SFMOMA and City College of San Francisco Partner on Historic Display of Monumental Diego Rivera Mural

What is Photogrammetry?

Scale bars for photogrammetry

Learn more about photogrammetry, a powerful imaging technique grounded in measurement science that can be used to create 3D content.

Video: Scientific Photogrammetric Imaging of a Large-scale Diego Rivera Fresco Mural

Video on imaging a large-scale Diego Rivera fresco mural

Watch this video to learn how the imaging team at CHI used photogrammetry to capture the mural, including the planning, equipment, tools, data collection, and image processing they employed to document the work.

Video: Mural Scholar Will Maynez Discusses Rivera and Pan American Unity

In September 2020, Friends of the San Francisco Library hosted a meeting featuring scholar Will Maynez as a speaker. Will discusses Rivera’s trajectory in San Francisco, including details about the people who assisted Rivera’s artistic endeavours in 1930, and in so doing, the development of his last U.S. work, Pan American Unity, a decade later.

San Francisco Chronicle Article about the Mural Imaging

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NBC Video Coverage of the Mural Imaging

NBC Bay Area News cropped photo

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

Consulting by CHI

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Capturing High-Resolution 3D Documentation of a Diego Rivera Mural

3D Photogrammetric Documentation of the 1940 Pan American Unity Fresco Mural at City College of San Francisco

CHI awarded a NEH CARES Grant to extend this project!
CHI is delighted to announce the award of a 6-month grant from the National Endowment for the Humanities (NEH) (June–December 2020) that dramatically extends CHI's existing documentation produced in 2018 for the Diego Rivera Mural Project at City College of San Francisco. Entitled “Bringing Diego Rivera's 1940 Pan American Unity Mural to the People, Virtually!”, the new grant funds additional mural imaging, digital public access to the mural, and integration with recently acquired conservation data from the mural. The project is a collaboration between City College of San Francisco, San Francisco Museum of Modern Art (SFMOMA), Stanford Libraries, and CHI. More information below.

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

The goals of the ongoing Diego Rivera Mural Project are to conserve and preserve the mural, expand the in-person and online viewing audience, enhance public experience and understanding of the mural, and use it as a focus for City College educational programs. Here is the story of the mural and how this historic collaboration came to be.

“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 high x 74 feet long, or 6.7 meters x 22.5 meters), 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 recognized the expediency of projecting a united front as he saw the seemingly unstoppable Nazis turn 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

After the exposition, the 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 free-standing mural. 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. The mural is wider than the theater building. Consequently its panels are installed in a crescent shape rather than on a flat plane as intended by Rivera. William Maynez, the mural’s historian and steward, who spent 30 years working in City College’s Physics Department, 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.”

For more than 15 years, City College has hoped to preserve and move the mural to a new Performing Arts Center building on campus. However, no site or funding was available until recently.

CHI's Initial Role in the Diego Rivera Mural Project

Maynez contacted Cultural Heritage Imaging (CHI) in 2011 and brought in the CHI imaging team (Mark Mudge, Carla Schroer, and Marlin Lum) to do an initial viewing of the work. Finally, in 2015, with funding from a group called Friends of the Diego Rivera Murals transferred to City College, Maynez 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’s aim from the start was to create a collection of data-rich 3D documentation using the computational photography technology photogrammetry, setting a data baseline against which future documentation can be compared. This will enable any changes in the mural's condition to be better understood and aid future conservation actions. Convinced of the cultural importance of the project, CHI, with the generous help of its donors, allotted significant additional resources to the project for planning, data collection, and image processing.

Watch the CHI Team in Action: Imaging the Mural in 2015

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 2000 images. The team rented a 20-foot scissor lift (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 lift. The team used a 50-megapixel, high-resolution camera to photograph the entire mural from a distance of 5 feet twice; first with a wide angle 24mm focal length 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 and resolution detail. This resolution was sufficient to disclose individual bristles in Rivera’s brushstrokes in the soft plaster. The total number of measured 3D points in the initial completed digital representation of the mural was 7.4 billion.

