Add GDALRegisterTransformer() to public gdal_alg.h API

and convert existing GCP_POLYNOMIAL, GCP_TPS, TPS and GeoLoc
transformers to use it.

This also allows custom spatial point transformers to be registered
and used by GDALCreateGenImgProjTransformer(), and in warped VRT.

alg/gdal_alg.h

@@ -96,21 +96,83 @@ typedef int (*GDALTransformerFunc)(void *pTransformerArg, int bDstToSrc,
                                    int nPointCount, double *x, double *y,
                                    double *z, int *panSuccess);
 
-/*! @cond Doxygen_Suppress */
+/** Signature to be set in GDALTransformerInfo::abySignature member */
 #define GDAL_GTI2_SIGNATURE "GTI2"
 
+/** Structure that must be instantiated as the first member of a transformer
+ * instance.
+ */
 typedef struct
 {
+    /** Signature. Should be filled with GDAL_GTI2_SIGNATURE. */
     GByte abySignature[4];
+
+    /** Class name. Should uniquely identify the transformer. */
     const char *pszClassName;
+
+    /** Transformer callback. */
     GDALTransformerFunc pfnTransform;
+
+    /** Cleanup callback. */
     void (*pfnCleanup)(void *pTransformerArg);
+
+    /** XML serialization callback. */
     CPLXMLNode *(*pfnSerialize)(void *pTransformerArg);
+
+    /** Callback to instanciate a transformer instance similar to the passed one. */
     void *(*pfnCreateSimilar)(void *pTransformerArg, double dfSrcRatioX,
                               double dfSrcRatioY);
 } GDALTransformerInfo;
 
-/*! @endcond */
+/* Custom transformers */
+
+/** Function pointer that instantiates a transformer instance for use by the
+ * GDALCreateGenImgProjTransformer() family of functions.
+ *
+ * The transformer instance should be a structure whose first member is an
+ * instance of GDALTransformerInfo.
+ *
+ * @param pszMethod Transformation method. May be NULL, in which case the
+ *                  callback is free to decide, from the content of the dataset
+ *                  and transformer options, if it can be instantiated.
+ * @param bIsSrcTransformer true if the transformer is used as the source
+ *                          transformer of GDALGenImgProjTransform.
+ *                          false if the transformer is used as the target
+ *                          transformer of GDALGenImgProjTransform.
+ * @param hDS source or target dataset, depending of bIsSrcTransformer.
+ *            The passed value is not NULL.
+ * @param papszTransformOptions Transformer options. May be NULL.
+ * @param[in,out] phSRS Pointer to a OGRSpatialReferenceH. The input *phSRS may be
+ *                   set. The callback may assign a new value to *phSRS (to be
+ *                   released with OSRRelease(). If the callback needs to free
+ *                   the passed *phSRS value, it must do so with OSRRelease().
+ * @return a new transformer instance, or NULL in case of error or if
+ * pszMethod == NULL and the callback does not recognize it should be
+ * instantiated.
+ */
+typedef void *(*GDALTransformerCreateForGenImgTransformer)(
+    const char *pszMethod, bool bIsSrcTransformer, GDALDatasetH hDS,
+    char **papszTransformOptions, OGRSpatialReferenceH *phSRS);
+
+/** Function pointer that takes a XML tree as argument and returns a
+ * transformer instance.
+ *
+ * The transformer instance returned should be a structure whose first member
+ * is an instance of GDALTransformerInfo.
+ *
+ * @param psTree XML tree
+ * @return a new transformer instance, or NULL in case of error
+ */
+typedef void *(*GDALTransformDeserializeFunc)(const CPLXMLNode *psTree);
+
+void CPL_DLL *
+GDALRegisterTransformer(const char *pszTransformName,
+                        GDALTransformerFunc pfnTransformerFunc,
+                        GDALTransformDeserializeFunc pfnDeserializeFunc,
+                        GDALTransformerCreateForGenImgTransformer
+                            pfnTransformerCreateForGenImgTransformerFunc);
+
+void CPL_DLL GDALUnregisterTransformer(void *pData);
 
