Information technology -- Computer graphics -- Computer Graphics Reference Model

Defines a set of concepts and their inter-relationships which should be applicable to the complete range of future computer graphics standard. May be applied to verify and refine requirements for computer graphics; to identify needs for computer graphics standards and external interfaces; to develop models based on requirements for computer graphics; to define the architecture of new computer graphics standards; compare computer graphics standards.

Technologies de l'information -- Infographie -- Modèle de référence

General Information

Status
Published
Publication Date
23-Sep-1992
Current Stage
6060 - International Standard published
Completion Date
24-Sep-1992
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ISO/IEC 11072:1992 - Information technology -- Computer graphics -- Computer Graphics Reference Model
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ISOAEC
INTERNATIONAL
11072
STANDARD
First edition
1992-10-01
Information technology - Computer graphics -
Computer Graphics Reference Model
Technologies de /‘information - lnfographie - Mod&/e de rbfbrence
Reference number
ISO/IEC 11072:1992 (E)
---------------------- Page: 1 ----------------------
ISO/IEC 11072rl992 (E)
Contents
Page
Foreword
Introduction
1 Scope
2 Definitions
3 The Computer Graphics Reference Model
3.1 Environment model
3.2 External relationships
3.3 Environment structure
3.4 Data elements
3.4.1 Introduction
3.4.2 Composition
3.4.3 Collection store
3.4.4 Token store
3.4.5 Aggregation store
3.4.6 Environment state
3.5 Processing elements
3 5.1 Absorption
3.5.2 Manipulation
3.5.3 Distribution
3.5.4 Assembly
3.5.5 Emanation
3.6 Characteristics of specific environments
3.6.1 Environment details
3.6.2 Output primitives
3.6.3 Input tokens
3.6.4 Properties
3.6.5 Transformations
3.6.6 Fan-in and fan-out
3.7 Relationship between output and input
3.8 Internal interfaces
0 ISO/IEC 1992

All rights reserved. No part of this publication may be reproduced or utilized in any form

or by any means, electronic or mechanical, including photocopying and microfilm,
without permission in writing from the publisher.
ISO/IEC Copyright Office l Case postale 56 l CH-1211 Genkve 20 l Switzerland
Printed in Switzerland
---------------------- Page: 2 ----------------------
ISO/IEC 11072:1992 (E)
Annexes
A Existing standards and the CGRM 22
Graphical kernel system-IS0 7942 22
A.1
Graphical kernel system for three 22
A.2
dimensions-IS0 8805
Programmer’s hierarchical interactive 24
A.3
graphics system - ISO/IEC 9592
A.4 Interfacing techniques for dialogues with 24
graphical devices-ISO/IEC 9636
A.5 Metafile for the storage and transfer of 26
picture description information-IS0 8632
B The relationship of computer imaging to computer 27
graphics
C The relationship of window systems to computer
graphics 30
Cl Introduction 30
co2 Window systems 30
c’3 . Windowing considered
as an operator
C.4 Windowing considered as part of the
computer graphics system
C.4.1 Overview
C.4.2 Operations on windows
C. 4.3 Operations on window content
C. 4.4 Displaying windows
C.4.5 Input
D Bibliography
---------------------- Page: 3 ----------------------
ISO/IEC 11072:1992 (E)
Foreword

IS0 (the International Organization for Standardization) and IEC (the International

Electrotechnical Commission) form the specialized system for worldwide standardization.

National bodies that are members of IS0 or IEC participate in the development

of International Standards through technical committees established by the respective

organization to deal with particular fields of technical activity. IS0 and IEC technical

committees collaborate in fields of mutual interest. Other international organizations,

governmental and non-governmental, in liaison with IS0 and IEC, also take part in the work.

