RFC 18: OGR Style Support in C API

Author: Daniel Morissette

Contact: dmorissette@mapgears.com

Status: Adopted (2007-12-05)


OGR has a number of C++ classes that deal with the encoding of style information and attaching that to features. More information is available in the Feature Style Specification document.

With GDAL/OGR version 1.4.x and older, it was not possible to deal with style information using the C API. This RFC proposes the addition of functions to the C API to manipulate style information in GDAL/OGR 1.5.

Implementation Details

  • The following enums will be moved from ogr_featurestyle.h to ogr_core.h:

  • The OGRStyleMgrH (corresponding to the OGRStyleMgr C++ class) will be added to the C API:

OGRStyleMgrH  OGR_SM_Create()
void          OGR_SM_Destroy(OGRStyleMgrH hSM)

const char   *OGR_SM_InitFromFeature(OGRStyleMgrH hSM)
int           OGR_SM_InitFromStyleString(const char *pszStyleString)
int           OGR_SM_GetPartCount(OGRStyleMgrH hSM)
OGRStyleToolH OGR_SM_GetPart(OGRStyleMgrH hSM)
int           OGR_SM_AddPart(OGRStyleMgrH hSM, OGRStyleTool *sPart)
  • The OGRStyleToolH (corresponding to the OGRStyleTool C++ class) will be added to the C API:

OGRStyleToolH OGR_ST_Create(OGRSTClassId eClassId)
void          OGR_ST_Destroy(OGRStyleToolH hST)
OGRSTClassId  OGR_ST_GetType(OGRStyleToolH hST)

OGRSTUnitId   OGR_ST_GetUnit(OGRStyleToolH hST)
void          OGR_ST_SetUnit(OGRStyleToolH hST, OGRSTUnitId eUnit, double dfGroundPaperScale)

int           OGR_ST_GetParamIsNull(OGRStyleToolH hST, int eParam)
const char   *OGR_ST_GetParamStr(OGRStyleToolH hST, int eParam)
int           OGR_ST_GetParamNum(OGRStyleToolH hST, int eParam)
double        OGR_ST_GetParamDbl(OGRStyleToolH hST, int eParam)
void          OGR_ST_SetParamStr(OGRStyleToolH hST, int eParam, const char *pszParamString)
void          OGR_ST_SetParamNum(OGRStyleToolH hST, int eParam, int nParam)
void          OGR_ST_SetParamDbl(OGRStyleToolH hST, int eParam, double dfParam)
const char   *OGR_ST_GetStyleString(OGRStyleToolH hST)

int           OGR_ST_GetRGBFromString(OGRStyleToolH hST, const char *pszColor,
                                     int *nRed, int *nGreen, int *nBlue, int *nAlpha);

Note: at implementation time, the OGR_ST_GetParamIsNull() has been removed and replaced by an 'int *bValueIsNull' argument on all the OGR_ST_GetParam...() functions in order to map more closely to the C++ methods.

  • NO wrappers will be needed for the following C++ classes which are handled internally by the OGR_ST_* wrappers above:

class OGRStylePen : public OGRStyleTool
class OGRStyleBrush : public OGRStyleTool
class OGRStyleSymbol : public OGRStyleTool
class OGRStyleLabel : public OGRStyleTool
  • Note that ogr_featurestyle.h also contains a OGRSTVectorParam enum and a corresponding OGRStyleVector class but this class is currently unused and may eventually be removed, so we will not implement support for it in the C API (and the OGRSTVectorParam enum will NOT be moved to ogr_core.h).

Python and other language bindings

The initial implementation will be for the C API only and will not be ported/tested with the Python and other scripting language bindings. This will have to wait for a later release.


Daniel Morissette will implement the changes to the C API described in this RFC for the GDAL/OGR 1.5.0 release.

The first test of the new C API functions will be the conversion of MapServer's mapogr.cpp to use them.

Voting History

+1 from all PSC members (FrankW, DanielM, HowardB, TamasS, AndreyK)