 /*! @cond Doxygen_Suppress */
 void CPL_DLL GDALDestroyTransformer(void *pTransformerArg);
@@ -246,6 +308,8 @@ int CPL_DLL GDALApproxTransform(void *pTransformArg, int bDstToSrc,
                                 int nPointCount, double *x, double *y,
                                 double *z, int *panSuccess);
 
+/* Warping related functions */
+
 int CPL_DLL CPL_STDCALL GDALSimpleImageWarp(
     GDALDatasetH hSrcDS, GDALDatasetH hDstDS, int nBandCount, int *panBandList,
     GDALTransformerFunc pfnTransform, void *pTransformArg,

alg/gdal_alg_priv.h

@@ -176,14 +176,7 @@ constexpr const char *GDAL_GEN_IMG_TRANSFORMER_CLASS_NAME =
 
 bool GDALIsTransformer(void *hTransformerArg, const char *pszClassName);
 
-typedef void *(*GDALTransformDeserializeFunc)(CPLXMLNode *psTree);
-
-void CPL_DLL *GDALRegisterTransformDeserializer(
-    const char *pszTransformName, GDALTransformerFunc pfnTransformerFunc,
-    GDALTransformDeserializeFunc pfnDeserializeFunc);
-void CPL_DLL GDALUnregisterTransformDeserializer(void *pData);
-
-void GDALCleanupTransformDeserializerMutex();
+void GDALCleanupTransformers();
 
 /* Transformer cloning */
 

alg/gdal_crs.cpp

@@ -102,9 +102,10 @@ struct GCPTransformInfo
     volatile int nRefCount{};
 };
 
+static CPLXMLNode *GDALSerializeGCPTransformer(void *pTransformArg);
+
 CPL_C_START
-CPLXMLNode *GDALSerializeGCPTransformer(void *pTransformArg);
-void *GDALDeserializeGCPTransformer(CPLXMLNode *psTree);
+void *GDALDeserializeGCPTransformer(const CPLXMLNode *psTree);
 CPL_C_END
 
 /* crs.c */
@@ -501,7 +502,7 @@ CPLXMLNode *GDALSerializeGCPTransformer(void *pTransformArg)
 /*               GDALDeserializeReprojectionTransformer()               */
 /************************************************************************/
 
-void *GDALDeserializeGCPTransformer(CPLXMLNode *psTree)
+void *GDALDeserializeGCPTransformer(const CPLXMLNode *psTree)
 
 {
     std::vector<gdal::GCP> asGCPs;
@@ -515,7 +516,7 @@ void *GDALDeserializeGCPTransformer(CPLXMLNode *psTree)
     /* -------------------------------------------------------------------- */
     /*      Check for GCPs.                                                 */
     /* -------------------------------------------------------------------- */
-    CPLXMLNode *psGCPList = CPLGetXMLNode(psTree, "GCPList");
+    const CPLXMLNode *psGCPList = CPLGetXMLNode(psTree, "GCPList");
 
     if (psGCPList != nullptr)
     {

alg/gdal_rpc.cpp

@@ -59,9 +59,10 @@
 
 // #define DEBUG_VERBOSE_EXTRACT_DEM
 
-CPL_C_START
 CPLXMLNode *GDALSerializeRPCTransformer(void *pTransformArg);
-void *GDALDeserializeRPCTransformer(CPLXMLNode *psTree);
+
+CPL_C_START
+void *GDALDeserializeRPCTransformer(const CPLXMLNode *psTree);
 CPL_C_END
 
 constexpr int MAX_ABS_VALUE_WARNINGS = 20;
@@ -2326,22 +2327,22 @@ CPLXMLNode *GDALSerializeRPCTransformer(void *pTransformArg)
 /*                   GDALDeserializeRPCTransformer()                    */
 /************************************************************************/
 
-void *GDALDeserializeRPCTransformer(CPLXMLNode *psTree)
+void *GDALDeserializeRPCTransformer(const CPLXMLNode *psTree)
 
