Overview
ISO/IEC 7942-4:1998 defines the Picture Part Archive (PPA) for the Graphical Kernel System (GKS‑94). It specifies a file format and three encodings for the storage and retrieval of GKS-94 picture parts, extending the GKS NDC metafile model and the Computer Graphics Metafile (CGM) conventions. The standard introduces a new delimiter element to mark the start of a picture part archive and describes how picture parts are structured, named, encoded and retrieved.
Key topics and technical requirements
- File structure: A PPA metafile follows the CGM/NDC metafile organization: a BEGIN PICTURE PART ARCHIVE METAFILE delimiter, a Metafile Descriptor, a sequence of independent picture parts, and an END METAFILE delimiter.
- New delimiter element: BEGIN PICTURE PART ARCHIVE METAFILE is defined as the first element and occurs exactly once per archive. Its identifier parameter is available for generator/interpreter use.
- Picture part representation: Each archived picture part is delimited by BEGIN PICTURE and END PICTURE elements. The archival name of a picture part is stored in the picture identifier parameter of the BEGIN PICTURE element.
- Functional set (PPA‑SET): The standard defines the PPA-SET - the allowed metafile elements for picture part archives. It reuses the GKS-94 NDC elements and adds the PPA-specific delimiter.
- Encodings: Three encodings are specified for interoperability:
- Character encoding
- Binary encoding
- Clear text encoding
Opcodes and representations for the new BEGIN PICTURE PART ARCHIVE METAFILE element are provided for each encoding.
- Formal grammar and annexes: Annex A provides the formal grammar (aligned with Part 2/NDC metafile grammar) and Annex B lists the new elements introduced by this part.
- Data types: Uses the same data type definitions as CGM Version 4.
Applications and who uses it
- Software developers and vendors implementing graphics subsystems that need to archive or exchange GKS-94 picture parts.
- CAD, GIS, scientific visualization, and image-processing systems that rely on interoperable, standardized metafile formats.
- Long-term archival and interchange of graphical assets where picture parts must be stored, named and later retrieved reliably.
- Toolchains converting between GKS/CGM-based metafiles and application-specific formats.
Related standards
Keywords: ISO/IEC 7942-4:1998, GKS-94, Picture Part Archive, PPA, CGM, metafile, character encoding, binary encoding, clear text encoding, archive picture part, graphical interchange.