1. Validation of a CGM Graphic Against the Web CGM Profile
Stuart Galt
The Boeing Company

2. Agenda
What is the WebCGM profile
The process
Hard parts

3. WebCGM
CGM (ISO 8632) vector and composite vector/raster graphic format.
WebCGM is a profile used to specify the subset of the CGM standard that will be used to exchange data on the web.
This is background and most everyone in the audience should know this. Feel free to ad-lib here.

4. WebCGM History 1987 - ISO approved CGM as a Standard
CGM became a registered mime type
W3C published “Scalable Graphics Requirements”
W3C published “Use of CGM as a Scalable Graphic Format”
WebCGM 1.0 approved in January

5. WebCGM is intelligent Contains graphic primitives
Lines, polylines, polygons
Rectangles, circles, ellipses, arcs
Graphical text
Non graphical content
Metadata
Hierarchies objects
Links

6. The Process 1 – Convert CGM to XML
2 – get the profile schema provided by your standards body
3 – Take the above two pieces and validate the CGM instance

7. CGM to XML CGM graphic primitives are converted directly
Containers are used to represent internal state changes

8. CGM primitives Circle centered (4252, 2385) radius 150 Binary CGM XML
<Circle> <Point x=“4252” y=“2385”/> <Radius r=“150”/> </Circle>

9. CGM primitives (cont’d)
Restricted Text
<RestrictedText final="yes"> <DeltaBox dx="400" dy="97"/> <Point x="914" y="1953"/> <StringFixed>NOTE:</StringFixed> </RestrictedText>

10. CGM primitives (cont’d)
Polygon
<Polygon> <Point x="3175" y="3879"/> <Point x="3085" y="3833"/> <Point x="3075" y="3866"/> </Polygon>

11. CGM States Container elements used to mark state changes
Metafile defaults
<MetafileDefaultsReplacement> …defaults go here … </MetafileDefaultsReplacement>
Can be thought of similar to the “GSAVE” command in postscript.
Major States :
PCS Picture Closed State
MDS Metafile Descriptor State
DR Defaults Replacement Mode
GSS Global Segment State
DSS Picture Descriptor Segment State
PDS Picture Descriptor State
POS Picture Open State
LSS Local Segment State
SDS Structure Descriptor State
SOS Structure Open State
Minor States :
FOS Figure Open State
TOS Text Open State
CPS Compound Path State
PRS Protection Region State
TAS Tile Array State

12. CGM States (cont’d) Picture Descriptor and Picture Open State
<Picture> <StringFixed>Picture Description</StringFixed>
…Picture descriptor data here…
<BeginPictureBody/> …Picture data here… </Picture>

13. CGM States Computer Graphics Metafile Metafile Descriptor Picture
EndMF
BegMF
Picture
Picture
Descriptor
Picture
Body
EndPic
BegPic
Picture
Body
Picture
Open
BegPicBody

14. Metafile Descriptor Metafile Decsriptor MFVersion MFDesc VDCType
opt
rep
MFVersion
MFDesc
VDCType
IntegerPrec
RealPrec
IndexPrec
ColrPrec
ColrIndexPrec
MaxColrIndex
ColrValueExt
MFElemlist
FontList
CharSetList
CharCoding
NamePrec
MaxVDCExt
SetPriExt
ColrModel
ColrCalib
FontProp
GlyphMap
SymLibList

15. Validating WebCGM Current state:
Profile validation tool exists for WebCGM in binary encoding
Metacheck
With a WebCGM schema defined
General purpose tools would be available

16. The Schema
Some elements are easy to represent
Some are hard

17. Metafile Version <element name="MetafileDescriptor">
<complexType>
<sequence>
<element name="MetafileVersion">
<attribute name=“version” use=“required”
type=“integer” minInclusive=“1”
maxInclusive=“4”/>
</element>
<all>
<element name="MetafileDescription" type="string"/>
...other required elements go here...
</all></sequence></complexType></element>
The Metafile Version tag may appear only once in the CGM file, and must
occur somewhere in the metafile descriptor section. There are a few other
required elements in this descriptor section and could be represented in a
schema by something like:
The above example is a long way of saying that the Metafile Version
tag must occur in the somewhere in the Metafile Descriptor section, but doesn't
say exactly where. In addition the allowable values for the required version
attribute are 1, 2, 3, or 4.

18. Non Graphical Text String
<simpleType name=“NonGraphicText”>
<restriction base=“string”>
<minlength value=“0”/>
<maxlength value=“254”/>
</restriction>
</simpleType>
A non-graphical text string is string that is not directly displayed
on the graphic. An example would be the text associated with the CGM begin
picture element. Non-graphical text strings are required to be less than 254
characters in length. By defining an element NonGraphicText type we can define
it as a type of string with a length from 0 to 254 characters
By defining the nongraphic text string attribute as:
type=“NonGraphicText”
We ensure that the string length requirement is met.<

19. The Hard
Some elements are restricted (or required based on the value of the metafile version
If version == 3 then a RestrictedTextType element must appear before the first RestrictedText element
If ColorSelectionMode is indexed color is index value, else color is RGB values
Currently the author does not know how to represent some of the more
complex restrictions in the WebCGM profile. There are several clauses that
restrict (or require) an element based on the value of the metafile version.
For example if you are in a version 3 metafile and you have a Restricted Text
element, then you are required to have a Restricted Text Type element before
the first Restricted Text element appears.
I don’t know how to represent regular expressions and branching type logic using schema. One of the talks at the conference is about regular expressions in schema…

20. The Hard Part Branching logic If (condition a) then foo else bar

21. Conclusion
Creation of a schema that enforces WebCGM profile allows simple XML schema validator to validate conformance of CGM graphics.
This approach could also be used to simplify validation of other binary structured formats.

22. What’s Ahead Complete mapping CGM binary to XML Finish WebCGM schema
Work to continue in CGM Open Consortium: