CGM(7P) — SUNPHIGS LIBRARY

NAME

CGM − contains information describing mapping of SunPHIGS functions to CGM elements.

DESCRIPTION

Overview

The Computer Graphics Metafile (CGM) standard defines a metafile for the capture of static picture definitions. Since use of the CGM was not intended to be restricted to PHIGS environments, a one-to-one mapping between the two standards is not possible in all cases. This document describes the generation of CGM in SunPHIGS environment.

Application Profiles

Application profile (AP) defines a subset of permissible values for parameters of elements to ease interoperability of implementations of a standard. There are several application profiles defined for CGM, among them are:

• TOP
• TOP-BASIC

The Technical Office Protocol (TOP) CGM AP defines the use of binary encoding, long-format command headers, and restricts the use of private ESCAPEs and GDPs. In addition, TOP-BASIC further restricts the range of primitive attributes to standardized values. For example, only line types of 1 through 5 may appear in the metafile. SunPHIGS , by default, generates TOP conforming metafiles. See WORKSTATION TYPE SET (3P).

Encodings

There are three standard encodings defined in CGM:

• Character Encoding
• Binary Encoding
• Clear Text Encoding

The Character Encoding is intended to provide an encoding of minimum size. It is particularly suitable for transfer through networks that cannot support binary transfers. The Binary Encoding provides an encoding that requires least effort to generate and interpret on many systems. The Clear Text Encoding provides an encoding that can be created, viewed, and edited with standard text editors. It is therefore also suitable for transfer through networks that support only transfer of text files. SunPHIGS is capable of generating any of the above three encodings. See WORKSTATION TYPE SET (3P).

Picture Generation

CGM is generated by the a workstation of type MO. See OPEN WORKSTATION (3P). At appropriate times, the PHIGS structures posted on an MO/CGM workstation are traversed and CGM elements are created. During PHIGS traversal, PHIGS primitives, which are in three dimensional space, are clipped to the view volume and then projected onto the view plane. Clipped PHIGS primitives may result in the generation of one or multiple CGM elements. Workstation transformation is applied to the clipped-and-projected primitives and appropriate CGM elements are generated.

CGM Construct

CGM created through SunPHIGS are typically in the following construct:

• BEGIN METAFILE
• Metafile Descriptor Elements
• BEGIN PICTURE
• Attribute Elements
• Graphical Primitive Elements
• END PICTURE
• END METAFILE

Metafile Descriptor Elements

Metafile Descriptor elements serves a similar purpose as the Workstation Description Table in PHIGS. By default, the following Metafile Descriptor elements appears at the beginning of every CGM metafile generated by SunPHIGS:

Metafile Elements	Default Value
METAFILE VERSION	1
METAFILE DESCRIPTION	"Sun Microsystems CGM, TOP/FULL conformance"
METAFILE ELEMENT LIST	list of metafile elements used in the metafile
VDC TYPE	INTEGER
INTEGER PRECISION	-32767 32767
REAL PRECISION	-32767.0 32767.0 4
INDEX PRECISION	0 127
COLOUR PRECISION	255
COLOUR INDEX PRECISION	255
COLOUR VALUE EXTENT	0 0 0 255 255 255
MAXIMUM COLOUR INDEX	255
METAFILE DEFAULTS REPLACEMENT	list of metafile defaults

Attribute Elements

Setting of the POLYLINE, POLYMARKER, TEXT, INTERIOR and EDGE representations on the MO/CGM workstations are recorded in the MO workstation state list. Because these representations cannot be encoded in a CGM metafile, PHIGS output primitives using bundled values are encoded in the metafile using individual attribute control. The following table shows the mapping of the SunPHIGS attribute control functions to CGM attribute elements:

SunPHIGS Attribute Functions	CGM Elements
SET LINETYPE	LINE TYPE
SET LINE WIDTH SCALE FACTOR	LINE WIDTH
SET POLYLINE COLOUR INDEX	LINE COLOUR
SET MARKER TYPE	MARKER TYPE
SET MARKER SIZE SCALE FACTOR	MARKER SIZE
SET POLYMARKER COLOUR INDEX	MARKER COLOUR
SET INTERIOR STYLE	INTERIOR STYLE
SET INTERIOR STYLE INDEX	PATTERN INDEX or HATCH INDEX
SET INTERIOR COLOUR INDEX	FILL COLOUR

Graphical Primitive Elements

The following table shows mapping of the SunPHIGS output primitives to CGM Graphical Primitive elements:

SunPHIGS primitives	CGM elements
POLYLINE, POLYLINE3, POLYMARKER,	One or multiple POLYLINE elements
POLYMARKER 3, TEXT, TEXT 3,
ANNOTATION TEXT RELATIVE,
ANNOTATION TEXT RELATIVE 3

FILL AREA, FILL AREA 3	POLYGON

FILL AREA SET, FILL AREA SET 3	POLYGON SET, and POLYLINE if edge flag is ON

CELL ARRAY, CELL ARRAY 3,	One or multiple POLYLINE elements
GDP - POLYLINE SET, GDP - POLYLINE SET 3