A crucial element of a successful photogrammetric process is obtaining a “good” photographic sequence, based on a few simple rules. The photogrammetric image capture and image processing techniques the CHI team uses are based on the work of Neffra Matthews and Tommy Noble, developed at the US Bureau of Land Management (BLM). The basics are explained on CHI's photogrammetry page in the “How to Capture” section. CHI's 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.

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 pigments onto the wet plaster.

2D From 3D

Registered shape and color: left side shows DEM, right side shows orthomosaic

Registered shape and color: left side shows Digital Elevation Model (DEM), right side shows orthomosaic

Photogrammetry produces 3D digital representations derived from archival quality photographs. By its nature, photogrammetry produces 3D models with perfect registration between color and surface. This is because the surface is built from the colored pixels in the source photographs.

The CHI team chose to create registered 2D orthomosaics and images of Digital Elevation Models built from the 3D data of the mural, because they show the shape and surface color. Because the mural’s 1.25-inch (3-centimeter) range of surface relief exists on a flat, planar substrate, this image pair conveys most of the information contained in the 3D mural model, and it allows for very high-resolution results to be shared over the Internet. Most very high-resolution 3D data available online is heavily reduced in size to make it shareable. By contrast, high-resolution 2D images are much easier to share and enable the user to zoom in to see fine details. As demonstrated by the mural’s recent pre-move conservation by the experts at SFMOMA, the orthomosaic/DEM pairs also form the perfect foundation for conservation and research analysis of the murals.

Orthomosaics are 2D images derived from the 3D model’s projection onto a plane or cylinder. Orthomosaics can provide very high-resolution images of the imaging subject. These images have no optical, perspective, or view-direction induced distortions. They are built from the same set of color-managed, high-resolution digital photographs that generated the associated 3D model.

Digital Elevation Models show the surface shape of a subject by using changing color to visualize height and depth. High-resolution orthomosaics and DEMs display each pixel representing the subject in its correct spatial location in a 2D image. In contrast, stitched images, no matter how carefully made, “pull and push” groups of pixels to make the constituent images “line up.”

Enter the San Francisco Museum of Modern Art (SFMOMA)

In 2017, City College and SFMOMA agreed to move the mural to the museum for a three-year period. The agreement called for SFMOMA to conserve the mural in preparation for the move. The timing of the move was to coincide with a planned exhibition, Diego Rivera’s America. During the exhibition, City College would build the new Performing Arts Center, and SFMOMA would then return the mural to its new home.

First Stage Project Outcomes

The image data CHI collected in December 2015 and January 2017, and the initial image processing of this data, is now complete. The collected and processed data produced a very high-resolution 7.4 billion sample 3D colored point cloud, and a 2.5D DEM of the mural surface. Using the point cloud and the original photographs of the mural, CHI produced a distortion-corrected 2D orthomosaic image of the mural with a resolution of 7.4 gigapixels, as well as a registered, equally sized image of the DEM, with a color-coded range to indicate surface-relief depth.

New Work Funded by the 2020 NEH Grant

NEH logo

The NEH grant CHI received in June 2020 has provided funding for additional mural imaging, integration of conservation data with the mural images, and the preparation of, and public access to, the mural image results via the Internet. The CHI team are thrilled to collaborate with Stanford Libraries to make the final image results available to the public in early 2021.

The original plan for the imaging was to shoot only certain areas of the mural. However, when the CHI team arrived on site, they discovered that the cleaning of the mural by the SFMOMA conservation team had returned the artwork to its original vibrant colors. The result was that there would be no way to match the colors in the original imaging with the colors captured by the new imaging. The team made a decision to reshoot the entire mural surface. What had been planned as 3 or 4 days on site turned into 6 long days. The CHI team were able to reuse some of the imaging from the prior work to aid in creating a scaled model, but only the newest images were used to generate color. Also, reimaging the entire mural has given CHI valuable data for comparison of the colors before and after conservation treatment. The cleaned mural looks fabulous, and we are excited to have a complete record of it.

Impact of 3D Photogrammetry on Mural Conservation and Research


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.

Kiernan Graves, owner of Site & Studio Conservation, LLC, is leading the conservation of the mural in partnership with SFMOMA. Kiernan is coordinating the integration of conservation data with CHI's mural imaging to allow public access of technical details of the mural.

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.