In the field of information technology, IS0 and IEC have established a joint technical

committee, ISO/IEC JTC 1. Draft International Standards adopted by the joint

technical committee are circulated to national bodies for voting. Publication as an International

Standard requires 75 % of the national bodies casting
approval by at least
a vote.
International Standard ISO/IEC 11072 was prepared by Joint Technical Committee
ISO/IEC JTC I, Information technology.
Annexes A to D of this International Standard are for information only.
---------------------- Page: 4 ----------------------
ISO/IEC 11072:1992 (E)
Introduction
The Computer Graphics Reference Model (CGRM) describes the conceptual

framework for computer graphics. Computer graphics is the creation of, manipulation

of, analysis of, and interaction with pictorial representations of objects and data using

computers.
The main purpose of the CGRM is to define concepts that shall be used to develop

computer graphics standards. Additional purposes are to explain relations between

SC24 standards and to provide a forum whereby areas outside computer graphics can

identify their relationships to computer graphics.

International Standards related to computer graphics include the following existing and

emerging areas:
a) Open Systems Interconnection - Basic Reference Model;
b) Virtual Terminal Protocols and Terminal Management;
c) File Transfer, Access and Management Protocols;
d) Office Document Architecture and Interchange;
e) Text and Office Systems;
f) Exchange of Product Model Data;
g) Character Sets and Coding;
h) Open Distributed Processing;
i) Image Processing and Interchange.

This International Standard shall be the basis for the development of specific standards

for computer graphics and will ensure their long term coherence based on objective

rational foundations. Existing computer graphics standards will not necessarily fit

precisely into the Reference Model. However, experience with current standards has

significantly influenced the model.
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~~ ~-
INTERNATIONAL STANDARD
ISO/IEC 11072rl992 (E)
Information technology - Computer graphics -
Computer Graphics Reference Model
1 Scope
This International Standard, the Computer Graphics Reference Model (CGRM),

defines a structure within which current and future International Standards for computer

graphics shall be compared and their relationships described.

This International Standard defines a set of concepts and their inter-relationships which

should be applicable to the complete range of future computer graphics standards.

This International Standard may be applied to:
verify and refine requirements for computer graphics;
b) identify needs for computer graphics standards and external interfaces;
c) develop models based on requirements for computer graphics;
d) define the architecture of new computer graphics standards;
e) compare computer graphics standards.

This International Standard does not define how computer graphics standards shall be

defined and developed. It does not specify the functional descriptions of computer

graphics standards, the bindings of those standards to programming languages, or the

encoding of graphical information in any coding technique or interchange format. It is

neither an implementation specification for systems incorporating computer graphics,

nor a basis for appraising the conformance of implementations.
---------------------- Page: 7 ----------------------
ISOAEC 11072:1992 (E)
2 Definitions

For the purposes of this International Standard, the following definitions apply. An

alphabetical list is given at the end of this clause.
2.1 computer graphics: The creation of, manipulation of, analysis of and
interaction with pictorial representations of objects and data using computers.

2.2 application: The external object that uses computer graphics. Applications are

not modelled in the CGRM, but their interactions with computer graphics are modelled.

2.3 operator: The external object that observes the contents of the display and

generates physical input values. Operators are not modelled in the CGRM, but their

interactions with computer graphics systems are modelled.
2.4 environment: A subdivision of the CGRM at a given level of abstraction. The

definition of the environment includes the definition of its data elements and processing

elements. Specific names are given to the five environments: construction, virtual,

viewing, logical and realization (see 3.6.1).
2.4.1 construction environment: The environment that interfaces to the
application.
2.4.2 virtual environment: The environment between the construction and
viewing environments.
2.4.3 viewing environment: The environment between the virtual and logical
environments.
2.4.4 logical environment: The environment between the viewing and the
realization environments.
2.4.5 realization environment: The environment that interfaces to the
operator.
2.4.6 higher environment: An environment closer to the application.
2.4.7 lower environment: An environment closer to the operator.
2.4.8 entity: An item of information stored within an environment or passed
between environments. Entities are divided into three classes: input, output and
control.
2.4.9 fan-in: The merging of entities from multiple, independent sources to

produce a single stream (without changing individual entities) to be processed by a

single environment.
2.4.10 fan-out: The generation of multiple, independent entities from a single

entity without change. The generated entities are sent to independent environments.

