Overview - ISO/IEC 8632-3:1999 (CGM Binary Encoding)
ISO/IEC 8632-3:1999 defines the binary encoding of the Computer Graphics Metafile (CGM). It maps the abstract elements and parameters from ISO/IEC 8632‑1 (functional specification) into explicit binary representations - bits, octets and words - so metafiles can be generated and interpreted with minimal processing overhead. The standard specifies how each CGM element is encoded, how precisions are recorded in the Metafile Descriptor, and rules that optimize speed and portability for computer graphics interchange.
Key topics and technical requirements
- Element encoding and structure
- Elements are encoded in one or more partitions; the first partition contains the opcode (Element Class + Element Id).
- Every element begins on a word boundary; padding with zero bits aligns subsequent elements.
- Each element includes an explicit parameter length (octet count), enabling fast scanning without full interpretation.
- Primitive data forms
- Signed and unsigned integers at 8, 16, 24, 32-bit precisions.
- Fixed-point reals (32- and 64-bit precisions) and floating-point reals (32- and 64-bit using ANSI/IEEE 754).
- Character encodings conform to ISO/IEC 646 and ISO 2022.
- Efficiency and packing
- Alignment of coordinate data and bit-order rules minimize assembly cost on common CPU architectures.
- Run-length encoding and packed list encoding for efficient raster/cell color data.
- Conformance and profiles
- Metafile conformance is profile-based. Unknown or non-standard elements must use GENERALIZED DRAWING PRIMITIVE or ESCAPE, and private elements are restricted in valid profiles.
- Index values for enumerations follow rules (standard/registered non-negative; profile-defined negative).
- Extensibility
- Element Class/Id layout supports future growth and registration of new graphical elements.
Applications and who uses it
ISO/IEC 8632-3 is used wherever portable, efficient storage or interchange of vector/raster picture description is required:
- CAD/CAM systems, technical illustration and engineering diagrams
- Scientific visualization, GIS and mapping systems
- Print workflows and high-fidelity rendering engines
- Graphics libraries, file format converters, and interoperability middleware
Primary users:
- Software developers and library implementers (CGM readers/writers)
- System integrators needing fast metafile processing
- Standards bodies and organizations registering CGM extensions
Related standards
- ISO/IEC 8632‑1 - Functional specification (element definitions)
- ISO/IEC 8632‑4 - Clear text encoding (alternate CGM encoding)
- ANSI/IEEE 754 - Floating point representation used by this part
- ISO/IEC 646 and ISO 2022 - Character encoding rules
Keywords: ISO/IEC 8632-3, Computer Graphics Metafile, CGM, binary encoding, metafile, IEEE 754, fixed-point, run-length encoding, element encoding, Metafile Descriptor.
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