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

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

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

Requirements. Cont 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

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

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

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

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

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

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

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

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

Properties of grobject ID - Unique ID (reqd) 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.

Properties of layer ID - Unique ID (reqd) LayerName - Name of the layer (reqd) LayerDesc - Description of the layer (optional)

Properties of para ID - Unique ID (reqd) 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)

Properties of subpara ID - Unique ID (reqd) 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)

WebCGM Content Model

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

WebCGM Hyperlinks Foo.cgm Foo.cgm#P3 Foo.cgm#P4.G2 Symbol Instances Foo.cgm P1 P2 P3 P4 P5 P6 P7 G1 G2

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

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

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

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

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

Whats not in WebCGM (but in CGM) Bundled attributes Segments Geometric and Interpolated fills Generalized text paths Protection regions

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

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