The main purpose of the PRC format is to provide a compact binary form for CAD data visualization; it provides the visual equivalent of 3D data and calculates the shape measurements with high accuracy without using specific CAD applications.
The PRC format is optimized for storing different types of CAD data such as coordinate systems, curves, surfaces, solids and so on. The format stores a wide variety of 3D data types:
PRC data can be stored in a file using regular or high compression. Regular compression avoids losses or transformation from the original CAD application format. It directly represents the 3D data. Using high compression leads to a significantly reduced file size with a representation that has a specified tolerance from the original data. Default tolerance value is 0.001 mm for exact geometry and 0.01 mm for tessellation.
A file structure represents independent assemblies or other 3D parts and can contain one or several product occurrences. Each file structure can be created with various CAD applications.
A product occurrence describes the assembly hierarchy and can include other product occurrences, therefore each file product occurrence can be considered as a tree.
There can be only one root product occurrence in the file structure. The root product occurrence is the ancestor of all other product occurrences and serves as an entry point for the file structure.
A product occurrence also can include:
The entry point for a .prc file is the model file. The model file defines the root product occurrence. It also can contain a collection of part definitions.
A part definition stores information about the starting point of the whole assembly description. A part definition can act as a prototype for a product occurrence. A part definition contains geometry data as an array of representation items and (optionally) a markups tree. The representation items are described as a combination of tessellation, topology and geometry information.
The model file has one header section where the common service information about the .prc file (original application that created the file, and start and end offsets of other file sections) is stored.
The file structure also has one header which contains special data about the structure:
Full .prc file structure is represented in the following table.
For example, version number 10001 means that the international standard of the PRC format corresponding to the first day of the 2010 year.
Since each file structure and file header in a .prc file can be written and read by different application versions, each of them can have their own format version numbers.
The following version number types are used in the PRC format:
If the current version of the PRC reading application is less than the minimum version for reading, the PRC reading application fails while processing the file and generates an error message.
If a minimum version for reading is lower than the authoring version, the PRC writing application should add a schema description to the .prc file. A schema stores information about the differences between the minimum for reading version and the authoring version: new fields or new entity types which were added between minimum version for reading and authoring version. Schema data allows the PRC reading application to skip information in a .prc file that is unknown in its minimum for read version.
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