FDO RFC 48 - Polygon Vertex Order

This page contains an change request (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	May. 06, 2010
Last Modified	Leaf Li Timestamp
Author	Orest Halustchak, Dan Stoica, Gavin Cramer, Leaf Li
RFC Status	Ready for Review
Implementation Status	Pending
Proposed Milestone	3.5.0.0
Assigned PSC guide(s)	Orest Halustchak
Voting History
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Motivation

Currently there are some inconsistencies and missing capability in dealing with the vertex order of polygon loops for different FDO Providers. Some of data sources use clockwise vertex order rule. Some of data sources use counterclockwise vertex order rule. For example, SHP file follows clockwise vertex order rule and Oracle follows counterclockwise rule.

When inserting or updating a geometry, the current strategy of FDO Provider is focused on the user being responsible for the geometry and not having the providers fix it. FDO Providers may even throw exceptions rather than automatically try to fix a user’s geometry. So FDO should provide a way for users to get the vertex order rule and the vertex order strictness rule so that users can insert or modify polygon correctly.

This purpose of this RFC is to resolve polygon vertex order issue. It outlines changes that are required in the FDO API.

Overview

Currently there are two inconsistencies in dealing with polygon vertex order. The first consistency is the vertex order rule in a polygon loop.

• For most systems where there is a specific rule for the order of vertices in a polygon boundary, the rule is that exterior loops follow a counterclockwise order whereas inner loops follow a clockwise order. For deeper nesting of loops, the order rule alternates from level to level, with the most exterior level following the counterclockwise rule. This is the rule that OGC has defined.
• The exception to this rule is the SHP file format specification. SHP follows a clockwise rule; however the SHP format was defined before OGC existed.

The second consistency is the vertex order strictness when inserting or updating a polygon.

• SQL Server 2008 Spatial has two data types that support geometry. They are Geometry and Geography. Both of these support polygons but the Geography type has a constraint that the vertex order around loops must be counterclockwise for outer loops and clockwise for inner loops. Polygons that fail this test will be rejected. The FDO SQL Server Spatial provider does not attempt to fix these polygons.
• Oracle also uses the counterclockwise loop order rule, but is not as stringent about enforcing it. It will allow polygons to be inserted with either order. Area calculations will always be positive. A validity function however will indicate which are valid and which are invalid.
• SHP follows a clockwise polygon loop vertex order in contrast to the other formats. While applications that process SHP files may be fairly lax in requiring that polygons follow a clockwise vertex order. ArcView can render polygons with counterclockwise loops correctly and display negative areas for them. OSGEO FDO SHP provider does not change the vertex order of polygons. However, the FME SHP provider does correct the vertex order of polygons.

We are not going to change the current strategy of FDO Provider to fix polygon vertex order error automatically by FDO Provider because users may want to save their modification to geometry temporarily before they fix all of geometry errors. Moreover, it may result in performance issue. So what FDO Provider should provide is the following two functionalities.

• Provides a way for users to get the vertex order rule and the vertex order strictness rule.
• Provides one utility method to help user fix polygon vertex order error. Users can call this method to fix polygon vertex order error anytime according to the polygon vertex strictness of a geometry.

Proposed Solution

Add the two capability functions in class FdoClassCapabilities to get the polygon vertex rule and the vertex order strictness of the geometry in a feature class. SQL Server 2008 adds a complication. SQL Server 2008 handles spatial data with two separate data types, geography for geodetic data and geometry for others. The geography type has the strict enforcement of the CCW order. The geometry type has the same rule, but does not have strict enforcement. We could have the capabilities tied to a particular geometry property, which would make the capability handling more complex for providers and for applications. Alternatively we can just make these schema level capabilities and have SQL Server advertise a general strict enforcement of the vertex order rule. Unfortunately, this messes up the capability-based copying of polygons from one provider to another when the source is SQL Server. Existing geometry data may not follow the rules if it uses the SQL Server geometry type. Another alternative is to have the capability function take a coordinate system definition as a parameter. But, that adds complexity as well, and requires providers to analyze the coordinate system definition. So, we’ll stick to defining the capability function for this against the geometry property.

/// \brief
/// The FdoPolygonVertexOrderRule enumeration defines the vertex order rule in a 
/// polygon loop. FdoPolygonOrderVertexRule values are typically counterclockwise, 
/// clockwise and none. None value means the vertex order rule is unknown, 
/// undefined or not care.
///
enum FdoPolygonVertexOrderRule {
	FdoPolygonVertexOrderRule_CCW,
	FdoPolygonVertexOrderRule_CW,
	FdoPolygonVertexOrderRule_None
};

/// \brief
/// The FdoClassCapabilities class describes various capabilities
/// for a particular FDO class definition and an FDO Provider datastore.
///
class FdoClassCapabilities  
{
    ......
/// \brief
/// Gets the vertex order rule of the specified geometry property.
///
/// \param geometryPropName
/// Input the geometry property name
/// \return
/// Returns the vertex order rule that the specified geometry property follows.
///
FDO_API FdoPolygonVertexOrderRule GetPolygonVertexOrderRule( 
FdoString* geometryPropName );
/// \brief
/// Sets the vertex order rule of the specified geometry property.
///
/// \param geometryPropName
/// Input the geometry property name to set vertex order rule
/// \param vertexOrderRule
/// Input vertex order rule that the specified geometry follows.
///
FDO_API void SetPolygonVertexOrderRule  ( 
FdoString* geometryPropName,
FdoPolygonVertexOrderRule vertexOrderRule );
/// \brief
/// Gets the vertex order strictness of the specified geometry property.
/// \param geometryPropName
/// Input the geometry property name
/// \return
/// Returns true if the vertex order  of the specified geometry property is enforced. 
///
FDO_API FdoBoolean GetPolygonVertexOrderStrictness  ( 
FdoString* geometryPropName );
/// \brief
/// Sets the vertex order strictness of the specified geometry property.
/// \param geometryPropName
/// Input the geometry property name
/// \param value
/// Input true if the vertex order  of the specified geometry property is enforced. Or input false. 
///  
FDO_API void SetPolygonVertexOrderStrictness ( 
FdoString* geometryPropName
FdoBoolean value );
......
};

Add a new function in class FdoSpatialUtility to fix the polygon vertex order error.

/// \brief
/// Spatial utility class
///
class FdoSpatialUtility
{
    ......
/// \brief
/// Fixes polygon vertex order error according to the specified vertex order rule.
///
/// \param geometry
/// Input the polygon geometry to be fixed.
/// \param vertexOrderRule
/// Input polygon vertex order rule that the specified geometry should follow.
/// \return
/// Returns the fixed polygon. If the input polygon already follows the specified
/// vertex order rule or the specified vertex order rule is 
/// FdoPolygonVertexOrderRule_None, returns NULL .  
///
FDO_API FdoIGeometry* FixPolygonVertexOrder  (
FdoIGeometry * geometry, 
FdoPolygonVertexOrderRule vertexOrderRule );
......
}

Managed FDO API

The FDO Managed Interfaces will be updated in a similar manner to reflect the proposed changes.

Provider Implementation

All FDO providers will be updated to return the correct polygon vertex order rule value and polygon vertex strictness value.

Test Plan

Existing FDO core unit tests will be expanded to test the proposed enhancements defined above. Provider specific unit tests will be added to test the proposed enhancements defined above.

Funding/Resources

Autodesk to provide resources / funding.