Handling minor extensions to Xdmf data format

[markcmiller86]

Ultimately, the Xdmf is implemented as XML. This means we can easily extend the Xdmf format in small ways by supporting additional XML attributes on the main XML tags that Xdmf defines. XML attributes are those name = value pairs that occur inside an XML tag as in...

   <myTag attr1=name1 attr2=name2>
   .
   .
   .
   </myTag>

For example, in a recent update to VisIt's Xdmf plugin to support xy-curves (which are topologically 1 dimensional things), we added support for a GridPurpose attribute to the <Grid> tag. Currently, the only supported value for that attribute in VisIt is "Curve". So, an Xdmf file with that new attribute in it might look like...

<?xml version="1.0" ?>
<!DOCTYPE Xdmf SYSTEM "Xdmf.dtd" []>
<Xdmf xmlns:xi="http://www.w3.org/2001/XInclude" Version="2.0">
    <Domain>
        <Grid Name="Curve" GridType="Uniform" GridPurpose="Curve">
            <Topology TopologyType="2DRectMesh" NumberOfElements="1 12"/>
            <Geometry Type="VXVY">
                <DataItem Format="XML" Dimensions="12">
                0 1 2 3 4 6 8 10 12 15 25 50
                </DataItem>
                <DataItem Format="XML" Dimensions="1">
                0
                </DataItem>
            </Geometry>
            <Attribute Name="Node Centered Values" Center="Node">
                <DataItem Format="XML" Dimensions="1 12">
                100 200 300 400 500 600 600 700 800 900 1000 1100
                </DataItem>
            </Attribute>
        </Grid>
    </Domain>
</Xdmf>

To handle introspection for this newly defined attribute, In the plugin code below, grid is an XdmfGrid*and one of the key Xdmf data modeling *elements.

    grid->Get("GridPurpose") &&
    StringHelpers::CaseInsensitiveEqual(grid->Get("GridPurpose"), "curve", 5))

But, all of Xdmf's data modeling elements inheret from a base type of XdmfElement which support Get() for an attribute name which may be defined on the associated tag in the Xdmf XML file.

visit/src/databases/Xdmf/avtXdmfFileFormat.C

Lines 1692 to 1706 in 2b36a3e

	// This lengthy if conditional is intended to really lock down the
	// manner in which an XY curve object is recognized in an Xdmf file.
	// It must be 2DRectMesh. The Y dimension must be of size 1 and
	// there must be an additional xml attrbute, GridPurpose, defined with
	// value "curve". That latter bit is outside Xdmf spec but because
	// it is handled as an xml attribute in the <Grid> tag, it should be ok.
	grid->GetTopology()->GetShapeDesc()->GetShape(dimensions);
	if (gridToExamine->GetTopology()->GetTopologyType() == XDMF_2DRECTMESH &&
	grid->GetGeometry()->GetGeometryType() == XDMF_GEOMETRY_VXVY &&
	dimensions[1]>1 && dimensions[0]==1 &&
	grid->Get("GridPurpose") &&
	StringHelpers::CaseInsensitiveEqual(grid->Get("GridPurpose"), "curve", 5)) {
	avtCurveMetaData *cmd = new avtCurveMetaData(attributeName.str());
	md->Add(cmd);
	curveToGridMap[attributeName.str()] = grid->GetName();

A similar approach could be taken to define other small metadata tidbits such as whether a mesh variable is intensive or extensive or handling cycle or state index in addition to time, or for defining VisIt expressions.