FDO RFC 28 - Add X,Y,Z and M Expression Functions

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	(1.0)
Submission Date	October 20, 2008
Last Modified	Robert FortinTimestamp
Author	Greg Boone
RFC Status	Draft
Implementation Status	Pending
Proposed Milestone	3.4.0.0
Assigned PSC guide(s)	Greg Boone
Voting History
+1	Greg Boone, Orest Haslustchak, Jason Birch
+0	Frank Warmerdam
-0
-1

Overview

This RFC follows from RFC 8 by adding geometric functions to retrieve the X, Y, Z and M value of a point.

Signatures

This RFC proposes the following signatures for the new functions:

double X(<geometry property>);
double Y(<geometry property>);
double Z(<geometry property>);
double M(<geometry property>);

All of the above functions accept a geometry property of type point only. if the geometry is not a point or if the dimension of the geometry prevent from returning a proper value (e.g. Z() function on a 2D geometry), a null value is returned. The new functions will be categorized as geometry functions (!FdoFunctionCategoryType_Geometry).

Example

FdoPtr<FdoISelect> select = (FdoISelect*)mConnection->CreateCommand (FdoCommandType_Select);
select->SetFeatureClassName (L"POLE");
FdoPtr<FdoIdentifierCollection> idColl = select->GetPropertyNames();

FdoPtr<FdoExpressionCollection> functionArgs = FdoExpressionCollection::Create();
FdoPtr<FdoIdentifier> arg1 = FdoIdentifier::Create(L"GEOMETRY");
functionArgs->Add(arg1);
FdoPtr<FdoFunction> expr = FdoFunction::Create(L"X", functionArgs);
FdoPtr<FdoComputedIdentifier> computedId = FdoComputedIdentifier::Create(L"POLE_X", expr);
idColl->Add(computedId);

FdoPtr<FdoIFeatureReader> reader = select->Execute ();

Standard Considerations

During the initial proposal of this RFC (which was focusing on returning the StartX/Y/Z and EndX/Y/Z of any geometry), many altertives were considered. The concensus was that FDO should try to adopt an existing standard. The function signatures adopted here are based on the OGC ​Simple Feature Access standard.

Other functions were considered for the initial RFC proposal but are outside the scope of this RFC:

StartPoint(): Point
EndPoint(): Point
NumPoints(): Int
PointN(N): Point

Using these additional functions in combination with the X,Y,Z and M functions, it would have been possible to return the start and end coordinate for other type of geometry.

Example:

To retrieve the X value for the first point of a line --> X(StartPoint(<geometry>) or X(PointN(1, <geometry>)
To retrieve the Y value for the last point of a line --> Y(EndPoint(<geometry>) or X(PointN(NumPoints(<geometry>, <geometry>)

Affected Providers

This enhancement is being targeted to only affect the following providers: SDF. However, if a shared generic implementation can be designed, these functions may also be exposed through the SHP provider.

Required FDO Enhancements

No FDO enhancements are required.

Implications

This change will not cause any side-effects. No existing provider needs to change unless the provider wants to expose the new capability.

Test Plan

Existing unit tests will be enhanced to test those changes.

Funding/Resources

Autodesk to provide resource / funding to implement the feature for the affected providers.