[move sift.cpp] Merge pull request opencv/opencv_contrib#2301 from ab-dragon:conditionally_compute_dog_pyramid

Build DoG Pyramid if useProvideKeypoints is false

The buildDoGPyramid operation need not be performed unconditionally. In cases where it is not needed, both memory and speed performance can be improved

original commit: e45887e1c0

modules/features2d/src/sift.cpp

@@ -1110,19 +1110,20 @@ void SIFT_Impl::detectAndCompute(InputArray _image, InputArray _mask,
     }
 
     Mat base = createInitialImage(image, firstOctave < 0, (float)sigma);
-    std::vector<Mat> gpyr, dogpyr;
+    std::vector<Mat> gpyr;
     int nOctaves = actualNOctaves > 0 ? actualNOctaves : cvRound(std::log( (double)std::min( base.cols, base.rows ) ) / std::log(2.) - 2) - firstOctave;
 
     //double t, tf = getTickFrequency();
     //t = (double)getTickCount();
     buildGaussianPyramid(base, gpyr, nOctaves);
-    buildDoGPyramid(gpyr, dogpyr);
 
     //t = (double)getTickCount() - t;
     //printf("pyramid construction time: %g\n", t*1000./tf);
 
     if( !useProvidedKeypoints )
     {
+        std::vector<Mat> dogpyr;
+        buildDoGPyramid(gpyr, dogpyr);
         //t = (double)getTickCount();
         findScaleSpaceExtrema(gpyr, dogpyr, keypoints);
         KeyPointsFilter::removeDuplicatedSorted( keypoints );