2.5 external interfaces: The interfaces between the computer graphics system and

the outside world, the interfaces communicate with the operator, application, data

capture metafile and audit trail metafile.
---------------------- Page: 8 ----------------------
ISO/IEC 11072:1992 (E)
2.5.1 operator interface: The interface between the realization environment and
the operator. This is the only interface between the operator and the graphics
system. \
2 S.2 application interface: The interface provided by the construction

environment to the application. This is the only interface between the application

and the graphics system.

2.5.3 data capture metafile: An external object for representing all or part of a

data element for storage, retrieval and transmission.
2.5.3.1 export: The process of generating a data capture metafile.
2.5.3.2 import: The action of setting part or all of a data element from a
data capture metafile.
2 S.4 audit trail metafile: An external object for representing the sequential
flow of information across the application interface.
2 o 6 processing element: A process in an environment: absorption, manipulation,
distribution, assembly, and emanation.
2 l 6o 1 absorption: A process which receives entities from the next higher
environment and processes them for use within its own environment. Specific

names are given to absorption at each environment level: preparation, production,

projection, completion and presentation.
preparation: The name given to absorption in the construction
2.6 l lo 1
environment.
2.6.1.2 production: The name given to absorption in the virtual
environment.
2.6.1.3 projection: The name given to absorption in the viewing
environment.
2.6.1.4 completion: The name given to absorption in the logical
environment.
2.6.1.5 presentation: The name given to absorption in the realization
environment.
2 l 6o2 emanation: A process which emanates token store and input control

entities to the next higher environment after processing them. Specific names are

given to emanation at each environment level: accumulation, abstraction, elevation,

generation and utilization.
2 l 6 l 2 l 1 accumulation: The name given to emanation in the realization
environment.
2.6.2.2 abstraction: The name given to emanation in the logical
environment.
2.6.2.3 elevation: The name given to emanation in the viewing
environment.
2.6.2.4 generation: The name given to emanation in the virtual
environment.
---------------------- Page: 9 ----------------------
ISO/IEC 11072:1992 (E)
2.6.2.5 utilization: The name given to emanation in the construction
environment.
2.6.3 distribution: A process which distributes the composition and output
control entities to the next lower environment.
2.6.4 assembly: A process which receives entities from the next lower
environment for use within its own environment.
2.6.5 manipulation: A process which accesses and changes the contents of
data elements.

2.7 data element: A store in an environment: composition, collection store, token

store, aggregation store, and environment state.
2.7.1 composition: A spatially structured set of output primitives in a given
environment. Specific names are given to the composition at each environment
level: model, scene, picture, graphical image and display.
2.7.1.1 model: The name given to the composition in the construction
environment.
2.7.1.2 scene: The name given to the composition in the virtual
environment.
The name given to the composition in the viewing
2.7.1.3 picture:
environment.
2.7.1.4 graphical image: The name given to the composition in the logical
environment.
2.7.1.5 display: The name given to the composition in the realization
environment.
2 l 7.2 collection store: A storage facility for collections.
collection: A set of output entities which are named and may be
2.7.2.1
structured. A collection may be manipulated to produce all or part of a
composition in the same environment.
A storage facility for aggregations.
2.7.3 aggregation store:
2.7.3.1 aggregation: A set of input entities which are named and may be
structured. An aggregation may be manipulated to produce one or more input
tokens in the token store in the same environment.
2.7.4 token store: A structured set of input tokens in a given environment.
Specific names are given to the token store at each environment: lexeme store,
information store, selection store, directive store and instruction store.
2.7.4.1 lexeme store: The name given to the token store in the realization
environment.
2.7.4.2 information store: The name given to the token store in the
logical environment.
---------------------- Page: 10 ----------------------
ISO/IEC 11072:1992 (E)
2.7.4 l 3 selection store: The name given to the token store in the viewing
environment.
directive store: The name given to the token store in the virtual
2.7.4.4
environment.
2.7.4.5 instruction store: The name given to the token store in the
construction environment.
2.7.5 environment state: Entities in the environment separate from other data
elements: composition, collection store, token store, aggregation store.
2.7 l 6 editing: The change of entities within data elements in an environment.

