[FEATURE]. Retroactive added feature tag to this commit to mark the f…

…act that we now use native provider stats gathering. Updated min max calculation of gdal provider with more efficient implementation. Use updated stats gathering routine kindly provided by Etienne Sky.

src/providers/gdal/qgsgdalprovider.cpp

@@ -883,24 +883,18 @@ void QgsGdalProvider::computeMinMax( int theBandNo )
   {
     return;
   }
-  int bApproxOK=false;
-  double pdfMin;
-  double pdfMax;
-  double pdfMean;
-  double pdfStdDev;
   GDALRasterBandH myGdalBand = GDALGetRasterBand( mGdalDataset, theBandNo );
-  double myerval =  GDALGetRasterStatistics ( 
-      myGdalBand, 
-      bApproxOK, 
-      TRUE, 
-      &pdfMin, 
-      &pdfMax, 
-      &pdfMean, 
-      &pdfStdDev
-      );
-  Q_UNUSED(myerval);
-  mMinimum[theBandNo-1] = pdfMin;
-  mMaximum[theBandNo-1] = pdfMax;
+  int bApproxOK=false;
+  int             bGotMin, bGotMax;
+  double          adfMinMax[2];
+  adfMinMax[0] = GDALGetRasterMinimum( myGdalBand, &bGotMin );
+  adfMinMax[1] = GDALGetRasterMaximum( myGdalBand, &bGotMax );
+  if( ! ( bGotMin && bGotMax ) )
+  {
+    GDALComputeRasterMinMax( myGdalBand, TRUE, adfMinMax );
+  }
+  mMinimum[theBandNo-1] = adfMinMax[0];
+  mMaximum[theBandNo-1] = adfMinMax[1];
 }
 
 double  QgsGdalProvider::minimumValue( int theBandNo ) const
@@ -1853,59 +1847,63 @@ QgsRasterBandStats QgsGdalProvider::bandStatistics( int theBandNo )
   myProg.type = ProgressHistogram;
   myProg.provider = this;
 
-  // Suggested by Etienne Sky to use getrasterstatistics instead of compute
-  // since computerasterstatistics forces collection each time
-  // where as getrasterstatistics uses aux.xml cached copy if available
-  // Note: there is currently no progress callback in this method
-  double myerval =  GDALGetRasterStatistics ( 
-      myGdalBand, 
-      bApproxOK, 
-      TRUE, 
-      &pdfMin, 
-      &pdfMax, 
-      &pdfMean, 
-      &pdfStdDev
-      );
-  //double myerval = GDALComputeRasterStatistics 	( myGdalBand,
-  //  bApproxOK,
-  //  &pdfMin,
-  //  &pdfMax,
-  //  &pdfMean,
-  //  &pdfStdDev,
-  //  progressCallback,
-  //  &myProg
-  //) ;
-  //end of first pass through data now calculate the range
-  myRasterBandStats.bandName = generateBandName( theBandNo );
-  myRasterBandStats.bandNumber = theBandNo;
-  myRasterBandStats.range =  pdfMax - pdfMin;
-  myRasterBandStats.minimumValue = pdfMin;
-  myRasterBandStats.maximumValue = pdfMax;
-  //calculate the mean
-  myRasterBandStats.mean = pdfMean;
-  myRasterBandStats.sum = 0; //not available via gdal
-  myRasterBandStats.elementCount = mWidth * mHeight;
-  myRasterBandStats.sumOfSquares = 0; //not available via gdal
-  myRasterBandStats.stdDev = pdfStdDev;
-  myRasterBandStats.statsGathered = true;
+  // double myerval = 
+  //   GDALComputeRasterStatistics ( 
+  // 				 myGdalBand, bApproxOK, &pdfMin, &pdfMax, &pdfMean, &pdfStdDev,
+  // 				 progressCallback, &myProg ) ;
+  // double myerval = 
+  //   GDALGetRasterStatistics ( myGdalBand, bApproxOK, TRUE, &pdfMin, &pdfMax, &pdfMean, &pdfStdDev);
+  // double myerval = 
+  //   GDALGetRasterStatisticsProgress ( myGdalBand, bApproxOK, TRUE, &pdfMin, &pdfMax, &pdfMean, &pdfStdDev,
+  // 				      progressCallback, &myProg );
+
+  // try to fetch the cached stats (bForce=FALSE)
+  CPLErr myerval = 
+    GDALGetRasterStatistics ( myGdalBand, bApproxOK, FALSE, &pdfMin, &pdfMax, &pdfMean, &pdfStdDev);
+
+  // if cached stats are not found, compute them
+  if ( CE_Warning == myerval )
+  {
+      myerval = GDALComputeRasterStatistics ( myGdalBand, bApproxOK, 
+					      &pdfMin, &pdfMax, &pdfMean, &pdfStdDev,
+					      progressCallback, &myProg ) ;
+  }
+
+  // if stats are found populate the QgsRasterBandStats object
+  if ( CE_None == myerval )
+  {
+
+    myRasterBandStats.bandName = generateBandName( theBandNo );
+    myRasterBandStats.bandNumber = theBandNo;
+    myRasterBandStats.range =  pdfMax - pdfMin;
+    myRasterBandStats.minimumValue = pdfMin;
+    myRasterBandStats.maximumValue = pdfMax;
+    //calculate the mean
+    myRasterBandStats.mean = pdfMean;
+    myRasterBandStats.sum = 0; //not available via gdal
+    myRasterBandStats.elementCount = mWidth * mHeight;
+    myRasterBandStats.sumOfSquares = 0; //not available via gdal
+    myRasterBandStats.stdDev = pdfStdDev;
+    myRasterBandStats.statsGathered = true;
 
