Changes between Initial Version and Version 1 of FDORfc51


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Timestamp:
Jul 27, 2010, 11:52:03 PM (14 years ago)
Author:
leodai
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  • FDORfc51

    v1 v1  
     1= FDO RFC 51 - Update WFS provider to support 1.1.0 =
     2
     3This page contains an change request (RFC) for the FDO Open Source project. 
     4More FDO RFCs can be found on the [wiki:FDORfcs RFCs] page.
     5
     6
     7== Status ==
     8 
     9||RFC Template Version||(1.0)||
     10||Submission Date|| May. 06, 2010 ||
     11||Last Modified|| Leo Dai - 07/06/10:10:13||
     12||Author||Leo Dai||
     13||RFC Status||Draft||
     14||Implementation Status||Pending||
     15||Proposed Milestone||3.6.0.0||
     16||Assigned PSC guide(s)||Orest Halustchak||
     17||'''Voting History'''||||
     18||+1|| ||
     19||+0|| ||
     20||-0|| ||
     21||-1|| ||
     22 
     23== Motivation ==
     24
     25Currently 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.
     26
     27When 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.
     28
     29This purpose of this RFC is to resolve polygon vertex order issue. It outlines changes that are required in the FDO API.
     30
     31== Overview ==
     32
     33Currently there are two inconsistencies in dealing with polygon vertex order. The first consistency is the vertex order rule in a polygon loop.
     34
     35    * 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. This is the rule that OGC has defined and applies to 2D polygons.
     36    * 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.
     37
     38The second consistency is the vertex order strictness when inserting or updating a polygon.
     39
     40    * 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.
     41    * 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.
     42    * 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. !ESRI !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.
     43
     44We 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.
     45
     46    * Provides a way for users to get the vertex order rule and the vertex order strictness rule.
     47    * Provides utility methods to help users fix polygon vertex order issues. Users can call these methods to fix polygon vertex order error anytime according to the polygon vertex strictness of a geometry.
     48
     49== Proposed Solution ==
     50
     51Add 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. These will apply to 2D polygons. For 3D polygons, e.g. ones that are completely vertical, these could be in either direction depending on which way is "up".
     52
     53SQL 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.
     54
     55{{{
     56/// \brief
     57/// The FdoPolygonVertexOrderRule enumeration defines the vertex order rule in a
     58/// polygon loop. FdoPolygonOrderVertexRule values are typically counterclockwise,
     59/// clockwise and none. None value means the vertex order rule is unknown,
     60/// undefined or not care.
     61///
     62enum FdoPolygonVertexOrderRule {
     63        FdoPolygonVertexOrderRule_CCW,
     64        FdoPolygonVertexOrderRule_CW,
     65        FdoPolygonVertexOrderRule_None
     66};
     67
     68/// \brief
     69/// The FdoClassCapabilities class describes various capabilities
     70/// for a particular FDO class definition and an FDO Provider datastore.
     71///
     72class FdoClassCapabilities 
     73{
     74    ......
     75    /// \brief
     76    /// Gets the vertex order rule of the specified geometry property.
     77    ///
     78    /// \param geometryPropName
     79    /// Input the geometry property name
     80    /// \return
     81    /// Returns the vertex order rule that the specified geometry property follows.
     82    ///
     83    FDO_API FdoPolygonVertexOrderRule GetPolygonVertexOrderRule(
     84                     FdoString* geometryPropName );
     85    /// \brief
     86    /// Sets the vertex order rule of the specified geometry property.
     87    ///
     88    /// \param geometryPropName
     89    /// Input the geometry property name to set vertex order rule
     90    /// \param vertexOrderRule
     91    /// Input vertex order rule that the specified geometry follows.
     92    ///
     93    FDO_API void SetPolygonVertexOrderRule  (
     94                     FdoString* geometryPropName,
     95                     FdoPolygonVertexOrderRule vertexOrderRule );
     96    /// \brief
     97    /// Gets the vertex order strictness of the specified geometry property.
     98    /// \param geometryPropName
     99    /// Input the geometry property name
     100    /// \return
     101    /// Returns true if the vertex order of the specified geometry property is enforced.
     102    /// Returns false if it is loose.
     103    ///
     104    FDO_API FdoBoolean GetPolygonVertexOrderStrictness  (
     105                     FdoString* geometryPropName );
     106    /// \brief
     107    /// Sets the vertex order strictness of the specified geometry property.
     108    /// \param geometryPropName
     109    /// Input the geometry property name
     110    /// \param value
     111    /// Input true if the vertex order of the specified geometry property is enforced.
     112    /// Input false if it is loose.
     113    /// 
     114    FDO_API void SetPolygonVertexOrderStrictness (
     115                     FdoString* geometryPropName
     116                     FdoBoolean value );
     117    ......
     118};
     119}}}
     120
     121Add the following three functions in class !FdoSpatialUtility to fix the polygon vertex order error. The first function is used to check the polygon vertex order and fix it as needed. The second function is used to reverse polygon vertex order directly. When the polygon vertex order is known, the second function can be used because it is more efficient.
     122{{{
     123/// \brief
     124/// Spatial utility class
     125///
     126class FdoSpatialUtility
     127{
     128    ......
     129    /// \brief
     130    /// Check whether the vertex order of the input polygon follows the specified
     131    /// vertex order rule. If not, fix it.
     132    ///
     133    /// \param geometry
     134    /// Input the polygon geometry to be fixed. It can be a polygon, multipolygon,
     135    /// curvepolygon, or multicurvepolygon.
     136    /// \return
     137    /// Returns the modified polygon. 
     138    ///
     139    FDO_SPATIAL_API static FdoIGeometry* FixPolygonVertexOrder (
     140                              FdoIGeometry * geometry,
     141                              FdoPolygonVertexOrderRule vertexOrderRule );
     142
     143    /// \brief
     144    /// Reverse the vertex order of the input polygon.
     145    ///
     146    /// \param geometry
     147    /// Input the polygon geometry to be reversed. It can be a polygon, multipolygon,
     148    /// curvepolygon, or multicurvepolygon.
     149    /// \return
     150    /// Returns the modified polygon.
     151    ///
     152    FDO_SPATIAL_API static FdoIGeometry* ReversePolygonVertexOrder (
     153                              FdoIGeometry * geometry );
     154
     155    /// \brief
     156    /// Get the vertex order of the input polygon.
     157    ///
     158    /// \param geometry
     159    /// Input geometry to be checked. It can be a polygon, multipolygon,
     160    /// curvepolygon, or multicurvepolygon.
     161    /// \return
     162    /// Returns the vertex order of the input polygon.
     163    ///
     164    FDO_SPATIAL_API static FdoPolygonVertexOrderRule CheckPolygonVertexOrder(FdoIGeometry* geometry);
     165    ......
     166};
     167}}}
     168
     169== Managed FDO API ==
     170
     171The FDO Managed Interfaces will be updated in a similar manner to reflect the proposed changes.
     172
     173== Provider Implementation ==
     174
     175All FDO providers will be updated to return the correct polygon vertex order rule value and polygon vertex strictness value. For King FDO providers, we will try to find some volunteers from the community to update them.
     176
     177== Test Plan ==
     178
     179Existing 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.
     180
     181== !Funding/Resources ==
     182
     183Autodesk to provide resources / funding.