2.8 output primitive: An atomic unit for graphical output in a given environment.

There may be more than one class of output primitive. Geometric and other properties

may be bound to an output primitive at its creation or later.

2.9 input token: An atomic unit for graphical input in a given environment. There

may be more than one class of input token. Geometry and other properties may be
bound to an input token at its creation or later.

2.9.1 property: A value that may be used by an output primitive or input token to

specify its geometry or other characteristics.
2.9.2 geometric property: A property which is subject to modification by
geometric transformations.
2.9.2.1 geometric transformation: A transformation that modifies the
geometry of an input token or output primitive.
2.9.2.2 geometry: A property of an input token or output primitive used to
define its shape, position, orientation and extent.

2 l 9 l 3 binding: The action of assigning a property to either an output primitive or

an input token.
2.9.4 unbinding: The action of un-assigning a property from either an output
primitive or an input token.
ping: The action of
2.9.5 clip constrainin g the geometric s hape and extent of
either an ou tput primiti ve or input token to be withi
n a specified region.
The following alphabetical list gives the sub-clause of each CGRM definition.
absorption 2.6.1
abstraction 2.6.2.2
accumulation 2.6.2.1
aggregation 2.7.3.1
aggregation store 2.7.3
application 22
application interface 2’5 . 2
assembly 2’6 . 4
audit trail metafile 2’5 . 4
binding 2’9 . 3
clipping 2’9 5
collection 21712.1
collection store 2.7.2
---------------------- Page: 11 ----------------------
ISO/IEC 11072:1992 (E)
completion 2.6.1.4
composition 2.7.1
computer graphics 21
construction environment 2’4 . 1
data capture metafile 2’5 . . 3
data element 27
directive store 217.4.4
display 2.7.1.5
distribution
2.6.3
editing 2.7.6
elevation 2.6.2.3
emanation 2.6.2
entity 2.4.8
environment 24
environment state 2’7 5
export 2:5:3.1
external interfaces 25
fan-in 2’4 9
fan-out 2:4: 10
generation
2.6.2.4
geometric property 2.9.2
geometric transformation 2.9.2.1
geometry 2.9.2.2
graphical image 2.7.1.4
higher environment 2.4.6
import 2.5.3.2
information store 2.7.4.2
input token 29
instruction store 217.4.5
lexeme store 2.7.4.1
logical environment 2.4.4
lower environment
2.4.7
manipulation
2.6.5
model 2.7.1.1
operator 23
operator interface 2’5 . . 1
output primitive 28
picture 2:7.1.3
preparation 2.6.1.1
presentation
2.6.1.5
processing element
production 2:6.1.2
projection 2.6.1.3
2.9.1
property
realization environment 2.4.5
scene 2.7.1.2
selection store 2.7.4.3
token store 2.7.4
unbinding 2.9.4
utilization 2.6.2.5
viewing environment 2.4.3
virtual environment 2.4.2
---------------------- Page: 12 ----------------------
ISO/IEC 11072:1992 (E)
3 The Computer Graphics Reference Model
3.1 Environment model
The CGRM defines computer graphics in terms of five abstract levels called

environments: construction, virtual, viewing, logical and realization (figure 1). The

internal model of each environment is identical. The symmetry between input and
output in the diagram reflects a symmetry of purpose rather than a symmetry of
complexity. The CGRM defines operations on data elements in each environment.

The CGRM defines computer graphics output in terms of output primitives which make

up a composition that is presented to the operator. The CGRM defines computer

graphics input in terms of input tokens which make up a token store that is accumulated

for the application in an appropriate form. Any connection between received input and

generated output is conceptually handled by the application. The application may

delegate this responsibility to specific environments. To allow complex graphical

compositions, the CGRM defines a storage facility-the collection store-from which

compositions may be derived. Similarly, a storage facility-the aggregation store-is

defined from which entries in the token store may be derived.
Application
Virtual Environment
Viewing Environment
Logical Environment
Realization Environment
I I
Figure 1 - Computer graphics environments
---------------------- Page: 13 ----------------------
ISO/IEC 11072:1992 (E)
3.2 External relationships
The overall structure of the reference model is illustrated in figure 2.
Application
Application

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..~........................