 #ifdef QGISDEBUG
-  QgsLogger::debug( "************ STATS **************", 1, __FILE__, __FUNCTION__, __LINE__ );
-  QgsLogger::debug( "VALID NODATA", mValidNoDataValue, 1, __FILE__, __FUNCTION__, __LINE__ );
-  QgsLogger::debug( "MIN", myRasterBandStats.minimumValue, 1, __FILE__, __FUNCTION__, __LINE__ );
-  QgsLogger::debug( "MAX", myRasterBandStats.maximumValue, 1, __FILE__, __FUNCTION__, __LINE__ );
-  QgsLogger::debug( "RANGE", myRasterBandStats.range, 1, __FILE__, __FUNCTION__, __LINE__ );
-  QgsLogger::debug( "MEAN", myRasterBandStats.mean, 1, __FILE__, __FUNCTION__, __LINE__ );
-  QgsLogger::debug( "STDDEV", myRasterBandStats.stdDev, 1, __FILE__, __FUNCTION__, __LINE__ );
+    QgsLogger::debug( "************ STATS **************", 1, __FILE__, __FUNCTION__, __LINE__ );
+    QgsLogger::debug( "VALID NODATA", mValidNoDataValue, 1, __FILE__, __FUNCTION__, __LINE__ );
+    QgsLogger::debug( "MIN", myRasterBandStats.minimumValue, 1, __FILE__, __FUNCTION__, __LINE__ );
+    QgsLogger::debug( "MAX", myRasterBandStats.maximumValue, 1, __FILE__, __FUNCTION__, __LINE__ );
+    QgsLogger::debug( "RANGE", myRasterBandStats.range, 1, __FILE__, __FUNCTION__, __LINE__ );
+    QgsLogger::debug( "MEAN", myRasterBandStats.mean, 1, __FILE__, __FUNCTION__, __LINE__ );
+    QgsLogger::debug( "STDDEV", myRasterBandStats.stdDev, 1, __FILE__, __FUNCTION__, __LINE__ );
 #endif
 
-  myRasterBandStats.statsGathered = true;
+    myRasterBandStats.statsGathered = true;
+
+  }
 
   return myRasterBandStats;
 
 } // QgsGdalProvider::bandStatistics
 
-
 /**
   Builds the list of file filter strings to later be used by
   QgisApp::addRasterLayer()