Sustainability of Digital Formats: Planning for Library of Congress Collections

The Product Representation Compact (PRC) file format represents the geometry, physical structure, and manufacturing metadata for a 3D model. Since December 2014, it has been an openly documented international standard (ISO 14739-1:2014). As the name implies, the format was developed to represent products. The intent was to support manufacturing workflows, for example, allowing a manufacturer to share a representation of a particular part with the supplier expected to build it or a parts manufacturer to assemble a detailed catalog of items available. According to a PRC FAQ from December 2008, "PRC is a compact 3D file format for representing 3D models and assemblies. It is optimized to store, load, and display various kinds of 3D data, especially for data that represents manufactured products. The data, conventionally generated with computer aided design (CAD) systems, includes product structure, geometry, and product manufacturing information (PMI)."

The format originated in 2003 with TTF, a French company specializing in CAD data exchange, which was acquired by Adobe Systems in 2006. An important use of the format is for interactive 3D visualization of a 3D model embedded in a document in the PDF family, starting with versions of PDF intended for engineering documents. See PDF/E. According to 7 Minutes with a PDF Standard, the first Adobe product to support the embedding of PRC data in a PDF was version 8 of Acrobat 3D, released in May 2007. See Notes below for information about support for PRC in later versions of Acrobat software. As of late 2019, the predominant use of the PRC format is within PDF documents.

High-level objectives for the design of the PRC format, drawn from various locations in the specification, are to:

• Achieve a high fidelity, visually equivalent representation of 3D design data produced by an advanced CAD [computer aided design] or PLM [product lifecycle management] system without requiring the original application.
• Allow applications to compute high accuracy product shape measurements.
• Complement native or open standard CAD and PLM formats as a compact, concise binary form for visualization and documentation.
• Be embeddable in PDF (ISO 32000) and other similar document formats for the purpose of 3D visualization and exchange.

According to the PRC FAQ from December 2008, these objectives are achieved by support for:

• Assembly and part relationships among 3D entities.
• Precise geometry representation (B-Rep). [See Wikipedia entry for Boundary Representation].
• Tessellated (i.e., polygonal mesh) representation of B-Rep geometry. [See Wikipedia entry for Tessellation (computer graphics)]
• A wide variety of high-order primitives that correspond to those used by CAD applications.
• Compression of meshes far more efficiently than native CAD formats. Files containing both precise geometry and tesselated geometry can be significantly smaller when converted to PRC.
• Representation of Product Manufacturing Information (PMI), used primarily to communicate the full design intent to suppliers and their shop floors. The use of PRC can reduce the need to create 2D drawings from 3D models for distribution to the supply chain.

Conceptually, a PRC file represents a product assembly and its parts. A PRC file has a single model file, which contains "root product occurrences that comprise the starting point for the entire assembly description." A "product occurrence" typically refers to a part definition, which has a tree of "representation items," defining the geometry and topology of the part. The product occurrence may also contain annotations and markup to define pre-determined views of the part and its substructures. Each part in the assembly is represented by a FileStructure that can be handled independently. Each FileStructure has an identifier unique among FileStructures in the PRC file and can have child nodes that are also product occurrences with corresponding FileStructures. Thus, by starting with the model file, the entire hierarchy of the assembly can be traversed using the unique identifiers for FileStructures.

Physically, a PRC file is a sequential binary file, designed for portability and efficient loading. A PRC file is composed of a global header section, which is uncompressed, followed by one or more FileStructures, each with its own uncompressed header followed by an individually compressed block of data. Important subsections of a FileStructure comprise: a tree that defines component parts, product occurrences, etc.; "precise" geometry data, and tesselation data. A FileStructure may also include embedded private data streams and/or UserData streams. A UserData stream is associated with a particular "writer" application identified by an ID that may be shared publicly. To optimize loading, FileStructures are ordered starting with parts, then sub-assemblies, etc. The final section in the PRC file is the model file data, also compressed.

The PRC format has data structures designed to store highly compressed geometry or tesselation data. See 3D Model: Beyond Normal below for more detail. The compilers of this resource have not been able to locate an informal introduction to this feature of the PRC specification. Comments welcome.

The PRC format originated as a proprietary format. In 2008, Adobe Systems, which had owned the technology since 2006, when it acquired Trade and Technologies France (TTF), submitted a specification for standardization through ISO in late 2008. PRC was approved as an international standard in 2014.

Maintained by a working group (WG7) under ISO/TC 171 SC2, the subcommittee for Document Management Applications, Application Issues. From 2002 to 2016, AIIM (The Association for Information and Image Management) acted as secretariat and U.S. Technical Advisory Group (TAG) to ISO/TC 171 SC 2 (see AIIM | U.S. TAG to ISO/TC 171 from 2015). In 2017, the 3D PDF Consortium was approved by the American National Standards Institute (ANSI) as a standards developer and has assumed the role of secretariat and U.S. TAG Administrator for ISO/TC 171 SC 2 (see 3D PDF Consortium Approved by ANSI as US TAG Administrator for PDF ISO Standards).

3D models in the PRC format can be legally embedded in a regular PDF document complying with ISO 32000-2:2017 (aka PDF 2.0, see PDF_2_0). Prior to the introduction of PDF 2.0, PRC was embeddable in PDF documents based on Adobe's PDF_1_7_ext03 extension to PDF 1.7.

According to What is a PRC File? from PDF3D.com, "PRC is widely used, and heavily depended on, by the manufacturing and engineering industries. This has recently spread to the medical, aerospace and earth sciences environments too including those involved in scanning, mining and geospatial exploration. Engineers, designers, scientists, analysts, managers, marketers and researchers all rely on the PRC encoding method to create PDF technical reports and presentations to demonstrate data from Computer-Aided Manufacturing (CAM), Computer-Aided Engineering (CAE), and Product Life-Cycle Management (PLM) systems to name but a few." PRC is not typically used for 3D models in the domains of entertainment or cultural heritage.