Interface
Computer Graphics
*Data Capture
Environments
Metafiles
Operator
,.,,.................................................. . . . . . ..I......

. . . . . . . . . . . . . . . . ..~...................................... ,...........................,,,..,

Interface
Operator
External interfaces
Figure 2 -

Data capture metafiles may be generated by external agents and imported through the

appropriate environment interface. Audit trail metafiles may be played back through the

audit trail metafile interface at a later time. Thus communication between a computer

graphics system and the “outside world” is described in the CGRM in terms of the
following external interfaces:
operator interface: The interface provided by the realization environment to the
operator. This is the only interface between the operator and the realization
environment, and consequently between the operator and any computer
graphics environment.
application inte$ace: The interface provided by the construction environment to
the application. This is the only interface between the application and the
construction environment, and consequently between the application and any
computer graphics environment.
data capture metafile inteeaces: The interfaces provided by each environment

for importing and exporting all or part of data elements: composition, collection

store, token store, aggregation store and environment state.
audit trail metafile interface: The interface provided to record or playback the
flow of information across the application interface.
---------------------- Page: 14 ----------------------
ISO/IEC 11072:1992 (E)

The external objects, which are not part of the computer graphics environment, are:

operator: The external object that observes the contents of the display in the
realization environment and provides physical input tokens.

application: The external object that interacts with the construction environment

through the application interface.

data capture metafile: The external object for representing all or part of a data

element for storage, retrieval, and transmission.

audit trail metafile: The external object for representing the flow of information

across the application interface.

Individual standards may define additional external interfaces (different from the

interfaces defined in the CGRM) to meet specific requirements of those standards.

However interfaces different from those defined in the CGRM shall be given unique

names distinct from those defined in the CGRM.

The CGRM does not specify the interfaces to either data capture metafiles or the audit

organization. It does
trail metafile nor does it specify their format and internal describe
their conceptual contents.
3.3 Environment structure

Each environment consists of data elements and processing elements as depicted in

figure 3. Entity flows are indicated by arrows:
a) An arrow coming from a data element means that entities in that data element
may be used by the processing element to which the arrow is directed.

b) An arrow directed towards a data element means that entities in that data element

may be set by the processing element from which the arrow is directed.
c) Dashed lines with arrows at each end coming from a data element mean that
some or all of the contents of this data element may be exported to, and
imported from, a data capture metafile.
d) An arrow between two processes means that entities may pass without storage
to the process to which the arrow is directed.
e) An open arrow between two processes means that only control entities may
pass between the two processes without storage.

No constraints are placed on the mechanisms by which the evolution of the behaviour

of processing elements is controlled. Control entities are part of the environmental

model though not explicitly shown in the diagram.
In each environment, there is a single interface for incoming entities from the

immediately higher environment concerning graphical output and a single interface for

incoming entities from the immediately lower environment concerning graphical input.

The same coordinate system is used for both input and output entities passing between

each pair of adjacent environments. The coordinate systems used by the composition,

collection store, token store and aggregation store within an environment are the same.

Consequently, all transformations occur in the absorption and emanation processes.

There are interfaces for storage and retrieval of all or parts of data elements in data

capture metafiles.
---------------------- Page: 15 ----------------------
ISO/IEC 11072:1992 (E)

