1. Interoperability for industrial-strength CGM graphics John C. Gebhardt
WebCGM
Interoperability for industrial-strength CGM graphics
John C. Gebhardt

2. Topics The requirements for a scalable graphics format for the WWW
Why a CGM-based solution?
An introduction to WebCGM
Examples
Implementation advice
Summary

3. Scalable Graphics Requirements
Open, readily available specification
Extensible
Widely implemented w/reference code
Good interoperability potential
Wide range of raster/vector graphic primitives
Flexible text and text font capabilities

4. Requirements. Con’t
Versatile control of properties - transparency, layering, line endings
Page-independent coordinate system
Linking to/from groups of elements
Pickable, active elements or groups
Extractable metadata

5. Why a CGM-based solution?
CGM is an ISO standard with hundreds of implementations
In 1999, the CGM standard will be available on the internet in HTML format
CGM is used by many industries for 2D graphics exchange and publishing
CGM Open members and others are united in support of the WebCGM profile

6. Why CGM? Rich set of raster and vector primitives
Versatile structuring capabilities
Compact binary encoding ideal for complex technical graphics
Established profiling mechanism
Established conformance testing capability

7. WebCGM Overview File Structure The roles of the picture
Groups within pictures
Group properties
Content model
Hyperlinking
Graphical Content
Encodings

8. WebCGM Overview Submitted to W3C on August 19, 1998
Issued as W3C Recommendation on January 21, 1999
Several editor and viewer/browser implementations under development
Meets “W3C Scalable Graphics Requirements”
A profile of ISO/IEC 8632:1992

9. WebCGM File Structure Metafile Picture Picture body Symbol Libraries
Begin
Metafile
Descriptor
End
Picture 1
Picture 2
...
Picture n
Metafile
Begin
Picture
Picture
Descriptor
Picture
Properties
End
Picture
Picture
Picture Body
Picture
body
Begin
Picture Body
Control, Graphical Primitive, and Attribute Elements +
Intelligent Groups with Properties
Symbol
Libraries
Begin
Metafile
Descriptor
End
Picture 1
Picture 2
...
Picture n

10. How pictures are used Independent graphical presentation
Opaque background
primitives rendered in order encountered
Overlays to other content
transparent background
Symbol definitions
One or more symbols per library
opaque or transparent background

11. Groups within pictures
Named, typed collections of graphical primitives with attached properties
Four group types are allowed:
graphical objects (grobject)
layers or levels (layer)
paragraphs (para)
text within paragraphs (subpara)
Groups can be navigation sources and targets

12. WebCGM Architecture Groups (grobject,layer, para, subpara)
with properties
Symbol
Instances
Pictures --
Overlays
Presentations
Symbol Definitions

13. Properties of grobject
ID - Unique ID (req’d)
Name - Name (optional)
LinkURI - Link (optional, multiple)
ScreenTip - Descriptive text for interactive display
Region - Optional spatial picking region
ViewContext - rectangle to zoom to when pointed to.

14. Properties of layer ID - Unique ID (req’d)
LayerName - Name of the layer (req’d)
LayerDesc - Description of the layer (optional)

15. Properties of para ID - Unique ID (req’d)
Content - Searchable text (optional)
LinkURI - Link (optional, multiple)
ScreenTip - Descriptive text for interactive display (optional)
Region - Spatial picking region (optional)
ViewContext - rectangle to zoom to when pointed to. (optional)

16. Properties of subpara ID - Unique ID (req’d)
Content - Searchable text (optional)
LinkURI - Link (optional, multiple)
ScreenTip - Descriptive text for interactive display (optional)
Region - Spatial picking region (optional)
ViewContext - rectangle to zoom to when pointed to. (optional)

17. WebCGM Content Model

18. WebCGM Content Model

19. WebCGM Content Model

20. WebCGM Content Model

21. WebCGM Content Model

22. Hyperlinking Bi-directional
From WebCGM Object to other objects, pictures, CGMs, HTML files, or any content supported by browser via one or more LinkURI properties
To WebCGM files, pictures, objects via a URI fragment

23. WebCGM Hyperlinks Foo.cgm Foo.cgm Foo.cgm#P3 Foo.cgm#P4.G2 G1 P1 G2 P2
Symbol
Instances P7

24. WebCGM Hyperlinks, long form
Foo.cgm#pword(<picture_id>,<pic_behavior>).objword(<obj_id>,<obj_behavior>)
Where: pword = picid|picseqno
objword = id|name
and : pic_behavior = _blank
|_replace
|_self
|_top
|<target frame name>
obj_behavior = view_context
|highlight
|highlight_all

25. Graphical Content Subset of the ATA GREXCHANGE profile
Highly expressive
Easily implemented
Includes Version 1,2, &3 elements

26. Graphical Primitives Line, polyline, polygon, polygon set
Rectangles, circles, ellipses, arcs, pie slices
Graphical text -- restricted, appended
Closed figures and compound lines
Polysymbols
Smooth curves -- Piece-wise cubic Bezier
Raster images -- tiled G4, PNG, jpeg

27. Attributes and Controls
Line width, style, color, cap and join
Fill with solid color, hatch pattern, bitmap
Edge attributes similar to line attributes
Line and fill patterns can be passed by name or precisely defined in the metafile
Text font, character set (including Unicode), size, orientation, skew, aspect ratio
Clipping to rectangular regions

28. Encodings Clear Text -- for education and testing
Binary -- for production use

29. What’s not in WebCGM (but in CGM)
Bundled attributes
Segments
Geometric and Interpolated fills
Generalized text paths
Protection regions

30. Implementation Advice
Join CGM Open
Download free viewer(s)
Buy the standard
Get a good toolkit
Look at the RAL source code
Download the CGM test suites from NIST

31. Summary
WebCGM is an excellent method for publishing intelligent hybrid 2D graphics on the WWW
It promises to deliver exceptional interoperability between editing tools and viewer/browsers
It will unleash a flood of new technical graphics publishing applications