maynez, historian and steward of the mural at City College, supported this work in a variety of ways, including clearing all the hurdles to get access to the mural and supporting the CHI team on site during image capture.

Tom Noble and Neffra Matthews of the US Bureau of Land Management National Operations Center, provided photogrammetry expertise to the CHI team.

Charles Walbridge and Dan Dennehy of the Minneapolis Institute of Art are donating computer processing time for the collected data.

Robert Kastler and Erik Landsberg of the Museum of Modern Art provided color management advice before and after the image collection.

Marlin Lum, Mark Mudge, and Carla Schroer, the CHI imaging team, planned and performed the imaging and are responsible for the processing, archiving, and final results from the data.