As of 2019, PRC and U3D are the only two formats supported by PDF standards for embedding 3D artwork in documents. PDF3D.com states that PRC is generally preferable because it is "significantly more feature rich than U3D, allows highly compressed geometric and visual representations to be added to 3D models and allows detailed structure, geometry and product manufacturing information (PMI) to be included." More specific reasons for using PRC are provided in a 2017 blog post 10 MBD Implementation Dos and Don’ts – Don’t Be Limited by Rudimentary 3D PDF and the User Guide for Tetra4D Converter 4.0.

The U.S. National Archives lists PRC as acceptable for the transfer of Computer Aided Design (CAD) files. In October 2018, the U.S. Department of Defense issued an update to its MIL-STD-31000 standard, which defines Technical Data Packages (TDP) as required in association with contracts. The purpose of a TDP is "to provide an authoritative technical description of an item which is clear, complete and accurate, and in a form and format adequate for its intended use." This update introduces the concept of the 3Di (3-Dimensional Intelligent) PDF viewable TDP. The definition for 3Di technical data is "A 3-dimensional viewable representation of an item provided in a widely available software format (e.g. ISO 32000-1 Portable Document Format (PDF)). This representation details the complete technical description of the required design configuration to include but not limited to geometry, topology, relationships, tolerances, attributes, metadata and other features necessary to define a component or assembly." "3D PDF with PRC" is mentioned in particular as an option for the format of a TDP.

Open source software libraries that support the reading and writing of PRC files include Asymptote and libPRC. Commercial software supporting PRC includes: SolidWorks MBD; a number of software products from PDF3D; HOOPS Publish and CADIQ (tool for validating and comparing 3D designs). Conversion tools that support PRC for input or output include: FME from Safe Software; Datakit; the ArcGIS Data Interoperability toolkit; Online CAD Converter; Tetra4D Converter, and HOOPS Exchange.

The format allows attributes to be stored in any FileStructure. To save space, common attributes are represented by integer codes. Attributes given standard codes (integers 2-19) are: Title; Subject; Author; Keywords; Comments; Template; Last Saved By; Revision Number; Total Editing Time; Last Printed; Create Time/Date; Last saved Time/Date; Number of Pages; Number of Words; Number of Characters; Thumbnail; Name of Creating Application; Security. User-defined attributes are also supported as key/value pairs.

The format also supports annotations, termed "markup data."

There is no support for embedding XML-based metadata in a recognized schema. However, there is support for storing private data in any component FileStructure.

Regular compression, using flate/deflate compression, can be used to directly represent CAD data without loss or transformation from the originating CAD system.

PRC can also store highly compressed geometry or tesselation data that is transformed in a lossy manner from the original CAD data. The standard describes a general approach to compression of tesselation data that uses general characteristics of a triangular mesh to produce a more compact representation based on a specified tolerance for approximation. The general approach is described in pseudocode, but the standard does not prescribe a particular methodology, since different approximation techniques might be relevant. Similarly, the PRC standard provides data substructures for storing an approximation to geometry defined by freeform curves and surfaces, but does not prescribe a single methodology for performing the approximation.

3D Model Characteristics: The quality and functionality factors used above are based on an analysis from 2008 in An Overview of 3D Data Content, File Formats and Viewers, a technical report (ISDA08-002) from the National Center for Supercomputing Applications (NCSA). According to this analysis, the key aspects of a 3D model are: geometry, appearance, scene, and animation. For brief descriptions of these factors, see the table for 3D model formats under Quality and functionality factors. The descriptions there are drawn from the 2008 report and from 2019 Most Common 3D File Formats (an article from All3DP), which uses the same factors to distinguish the capabilities of formats.

All 3D formats support some method for representing the geometry (the shape) of an object; support for other aspects is optional depending on the primary use of the format. Table 3 in the 2008 report, An Overview of 3D Data Content, File Formats and Viewers, compares nine popular 3D formats in use in 2008 based on these four aspects of 3D models.

Support for 3D formats in versions of Adobe's Acrobat software: Acrobat 3D was introduced in 2006 and was the name used for products released in parallel with versions 7 and 8 of Acrobat Pro. U3D was supported in version 7; the first support of PRC was in version 8 in 2007. Acrobat 3D was a special version of Acrobat with additional features, including direct import from files in some proprietary CAD formats. It was replaced in 2009 by Acrobat 9 Pro Extended, which offered conversion from selected proprietary CAD formats and allowed import of files in U3D or PRC formats . Starting with Acrobat X, Adobe stopped supporting conversion from proprietary CAD formats in the Acrobat products it sold, relying instead on add-on products from Tetra 4D, first using the name 3D PDF Converter and more recently Tetra4D Converter. Since then versions of Acrobat Pro for Windows have supported the import of U3D and PRC formats. Comments welcome.

Developed originally by TTF (Trade and Technologies France), a French company, and demonstrated in 2003. TTF was acquired by Adobe Systems in 2006. PRC was developed for standardization through ISO under the auspices of the AIIM Standards program in the PDF/E committee. Published as ISO 14739-1:2014. See announcement from PDF Association.

A new version of ISO 14739-1 is in the final stages of approval through ISO as of October 2019. It will probably be published in 2020. Although there were plans for a PDF/E-2 update based on ISO 32000, as of late 2019, the expectation is that the next edition of PDF/A, to be based on PDF 2.0 will have an application profile aimed at engineering documention.