Overview
ISO/IEC 8632-4:1999 specifies the Clear Text Encoding for the Computer Graphics Metafile (CGM) - a standardized metafile format for the storage and transfer of picture description information. This part of ISO/IEC 8632 defines how each CGM element (as described in ISO/IEC 8632-1) is represented in a human-readable text form so metafiles can be created, inspected, and edited with ordinary text editors. The encoding includes allowed abbreviations, the overall metafile format, comment conventions, and rules to make files both easy to type and machine-parseable.
Key Topics
- Human-editable, machine-readable format: Clear Text Encoding is optimized for readability and editability while remaining parsable by software.
- Element encodings: A clear text representation is defined for every CGM element (metafile descriptor, picture descriptor, primitives, attributes, control and escape elements).
- Metafile format and grammar: Character repertoire, separators (element and parameter), comment insertion, and a formal grammar are specified to ensure consistent parsing.
- Parameter type encodings: Rules for encoding integers, reals, strings, enumerations, derived types, bitstreams, and structured data records.
- Name formation and abbreviations: Standardized abbreviations and name-construction rules to balance brevity and clarity.
- Conformance and profiles: Metafile conformance is determined via profiles; undefined elements must be represented via GENERALIZED DRAWING PRIMITIVE or ESCAPE, and registered/profile-defined values govern reserved ranges.
- Interoperability rules: Character set considerations and recommendations (subsets of ISO/IEC 646 and compatibility with ISO 2022) to maximize portability across systems.
Applications
- Software developers and CAD/graphics tool vendors: Implement CGM import/export, debugging and conversion tools that accept editable clear-text metafiles.
- Technical illustrators, documentation teams, archivists: Manually create or edit picture-descriptive metafiles for diagrams, schematics, or long-term preservation.
- Systems integrators and standards implementers: Ensure interoperability between systems by using a human-readable interchange format for picture data.
- Educational and research use: Teach graphics primitives and metafile structure without needing binary editors.
Practical benefits: easy hand-editing, convenient inspection, embedded comments, platform portability, and suitability for workflows that require human oversight (e.g., version control, review, automated transformations).
Related Standards
- ISO/IEC 8632-1 - Functional specification (defines the CGM elements the clear text maps to)
- ISO/IEC 8632-3 - Binary encoding of CGM (for CPU-efficient representations)
- ISO/IEC 646 and ISO 2022 - Character repertoire and encoding techniques referenced for text portability
Keywords: ISO/IEC 8632-4, CGM Clear Text Encoding, Computer Graphics Metafile, metafile format, picture description, human-editable CGM, graphics interoperability.