GDP - CIRCLE, GDP - CIRCLE 3	POLYGON SET, and POLYLINE if edge flag is ON

GDP - ANNOTATION CIRCLE,	CIRCLE, and POLYLINE if edge flag is ON
GDP - ANNOTATION CIRCLE 3

GDP - CIRCULAR ARC, GDP - CIRCULAR ARC 3	One or multiple POLYLINE elements

GDP - ANNOTATION CIRCULAR ARC,	CIRCULAR ARC CENTRE
GDP - ANNOTATION CIRCULAR ARC 3

GDP - CIRCULAR ARC CLOSE,	POLYGON SET, and POLYLINE if edge flag is ON
GDP - CIRCULAR ARC CLOSE 3

GDP - ANNOTATION CIRCULAR ARC CLOSE,	CIRCULAR ARC CENTRE CLOSE
GDP - ANNOTATION CIRCULAR ARC CLOSE 3

GDP - ELLIPSE, GDP - ELLIPSE 3	POLYGON SET, and POLYLINE if edge flag is ON

GDP - ANNOTATION ELLIPSE,	ELLIPSE, and POLYLINE if edge flag is ON
GDP - ANNOTATION ELLIPSE 3

GDP - ELLIPTICAL ARC,	One or multiple POLYLINE elements
GDP - ELLIPTICAL ARC 3

GDP - ANNOTATION ELLIPTICAL ARC,	ELLIPTICAL ARC
GDP - ANNOTATION ELLIPTICAL ARC 3

GDP - ELLIPTICAL ARC CLOSE,	POLYGON SET, and POLYLINE if edge flag is ON
GDP - ELLIPTICAL ARC CLOSE 3

GDP - ANNOTATION ELLIPTICAL ARC CLOSE,	ELLIPTICAL ARC CLOSE, and POLYLINE if edge
GDP - ANNOTATION ELLIPTICAL ARC CLOSE 3	flag is ON

Graphical Primitive Elements (continued)

SunPHIGS primitives	CGM elements
POLYLINE SET 3 WITH DATA†	One or multiple POLYLINE elements

NON-UNIFORM B-SPLINE CURVE†	POLYMARKER or POLYLINE

FILL AREA 3 WITH DATA†	POLYGON

FILL AREA SET 3 WITH DATA,†	POLYGON SET, and POLYLINE if
TRIANGLE STRIP 3 WITH DATA,†	edge flag is ON. Vertex colours, vertex
QUADRILATERAL MESH 3 WITH DATA,†	normals, facet colours, facet normals,
POLYHEDRON 3 WITH DATA,†	and PHIGS+ rendering attributes (such as
NON-UNIFORM B-SPLINE SURFACE†	lighting, shading, and depth cueing)
	are ignored.

† These are SunPHIGS Extensions based on PHIGS+ and are not part of the PHIGS standard.

Line Types mapping

There are 6 line types defined in SunPHIGS 1.0 but only 5 are defined in CGM. Line types that are not defined in CGM, are mapped to the line types that resemble them most closely. The following table shows mapping of SunPHIGS line types to CGM line types:

SunPHIGS Line Types		CGM Line Types
Value	Meaning	Value	Meaning
1	Solid	1	Solid
2	Dashed	2	Dashed
3	Dotted	3	Dotted
4	Dash-dotted	4	Dash-dotted
0	Long-dashed	2	Dashed
-1	Dot-dash-dotted	5	Dash-dot-dotted
-2	Center	5	Dash-dot-dotted
-3	Phantom	5	Dash-dot-dotted

Hatch Styles Mapping

There are 24 hatch styles defined in SunPHIGS 1.0, but only 6 are defined in CGM. Hatch styles that are not defined in CGM, are mapped to the hatch styles that resemble them most closely. The following table shows mapping of SunPHIGS hatch styles to CGM hatch styles:

SunPHIGS Hatch Styles		CGM Hatch Styles
Value	Meaning	Value	Meaning
-1	Horizontal	1	Horizontal
-2	Vertical	2	Vertical
-3	45 Degree-angled	3	45 Degree-angled
-4	135 Degree-angled	4	135 Degree-angled
-5	Rectangular Grid	5	Rectangular Grid
-6	Diagonal Grid	6	Diagonal Grid
-51	Double-spaced Horizontal	1	Horizontal
-52	Double-spaced vertical	2	Vertical
-53	Double-spaced 45 Degree-angled	3	45 Degree-angled
-54	Double-spaced 135 Degree-angled	4	135 Degree-angled
-55	Double-spaced Rectangular Grid	5	Rectangular Grid
-106	Double-spaced Diagonal Grid	6	Diagonal Grid
-101	Double-spaced Transparent Horizontal	1	Horizontal
-102	Double-spaced Transparent Vertical	2	Vertical
-103	Double-spaced Transp. 45 Degree-angled	3	45 Degree-angled
-104	Double-spaced Transp. 135 Degree-angled	4	135 Degree-angled
-105	Double-spaced Transp. Rectangular Grid	5	Rectangular Grid
-106	Double-spaced Transp. Diagonal Grid	6	Diagonal Grid

LIMITATIONS

CGM does not have facilities to define the placement of the image on a physical display surface. Hence the workstation viewport is discarded. Hidden line and hidden surface removal (HLHSR) are not supported in MO/CGM workstation.

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