 {
     char **papszOptions = nullptr;
 
     /* -------------------------------------------------------------------- */
     /*      Collect metadata.                                               */
     /* -------------------------------------------------------------------- */
-    CPLXMLNode *psMetadata = CPLGetXMLNode(psTree, "Metadata");
+    const CPLXMLNode *psMetadata = CPLGetXMLNode(psTree, "Metadata");
 
     if (psMetadata == nullptr || psMetadata->eType != CXT_Element ||
         !EQUAL(psMetadata->pszValue, "Metadata"))
         return nullptr;
 
     char **papszMD = nullptr;
-    for (CPLXMLNode *psMDI = psMetadata->psChild; psMDI != nullptr;
+    for (const CPLXMLNode *psMDI = psMetadata->psChild; psMDI != nullptr;
          psMDI = psMDI->psNext)
     {
         if (!EQUAL(psMDI->pszValue, "MDI") || psMDI->eType != CXT_Element ||

alg/gdal_tps.cpp

@@ -47,9 +47,10 @@
 #include "gdal_priv.h"
 #include "gdalgenericinverse.h"
 
+static CPLXMLNode *GDALSerializeTPSTransformer(void *pTransformArg);
+
 CPL_C_START
-CPLXMLNode *GDALSerializeTPSTransformer(void *pTransformArg);
-void *GDALDeserializeTPSTransformer(CPLXMLNode *psTree);
+void *GDALDeserializeTPSTransformer(const CPLXMLNode *psTree);
 CPL_C_END
 
 struct TPSTransformInfo
@@ -438,13 +439,13 @@ CPLXMLNode *GDALSerializeTPSTransformer(void *pTransformArg)
 /*                   GDALDeserializeTPSTransformer()                    */
 /************************************************************************/
 
-void *GDALDeserializeTPSTransformer(CPLXMLNode *psTree)
+void *GDALDeserializeTPSTransformer(const CPLXMLNode *psTree)
 
 {
     /* -------------------------------------------------------------------- */
     /*      Check for GCPs.                                                 */
     /* -------------------------------------------------------------------- */
-    CPLXMLNode *psGCPList = CPLGetXMLNode(psTree, "GCPList");
+    const CPLXMLNode *psGCPList = CPLGetXMLNode(psTree, "GCPList");
 
     std::vector<gdal::GCP> asGCPs;
     if (psGCPList != nullptr)

alg/gdalgeoloc.cpp

@@ -69,9 +69,10 @@ constexpr float INVALID_BMXY = -10.0f;
 #warning "Remove me before committing"
 #endif
 
+static CPLXMLNode *GDALSerializeGeoLocTransformer(void *pTransformArg);
+
 CPL_C_START
-CPLXMLNode *GDALSerializeGeoLocTransformer(void *pTransformArg);
-void *GDALDeserializeGeoLocTransformer(CPLXMLNode *psTree);
+void *GDALDeserializeGeoLocTransformer(const CPLXMLNode *psTree);
 CPL_C_END
 
 /************************************************************************/
@@ -2155,13 +2156,13 @@ CPLXMLNode *GDALSerializeGeoLocTransformer(void *pTransformArg)
 /*                   GDALDeserializeGeoLocTransformer()                 */
 /************************************************************************/
 
-void *GDALDeserializeGeoLocTransformer(CPLXMLNode *psTree)
+void *GDALDeserializeGeoLocTransformer(const CPLXMLNode *psTree)
 
 {
     /* -------------------------------------------------------------------- */
     /*      Collect metadata.                                               */
     /* -------------------------------------------------------------------- */
-    CPLXMLNode *psMetadata = CPLGetXMLNode(psTree, "Metadata");
+    const CPLXMLNode *psMetadata = CPLGetXMLNode(psTree, "Metadata");
 
     if (psMetadata == nullptr || psMetadata->eType != CXT_Element ||
         !EQUAL(psMetadata->pszValue, "Metadata"))