Some or all of a data element may be exported to a data capture metafile. All or part of

the contents of a data capture metafile may be added to the appropriate current data

element, or may replace all or part of it. A data element exported to a data capture

metafile from one environment may only be imported into a data element of the same

type in the same environment. A data capture metafile may only capture data elements

from a single environment.
I m---
Emanation
Absorption
~~~~~~~&\
--m-
Assembly
Distribution
all processes may use or set environment state
0 = processing elements
control and data
q = processing elements with
a---
control only
transformation
.-III-m-
data capture metafile interfaces
0 = data elements
Figure 3 - Environment model
---------------------- Page: 16 ----------------------
ISO/IEC 11072:1992 (E)
3.4 Data elements
3.4.1 Introduction

Each data element in an environment is affected by those processes having an arrow

directed towards that data element (see figure 3). The operations on each data element

may include editing (for example, create, insert, replace or delete) individual entities,

groups of entities or the complete data element. Complete data elements or a subset of

entities within a data element may be exported and imported. The entities within a data

element may be inquired.
3.4.2 Composition

The composition is a spatially structured set of output primitives in a given environment

ready for distribution. The composition represents the “output working set” for the

environment. The CGRM places no constraints on the ordering of output primitives in

the composition other than that the specification be well-defined and unambiguous.

The contents of the composition may be generated by manipulating the collection store

(possibly as a result of input actions) in a given environment.
Specific names are given to compositions in each environment:
a) construction: model;
b) virtual: scene;
picture;
c) viewing:
graphical image;
d) logical:
e) realization: display.
3.4.3 Collection store

A collection is a set of output entities which are named and may be structured. A

collection is intended for use within an environment. Collections are stored in a

collection store. Structure may exist within collections which relate one entity to

another. The manipulation process may change the contents of the composition using

the contents of the collection store.
3.4.4 Token store

The token store is a structured set of input tokens in a given environment ready for

emanation. The token store represents the “input working set” of the environment.

Changes of input token class occur by manipulation of entries in the aggregation store

to create new entries in the token store. Consequently, coordinate transformations to the

coordinates of the next higher level occur during emanation and not assembly. The

CGRM places no constraints on the ordering of input tokens other than that the

specification be well-defined and unambiguous. Input tokens may result from assembly

or manipulation.
Specific names are given to token stores in each environment:
a) realization: lexeme store;
b) logical: information store;
c) viewing: selection store;
d) virtual: directive store;
e) construction: instruction store.
---------------------- Page: 17 ----------------------
ISO/IEC 11072:1992 (E)
3.4.5 Aggregation store

An aggregation is a set of input entities which are named and may be structured. An

aggregation is intended for use within an environment. Aggregations are stored in an

aggregation store. Structure may exist within aggregations which relate one entity to

another. The manipulation process may use the contents of the aggregation store to

change the contents of the token store .
3.4.6 Environment state

An environment may contain an environment state separate from the other data elements

in the environment. When a process sets or uses environment state entities, it shall set

or use environment state entities in its own environment. Environment state entities

may be propagated to adjacent environments through control operations. Some or all of

the environment state entities in an environment may be exported to or imported from a

data capture metafile.

State information may be used and shared by all processes within an environment, as

well as by all operations on data elements within an environment. Environment state

entities may be used to determine whether or not a particular operation shall be
performed.
There may be environment state entities which cannot be inquired from other
environments.
3.5 Processing elements
3.5.1 Absorption
Absorption is the process which receives output entities from the next higher

environment and applies the geometric and other transformations necessary to produce

the entities in the form appropriate to its environment. Absorption transforms that part

of the composition distributed from the next higher environment into a composition or

collection(s) in its environment. This may involve transforming entities with geometric

properties into the appropriate local coordinate system, elaborating output primitives

into one or more replacement output primitives, as well as applying any clipping or

non-geometric operations appropriate to the environment.

The absorption process in one environment may automatically cause changes made to

its composition to be distributed to cause changes to be made to the composition at the

lower environment. Alternatively, absorption may occur only when explicitly

requested, for example by the next higher environment. The CGRM does not constrain

when output primitives are received from the next higher environment.
Specific names are given to absorption in each environment:
a) construction: preparation;
b) virtudl: production;
c) viewing: projection;
d) logical: completion;
e) realization:
presentation.

Control information for input or output control may be consumed by the environment

and change the environment state to dictate the behaviour of processing elements.

Control entities destined for a lower environment may be modified by the absorption

process. The results of the absorption process may be directed to the environment

state, collection store, composition, directly to the next lower environment throu

...

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