Scientific Imaging of Diego Rivera's 'Pan American Unity' Mural

Composite view of one section of the mural with full color, 3d photgrammetry, and a blend of the two.

Stanford Libraries is proud to announce a new Spotlight exhibit: Diego Rivera's San Francisco Masterpiece - Virtual Preservation of "Pan American Unity". The exhibit is devoted to rich scientific imaging of Diego Rivera's 1940 mural Unión de la Expresión Artistica del Norte y Sur de este Continente (The Marriage of the Artistic Expression of the North and of the South on this Continent), also known as Pan American Unity. It highlights 3D photgrammetric documentation of the mural created by Cultural Heritage Imaging as part of an arrangement between City College of San Franscisco and SFMOMA to display the mural at SFMOMA from 2021 to 2023. This exhibit takes advantage of both the International Image Interoperability Framework (IIIF) and the Mirador viewer to allow users to explore the mural's colors and surface textures, the progress of the work as it was created, and work done by SFMOMA and Site & Studio Conservation to describe the condition of the mural.

Color and Shape

Full mural view in full color

The team at Cultural Heritage Imaging (CHI) produced 3D documentation of the mural, made up of 8.1-billion shape and color measurements, which contains a wealth of information about the surface shape and color of the artwork. They then used the 3D data to make two special types of high-resolution 2D images; orthomosaics depicting the mural color and Digital Elevation Models (DEMs) showing the fresco’s surface shape. The full resolution of the orthomosaic (color) and Digital Elevation Model (DEM) (shape) is 163,864 pixels across by 49,177 pixels tall totaling 8.06 billion pixels or 8.06 gigapixels. While Stanford Libraries has preserved these very large images, a requirement to prevent full download of the artwork means that we can only display a reduced-size version of the images at this time.

However, even at a reduced size, the combination of technologies we support allows for a deep investigation of the mural in ways that are impossible when viewing the art in person. While the piece measures 22 X 74 feet, a viewer can use the digital images to narrow in on tiny details and begin to understand the methods that Rivera used to produce the work. For instance, in the example below, we can see a change made to the artwork during the course of production - termed pentimento - invisible in the color image but made apparent by the 3D imaging on the right.

Displaying pentimento in color and shape

While typically we might expect side-by-side comparison of the different imaging techniques, IIIF and Mirador allow us to produce a layered view of the two images that gives the user the freedom to navigate between the layers.

Display showing user control of color and shape layers to investigate pentimento

Production and Conservation

The CHI team also worked with SFMOMA and Site & Studio Conservation to produce overlays depicting areas of the mural that Rivera worked on each day. Not only is the overlay "map" of daily production a fascinating tour of the creation of the mural, but comparing the boundaries of each section with the underlying 3D surface shape allows us to investigate aspects of the mural that are obscured by the vibrant colored image.

Detail of panel 2 of the mural depicting in the background San Francisco and in the foreground swimmers, spectators and an artist carving

For instance, thanks again to the use of IIIF to superimpose mutliple image layers and provide a way for a user to fade between the layers, the surface outlines of each section of daily work become immediately apparent.

A view fading between the giornata map and the underlying 3D showing the physical outlines of each section of daily work on the mural


Stanford Libraries and the Mural

The Libraries' role in this exciting project is to serve as a preservation and hosting platform for the rich data produced by CHI and their partners. The full-scale images for each layer have been preserved in the Stanford Digital Repository, the Spotlight exhibit platform is being used to host this exhibit of CHI's data, and our IIIF support and Mirador viewer make the dynamic image overlays possible. This project demonstrates exciting new directions for Stanford Libraries' unique combination of curatorial interests and digital infrastructure in terms of the types of stories we can tell about cultural heritage materials through advanced imaging and software applications.

Close up of globe from Panel 5 of the mural



Any collaborative project relies on a number of committed participants. The full list of partners that contributed to the production of this exhibit, including people from multiple Stanford Libraries departments, can be found here: