FDO RFC 66 - Support coordinate system transformation when serialize XML

This page contains a request for comments document (RFC) for the FDO Open Source project. More FDO RFCs can be found on the RFCs page.

Status

RFC Template Version	1.1
Submission Date	April 19, 2013
Last Modified	Christine Bao - April 25, 2013
Author	Christine Bao
RFC Status	For Review
Implementation Status	In Progress
Proposed Milestone	3.8.1
Assigned PSC guide(s)	Greg Boone

Overview

This proposal adds coordinate system transformation support when serialize XML using FdoXmlFeatureSerializer::XmlSerialize(...).

Motivation

FdoXmlFeatureSerializer is used for serializing FDO features into XML, for example it can serialize FDO features into WFS GML format.

void FdoXmlFeatureSerializer::XmlSerialize( 
	FdoIFeatureReader*              reader, 
	FdoXmlFeatureWriter*            writer,
        FdoXmlFeatureFlags*             flags)

However currently it can only reads the features and write the native data in data source coordinate system. If user request the data in another coordinate system, it fails to return the right value. For example, if the data source SDF's coordinate system is ca83, and published as WFS service. Customer can request this WFS in LL84 coordinate system. Currently the returned WFS GML still ca83, not expected as customer wants.

Proposed Solution

FDO does not have coordinate system transformation capability. It should provide a way to let the caller to coordinate system transformation. A solution is to define a pseudo method in FDO:

class FdoCoordinateSystemTransform : public FdoDisposable
{
public:    
    FDO_API  virtual FdoIDirectPosition* CoordinateSystemTransform(FdoIDirectPosition* sourceGeometry);
};

And FdoXmlFeatureFlags adopt this if the caller wants to do coordinate system transformation.

    FDO_API virtual void SetSrsName(FdoString* srsName); 
    FDO_API virtual FdoString* GetSrsName();
    FDO_API virtual void SetCoordinateSystemTransform(FdoCoordinateSystemTransform *transform);
    FDO_API virtual FdoCoordinateSystemTransform* GetCoordinateSystemTransform();

This srs name and transform is called during FDO serializing XML. The default value is NULL, so if caller does not set it, it's ignored and work as before.

The caller can make a class inherit from FdoCoordinateSystemTransform and implement real CoordinateSystemTransform(...) function, set the instance to FdoXmlFeatureFlags, and the feature data is transformed.

Managed FDO API

The managed FDO API is a little tricky because it needs to allow the caller create managed child class and implement the coordinate system transformation in its own child class. A solution is to make a VirtualFdoCoordinateSystemTransform which inherits from VirtualObject, a template in OSGeo.Fdo.Common for forwarding unmanaged methods to managed one:

class VirtualFdoCoordinateSystemTransform : public VirtualObject <NAMESPACE_OSGEO_FDO_XML::CoordinateSystemTransform, FdoCoordinateSystemTransform>
{
public:
	FdoIDirectPosition* CoordinateSystemTransform(FdoIDirectPosition* sourceGeometry);

private:
	enum WrapperCallBits
	{
		TransformBit              = 0x01,
	};
	mutable FdoInt32 wrapperCallBits;
};

FdoIDirectPosition* VirtualFdoCoordinateSystemTransform::CoordinateSystemTransform(FdoIDirectPosition* sourceGeometry)
{
	if (!WrapperCallWrapper::IsCalling(wrapperCallBits, TransformBit))
	{
		WrapperCallWrapper ctx(wrapperCallBits, TransformBit);
		
        NAMESPACE_OSGEO_GEOMETRY::IDirectPosition ^managedSourceGeometry =  gcnew NAMESPACE_OSGEO_GEOMETRY::IDirectPositionImp(IntPtr(sourceGeometry), true);
		NAMESPACE_OSGEO_GEOMETRY::IDirectPositionImp^ destGeometry = static_cast<NAMESPACE_OSGEO_GEOMETRY::IDirectPositionImp^>(GetWrapper()->CoordinateSystemTransformPosition(managedSourceGeometry));
		return static_cast<FdoIDirectPosition*>(destGeometry->GetDisposableObject().ToPointer());
	}
	else
	{
		return 0;
	}
}

And the managed CoordinateSystemTransform can be initialized in this way:

NAMESPACE_OSGEO_FDO_XML::CoordinateSystemTransform::CoordinateSystemTransform() : Disposable(IntPtr(new VirtualFdoCoordinateSystemTransform()), true)
{
	static_cast<VirtualFdoCoordinateSystemTransform*>(GetImpObj())->SetWrapper(this);
}

By doing this the child class of CoordinateSystemTransform can do the coordinate system transformation.

Above is pseudo code as a reference. Currently there is no concrete requirement in managed side, so the managed API is not added and tested.

Implications

Existing code is not implicated.

Test Plan

Add unit test in FDO to do pseudo coordinate system transformation.

Test in MapGuide to do real coordinate system transformation.

Funding/Resources

Autodesk